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STATE OF CALIFORNIA
DEPARTMENT OF NATURAL RESOURCES
Geology of the
San Jose-Mount Hamilton Area,
California
BULLETIN 157
1951
DIVISION OF MINES
FEHBY BUILDING, SilH FRANCISCO
STATE OF CALIFORNIA
EARL WARREN. Governor
DEPARTMENT OF NATURAL RESOURCES
WARREN T. HANNUM. Director
DIVISION OF MINES
FERRY BUILDING. SAN FRANCISCO 11
OLAF P. JENKINS, Chief
SAN FRANCISCO
BULLETIN 157
APRIL 1951
GEOLOGY OF THE
SAN JOSE-MOUNT HAMILTON AREA,
CALIFORNIA
By MAX D. CRITTENDEN, JR.
UNIVERSITY OF CALJFORMA
LIBRARY
COLLEGE OF AGRICULTURE
DAVIS
LETTER OF TRANSMITTAL
To His Excellency
THE HONORABLE EARL WARREX
Governor of the State of California
Sir : I have the honor to transmit herewith Bulletin 157, Geology of
the San Jose-Mount Hamilton Area, California, prepared under the
direction of Olaf P. Jenkins, Chief of the Division of !Mines, Department
of Natural Resources. The report is accompanied by a detailed colored
lithographed map of the parts of the quadrangles covered. It also eon-
tains a number of photographs, cross-sections, and various other illus-
trations. Detailed descriptions are given of the rocks and their composi-
tion, structure, and history, together with the economic minerals —
quicksilver, manganese, and road material — found in them. The subject
of ground water conditions is also treated. -
In preparing this report, the author, ]\Iax D. Crittenden, Jr.. carried
on his tield work and laboratorv studies in connection with fulfilling the
requirements of his doctorate at the University of California. The proj-
ect was done in cooperation with the Division of Mines and represents
one of a series of such projects of mapping the geology of the State.
Respectfully submitted,
Warren T. Hannum. Director
Department of Natural Resources
November 27. 1950
73350
(3)
CONTENTS
Page
ABSTRACT "^
INTRODUCTION '^
PHYSIOGRAPHY ^^
STRATIGRAPHY ^^
IP)
Jurassic system
Franciscan formation . ^'^
Knoxville (?) formation 26
Cretaceous system
Oakland conglomerate (?) ^^
Berryessa formation
Tertiary system ^_
Temblor formation *^
Monterey formation **'
on
Briones sandstone •^'
Orinda formation
Tertiary-Quaternary (?) system 42
Packwood gravels ^"
Quaternary system '
Santa Clara formation "*•''
44
Terrace gravel
Landslides '
Alluvium ^^
47
Igneous rocks
Alum Rock rhyolite ■*'
Andesite
STRUCTURE ^^
Faults ^^
Calaveras fault system ^'^
Older faults of the Calaveras zone 53
Hayward fault system ^^
Older faults of the Hayward zone 54
Silver Creek fault 55
Other longitudinal faults 56
Faults east of Calaveras Dam 57
Cross faults ^"^
Folds ^"^
pro
Structural dynamics
GEOLOGIC HISTORY ^^ "
ECONOMIC GEOLOGY ^
^ . , ., 60
Quicksilver
^q
Manganese
64
Road metal and rip-rap "^
65
Sand and gravel
Ground water
BIBLIOGRAPHY ^^
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ILLUSTRATIONS
Page
Plait' 1. Geologic nijip of llic Sail Jose-Mount Hamiltou area, Cali-
fornia In pocket
2. ISIap of San Jose-Mount Hamilton area showing toposraphy.
location of mineral deposits, and location of geologic
sections - In pocket
3. Geologic sections through the San Jose-Mount Hamilton
area In pocket
4. Photo showing gently sloping summit of Oak Ridge from
summit of Poverty Ridge 32-33
5. A, Photo showing subdued topography of San Felipe Hills.
B, Photo showing smooth summit of Poverty Ridge 32-33
6. A. Photo showing view down Arroyo Hondo from point west
of junction of Smith Creek and Isaliel Creek. B, Photo
showing subdued topography of Ia)s Beullis Hills and the
north slopes of Alum Rock Canyon 32-33
7. Photo showing north rim of Alum Rock Canyon and Los
Buellis Hills 32-33
8. Photo showing view southwest across San Felipe Valley 32-33
9. A, Photo showing nodule of bladed actinolite and talc
wrapped by chlorite and actinolite. />', Photo showing
crumpled chert replaced by glaueophane schist 32-33
10. Photo showing opaline chert of the middle member of the
Monterey formation 32-33
n. Photo showing badland erosion of Santa Clara formation
east of San Felipe Road 32-33
Figure 1. Relief map of California showing location of the San Jose-Mount Ham-
ilton area 8
2. Profiles of Penitencia Creek in Alum Rock Canyon 14
3. Profile of Oak Ridge and Valpe Kidge showing gentle northward slope
and relationship to the Livermore gravels 16
4. Generalized stratigraphic column of the San Jose-Mount Hamilton area 22
5. Distribution of glaueophane schist in the San Jose-Hamilton area 24
6. Photomicrographs of Franciscan, Berryessa, and Briones sandstones ._ 28
7. Photomicrograph of hornblende andesite from the Orinda formation 29
8. Map showing distribution of Alum Rock rhyolite 46
9. Photomicrograph of andesite from Silver Creek 49
10. Structural features of the San Jose-Mount Hamilton area 50
11. Map showing distribution of Cretaceous and Miocene rocks along the
Calaveras fault zone 51
12. Section through Silver Creek fault zone along line E-E', plate 2 56
13. Geologic map of the Silver Creek mine area 61
14. Geologic map of the Hillsdale mine area near San Jose 62
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GEOLOGY OF THE
SAN JOSE-MOUNT HAMILTON AREA, CALIFORNIA*
By Max D. Crittenden, Jr.
ABSTRACT
The San Jose-Mount Hamilton area is in the central Coast Ranges of California,
about ~>0 miles southeast of San Francisco ; it extends from Copernicus Peak, near
Mount Hamilton, to the edge of the Santa Clara Valley.
The oldest rocks exposed are arkosic sandstone and shale, and small amounts of
thin-bedded chert and altered volcanic rocks, of the Upper Jurassic Franciscan forma-
tion. The Knoxville formation (Upper Jurassic) is in fault contact with the Francis-
can. Large masses of serpentine and smaller plugs and dikes of gabbro and diorite
intrude these sediments ; glaucophane schists and their unusual mineralogical assem-
blages are developed locally.
As much as 5,000 feet of Oakland conglomerate ( ?), succeeded by fine-grained
sediments of the Berryessa formation, were deposited east of San Francisco Bay ; scant
fossil evidence suggests that both these units are Lower Cretaceous in age. Between the
Cretaceous and middle Miocene, there is an interval unrecorded in sediments, during
which the dominant trends and major structural units of the Coast Ranges were blocked
out. The west edge of the Diablo Range and the block now beneath the Santa Clara
Valley were uplifted and sti-ipped of Cretaceous rocks. The faults thus formed estab-
lished lines of weakness which were later followed by faults with largely horizontal
displacement ; the narrow block between them became a persistently negative belt
called the Tularcitos trough.
Seas again advanced over the area, depositing the Temblor and Monterey forma-
tions during the middle Miocene, and the Briones sandstone during the upper Miocene.
Following this, the core of the Diablo Range was uplifted and folded, exposing the
Franciscan formation over large areas during the deposition of the Orinda formation
in the Tularcitos trough. The final and strongest folding occurred sometime during the
Pliocene when rocks of the Tularcitos trough underwent strong compression and over-
thrusting by the Franciscan core of the range, and were elevated to become part of the
Diablo Range.
During the early Pleistocene the conglomerates of the Pack wood gravel and Santa
Clara formation accumlated in the ancestral Santa Clara Valley. Horizontal move-
ments on the Calaveras and Hayward faults began, cutting across the axes of earlier
folds. Twice-renewed uplift of the Diablo Range brought the mountains to their present
altitude.
Quicksilver deposits in Silver Creek and Oak Hills are believed to be associated
with late Pleistocene to Recent volcanism. Minor deposits of manganese occur in the
cherts of the Franciscan formation.
INTRODUCTION
Location. The San Jose-Mount Hamilton area is in the Coast
Ranges of California about 50 miles southeast of San Francisco. The por-
tion mapped is on the west slope of the Diablo Range, between the latitude
of 37°15'N. and 37°30'N. ; it extends from a line through Copernicus
Peak on Mount Hamilton west to the edge of Santa Clara Valley.
Field Work. The work was started in December 1940 ; about a
month was spent in the field between then and June 15, 1941, and a few
days were spent in January 1942. Work was resumed on weekends in
October 3946, and continued full time from March 1 to June 1, 1947, and
from November 20, 1947, to May 1, 1948.
Mapping was begun using the topographic maps of the U. S. Geolog-
ical Survey on a scale of 1 : 62,500. After the war, maps made by multi-
plex by the U. S. Army Engineers became available, and work was con-
tinued on this base. In the spring of 1947, aerial photographs on the scale
* Submitted in partial fulfillment of the requirements for the degree Doctor of
Philosophy, University of California, Berkeley, California. Manuscript submitted for
publication May 1949.
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8
I8S°00'
SAN JOSE-MOUNT HAMILTON AREA
Ite^O*' 122*00 ' 121*30-
[Bull. 157
i2r1oo"
38°30'
3«°30'
37»J0'
37'^30'
I23°00'
I22''00'
SCALE
K) 5 10
121*30'
121*00'
20 MILES
Figure ]. Index map showing location of tlie San Jose-Mount Hamilton area
of 1 : 20,000 were obtained, and most of the recent work has been done
directly on the photos. The original maps, aerial photographs, and speci-
mens are on file in the Department of Geology of the University of Cali-
fornia at Berkeley.
Acknowledgments. It is a pleasure to acknowledge the courtesy and
helpfulness of the many residents of the area who permitted the author
to drive or walk over their property. He is particularly indebted to Mr.
Wright of the Grant Ranch, Mr. Douglas C. Alexander and Mr. Paul
C. Hoover of Rancho Los Huecos, Mr. Shelton of the Richmond Ranch,
Mr. W. S. Holmes of the Paradise and Holmes Ranches, Mrs. A. F.
Kirschner, Mr. N. A. Eckert, and Mr. Walter Yount of the San Francisco
Water Department, Mr. E. 0. Wool, Mr. John Rogers and Mr. John
Tiernan ; and Mr. Lee Ogier for the genuine interest and hospitality I
enjoyed at the Ogier Ranch on Alameda Creek.
To Dr. Wayne E. Kartchner, of San Jose State College, gratitude is
expressed for the loan of a petrographic microscope. Acknowledgment is
also made to Prof. N. L. Taliaferro, under whose direction the work was
done, for his interest and assistance.
To the regents of the University of California, the author is indebted
for financial assistance during 1947 and 1948 which made possible the
completion of the work.
HMBBBH
1951] ENTRODUCTIOX 9
Geography. The San Jose-Mount Hamilton area is on the west
edge of the Diablo Range, about 50 miles south-southeast of San Fran-
cisco. Santa Clara Valley, a continuation of the trough of San Francisco
Bay, slopes upAvard imperceptibly to the south, attaining an altitude of
184 feet at Edenvale. To the east, broad, low alluvial fans with a slope
seldom exceeding 100 feet per mile, reach altitudes of 300 and 500 feet at
the foot of the mountains. Here and there the orchards and fields of the
Santa Clara Valley are interrupted by isolated hills such as Oak Hills
south of San Jose, whose rocky bedrock surfaces indicate that they are
the tops of ridges or hills only partly covered by alluvium.
The mountains rise abruptly from the alluvium to altitudes of 1.000
to 2.000 feet. Their average slope is 600 to 1.000 feet per mile. From "^arm
Springs to Evergreen the mountain front is a relatively smooth slope,
broken only by the canyons of Arroyo de Los Coches. Berryessa Creek,
and Penitencia Creek. Except for those larger streams, the creeks floAving
down the surface are short, steep, and intermittent, and are in shallow,
poorly developed canyons. The major streams of the area form a modified
trellis pattern with a northwest trend. The canyons are very steep, reach-
ing a maximum in Arroyo Hondo, which is nearly 3000 feet deep west of
^Mount Day. Most of the area west of ]\Iount Hamilton drains northward
through Calaveras Vallev or Arrovo Hondo, into Alameda Creek, which
reaches San Francisco Bay tkrough Xiles Canyon. A small area near San
Felipe Valley drains south into Coyote Creek, which flows into the Santa
Clara Valley near Morgan Hill.
Immediately east of the Santa Clara Valley is a fairly continuous
ridge, separated from the rest of the mountains by the Calaveras fault.
East of Warm Springs this ridge rises to an elevation of 2608 feet, form-
ing ]\Ionument Peak. It continues south to Los Buellis Hills and the high
points along Sierra Road, but is cut through completely by Alum Rock
Canyon of Penitencia Creek. To the south, it continues from Alum
Rock triangulation station through Masters Hill and the ridge east of
Evergreen. East of this ridge, a series of aligned creeks and valleys,
extending through Calaveras Valley. Halls Valley, and San Felipe Valley,
marks the course of the Calaveras fault zone. This prominent feature has
provided reservoir sites at three places within the area mapped, and in
Coyote reservoir a short distance to the south of the area.
Viewed from a high point like ]\Iount Lewis, near the north edge
of the area, the ridge crests rise successively higher east from the valley
toward Mount Hamilton, Mount Isabel. Mount Day. and Black Mountain,
all about 4.000 feet in elevation. East of here, the average elevation
decreases, and the area slopes into the interior valleys of upper Isabel
Creek and Arroyo Bayo.
The climate of the area is of Mediterranean type, with mean tempera-
tures ranging from about 40 to 70 degrees. AA'erage rainfall is 16-17
inches in San Jose and 32 inches at Mount Hamilton. Most of the rain-
fall occurs during the months of December. January, and February : the
months of June. July, and August are commonly almost dry.
The smooth lower slopes of the mountains are mostly cultivated or
are open grassland. Steep dry slopes are commonly covered by a sage-
brush association characterized by Artemisia and the bush monkey flower
(Mimulus). Within the range, the southwest slopes and ridgetops are
open and grassy, but scattered oaks are abundant ; the north-facing
10 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
slopes and dee]) canyons are covered with brush or woods in which vari-
ous types of oak predominate. The summits of some of the higher ridges,
such as Mount Lewis and Mount Day and the southern parts of the San
Felipe Hills have developed local mixed forests of coulter and yellow
pine and black oak. The interior of the Diablo Range north and east of
Mount Hamilton is dominated by a dense cover of chaparral, in which
manzanita (Arctostaphylos), buckthorn (Rhanmus), chamise (Adenos-
toma), mountain mahogony (Cercocarpus), and Baccharis are common
types.
The flora of the Mount Hamilton range has been studied by Shar-
smith,^ who discusses the effect of lithology on its development. This
relationship is most striking in the case of serpentine which usually shows
a characteristic flora consisting of species which are to be found in many
dry rock areas, species which occur mostly on serpentine, and a few
species whose occurrence is restricted to serpentine. One example is the
long narrow area of serpentine northwest of Mount Hamilton which is
characterized by a thick growth of chaparral, contrasting strongly with
the surrounding open grassland and forests. The influence of the strati-
fied rocks on vegetation is less marked than in many areas of the Coast
Ranges.
Roads are abundant and fairly closely spaced throughout most of
the area. Several paved county roads extend up the mountain front east
of Santa Clara Valley. East of this ridge, fire control and ranch roads
extend along most of the ridges. Only a few of the deep canyons are
accessible by road. The area of Calaveras Dam is accessible either from
Milpitas or Sunol. The highway from San Jose to Mount Hamilton
provides a good section across the entire area near the center. The Dry
Creek road southeast of Evergreen and ranch roads leading east to Isabel
Valley from San Felipe Valley make access to this area relatively easy.
Previous Work. The earliest geological study of the San Jose-
Mount Hamilton area was made by J. D. Whitney in 1865 as part of
the Geological Survey of California.^ The sandstone, chert, and meta-
morphic rocks of the Franciscan formation were recognized and distin-
guished from the fossiliferous Tertiary rocks in Alum Rock Canyon;
serpentine and its differentiates were pointed out.
The first detailed studv was bv students of the Stanford Geological
Survey in 1905 and 1906, in 1911 and 1912, and in 1922 and 1923. Two
short papers, the first dealing with stratigraphy ^ and the second with
petrography ^ appeared about this time. E. C. Templeton also completed
a study of the San Jose quadrangle in 1911.*^ All this early work was
compiled and extended by F. P. Vickery ^ whose work is the first general
1 Sharsmith, Helen K., The flora of the Mount Hamilton range of California:
Am. Midland Naturalist, vol. 34, no. 3, pp. 289-367, 194.5.
- Whitney, J. D., Geological survey of California, vol. 1, pp. 47-52, Philadelphia,
Sherman and Company, 1865.
3 Crandall, Roderic, The Cretaceous stratigraphy of the Santa Clara Valley : Am.
Jour. Sci., 4th ser., vol. 24, p. 41, 1907.
* Carev, E. P., and Miller, W. J., The crystalline rocks of the Oak Hills near San
Jose : Jour. Geology, vol. 15, pp. 161-166, 1907.
^ Templeton, B. C, The geology and stratigraphy of the San Jose quadrangle,
unpublished Doctor's Thesis, Stanford University, 1913.
"Vickery, P. P., The structural dynamics of the Livermore region: Jour. Geology,
vol. 33, pp. 608-628, 1925.
rrrMWM^'^^iMflH
1951] PHYSIOGRAPHY 11
report on the area to be published. AVillis '^ mentions briefly the physi-
ographic features of the Mount Hamilton Range in a discussion of the
physiography of the Coast Ranges.
The structural dynamics of parts of the Ilayward and Calaveras
fault zones have been discussed by B. L. Clark. "^ The general geology of
the Diablo Raiige, and a section through the Alum Rock Canyon area
were recently published by Taliaferro.^ Interesting mineralogical occur-
rences in the area have been described by A. F. Rogers.^"
PHYSIOGRAPHY
General Statement. The physiographic development of the San
Jose-Mount Hamilton area is approaching the mature stage. Ridges are
commonly narrow, canyons are steep, and streams are approaching grade
in their lower reaches. The development is far from uniform, however,
and complicated by many structural irregularities. It was immediately
apparent in the field that the present form is the product of more than
one period of erosional development, and that its features have been
inherited from older cycles of erosion onh^ partly preserved.
Present Erosion Cycle. Canyon cutting, and rapid headward
growth of stream valleys now is the dominant process over almost all
the area. Arroyo Hondo, Smith Creek north of the Smith Creek Ranger
Station, Isabel Creek west of its junction with Bonita Creek, and Alameda
Creek northwest of the Ogier Ranch are all youthful streams, flowing in
steep narrow canyons. There is no suggestion of recent lateral cutting.
From Poverty Ridge to the San Felipe Hills, the streams flowing into
Arroyo Hondo and Smith Creek are developing headward rapidly by
gullies cut in th^ smoother ridge tops. Similar gullies are found on the
divide between Dry Creek and San Felipe Valley where the streams
flowing toward the south are cutting into smooth slopes and alluvium-
filled valleys. The headwaters of Arroyo Aguague north of Halls Valley
are steep and narrow, and deep gullies were being cut in the mature
surfaces of Halls Valley before they were interrupted by damming.
Older Broad Valleys. Evidence for an earlier period of canyon
cutting is clearly discernable in many places. Up Arroyo Hondo, for
example, near its junction with Calaveras Vallej^ a distinct break in
the slope of the walls is observable, and the profile of the interlocking
spurs outlines a much less rugged canyon whose floor was about 1000
feet above the present stream. Distinct benches preserved on the spurs
east of the canyon are accordant with the smooth-surfaced ridge between
Isabel Creek and Smith Creek. Farther upstream these benches disap-
pear, and the canj^ons are noticeably less rugged. Smith Creek, east of
San Felipe Hills, flows in a steep but not precipitous canyon, and has
T Willis, Robin, Physiography of the California Coast Ranges : Geol. Soc. America
Bull., vol. 36, no. 4, pp. 641-678, 1925.
8 Clark, B. L., Tectonics of the Coast Ranges of middle California: Geol. Soc.
America Bull., vol. 41, pp. 747-828, 1930.
9 Taliaferro, N. L., The Franciscan-Knoxville problem: Am. Assoc. Petroleum
Geologists Bull., vol. 27, no. 2, pi. 2, section 9, 19 43.
1" Rogers, A. F., An interesting occurrence of manganese minerals near San Jose,
California: Am. Jour. Sci., 4th ser., vol. 48, pp. 443-449, 1919.
Rogers, A. F., The crystallography of hydromagnesite : Am. Jour. Sci., 5th ser.,
vol. 6, pp. 37-47, 1923.
Rogers, A. F., Euhedral magnesite crystals from San Jose, California : Am. Mm-
eralogist, vol. 8, no. 8, pp. 138-140, 1923.
Rogers, A. F., Kempite, a new manganese mineral from California ; Am. Jour.
Sci., 5th ser., vol. 8, pp. 145-150, 1924.
r
12 SAN JOSE-MOUNT HAMILTON AREA fBllll. 157
developed narrow valley flats that are most marked in Horse Valley.
Isabel Creek undergoes a similar change of character at the east edge
of the map as it approaches Isabel Valley ; Alameda Creek in its course
between Valpe Ridge and Packard Ridge is similarly open. The relatively
open, smooth topography of Packard and Seeboy Ridges is probably
typical of that tlironghont the range during this cycle of development.
Oak Ridge Surface. Traveling along the crest of Poverty Ridge, or
Oak Ridge, one is struck immediately by the extensive areas of flat or
gently rolling topography, which are remnants of a still older period
of erosion. The smooth surfaces end abruptly at the edges of steep
canyons, and here and there have given way, as in the great Oak Ridge
slide. Scattered remnants of this old surface are preserved throughout
the area, and it is strikingly evident in the profile of Oak Ridge as seen
from the west, near the summits of Mount Day and Black Mountain,
and on the summit of Mount Lewis. Extending these remnants across
the intervening valleys, one can reconstruct a gently sloping mature
surface characterized by smoothly rounded contours, and with a relief
that probabh^ did not exceed 1,500 feet. There is no evidence that the
surface continues east of Mount Lewis and Black Mountain; however,
w^ell-developed flats continue south on both sides of Smith Creek, and
grade into the rolling topography of the San Felipe Hills. Although
correlation across the Calaveras fault zone is somewhat uncertain, smooth
surfaces on the east side of Monument Peak and along Sierra Road
west of Los Buellis Hills are tentatively regarded as extensions of the
Oak Ridge surface.
Silver Creek Surface. The summit of the ridge between Silver
Creek and Santa Clara Valley has a remarkably smooth and level surface
that is best developed south of the area mapped. East of Edenvale, the
surface is less well developed, and consists of rounded hills with accord-
ant summits. This surface is separated from the main part of the range
by two large fault zones, both of which have been active recently. For
this reason, it cannot be correlated definitely with either of the other
surfaces.
Isolated Surfaces. Two small isolated areas of old-age topography
occur along the Calaveras fault zone. The most prominent of these is a
gently sloping surface at the south end of San Felipe Valley. This almost
plane surface is bounded on the west by a branch of the Calaveras fault
system, and on the east by San Felipe Creek. It is being dissected by
gullies from all sides, but particularly from the east and south. Another
area of subdued relief is that extending north from Halls Valley along
the Calaveras fault zone. This gently rolling surface rises slowly from
Halls Valley northward, is interrupted by Arroyo Aguague and con-
tinues into the area around the Haskins triangulation station. This con-
tinuity suggests strongly that these surfaces are downwarped parts of
the Oak Ridge surface. The most striking feature of these areas, however,
is the tilting and warping they show, and the change in the drainage
pattern which has resulted. The surface near San Felipe Valley slopes
northward, but is being dissected by a southward-flowing stream, San
Felipe Creek. In Halls Valley, the surface slopes gently southward and
the headwaters of San Felipe Creek are being pirated rapidly by the
steeper, shorter Arroyo Aguague.
1951] PHYSIOGRAPHY 13
Correlation of Erosion Cycles and Recent Sediments. The erosional
features described in the preceding paragraphs indicate that the moun-
tains east of San Jose have been abruptly elevated twice within relatively-
recent geological time. Moreover, the magnitude of the displacements is
sufficient to suggest that these events should also be recorded in the
sediments accumulating wathin and around the margins of the adjoining
Santa Clara and Livermore Valleys.
The present cycle of erosion can be correlated readily with alluvial
cones which extend out from the mouths of the main streams and which
coalesce to form a broad alluvial apron along the whole mountain front.
These deposits, w^hich are relatively coarse near the mountains and con-
tain some coarse debris well out in the valley, are presumed to rest on
older sediments, which are somewhat finer grained. Logs of deep wells
in San Jose show a marked decrease in amount of gravel near the bottom.
The older valley stage of erosion is tentatively correlated with the
period of sedimentation which produced the Santa Clara formation. This
correlation is based largeh^ on the relations observed near the mouth
of Peniteneia Creek and must be regarded as highly tentative until data
from other areas are accumulated. The multiple transverse profile of
Peniteneia Creek through Alum Rock Canyon is evident in the view
down the canyon (pi. 6B) and in the ridge profile (fig. 2, A-A', B-B',
C-C) obtained from the map (pi. 1). The older valley surfaces were
projected to the center of the stream for each profile, giving three points
on the old broad valley stream. These points were then transferred to
the longitudinal profile (fig. 2) whose emergence at the front of the
range indicates roughly the base level during this stage of stream cut-
ting. The older valley stream apparently reached the front of the range
at an altitude of about 1100 feet. This is reasonably close to the level
of the highest exposures of the Santa Clara formation at 1300 feet.
The most serious difficulty with this correlation is the persistent east
dip of the Santa Clara formation which averages about 10 degrees, but
locally may be as much as 30 degrees. If this is attributed to tilt, it must
have affected the entire block, including the Oak Ridge surface. How-
ever, there is no evidence of eastward tilting of that surface, so either
the dips are initial or the Santa Clara formation is older than any of
the erosion surfaces preserved in the area. The latter alternative seems
untenable inasmuch as the Santa Clara formation is resting on the eroded
mountain front which truncates the Oak Ridge surface. It is suggested
that the Santa Clara formation was rapidly deposited by a large stream
which piled material against the sloping mountain front. This process
could account, at least in part, for the dips observed.
There are no known high-level recent sediments in the Santa Clara
Valley which might be correlated with the Oak Ridge erosion surface.
However, the gentle northward slope of the surface on Oak Ridge and
Valpe Ridge suggests that this surface might be correlated with some
of the relatively recent sediments in Livermore Valley. Figure 3 shows
the uniformity and extent of the surface developed on Oak Ridge, Valpe
Ridge, and the slopes leading into San Antonio and Valleeitos Creeks in
the southern end of the Pleasanton quadrangle. It also show's the relation-
ship of this surface to the gently tilted Livermore gravel and lends some
support to the theorj^ that the surface on which the gravel was deposited
is an extension of the Oak Ridge surface. Serious difficulty is encountered,
14
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
LONGITUDINAL PROFILE
Highest Sonto Cloro
--'-'■'Treser
_„-'--''6lder Volley '^tnge ^ —
■^°&-Penitentia Creek
T^^SS-Arroyo Aguague
It Cvclfi
, 1
A-A' B-B' C-C'
*
A' 1
.^^-^^^
■1000
B ^ ^ ^ B'
1000-
^^ TRANSVERSE PROFILES DOWN SPURS
C
2000-
' -^ s x_.^ j
J
1000-
SCALE
5000 FT
Figure 2. Profiles of Penitencia Creek in Alum Rock Canyon
however, if the Livermore gravel is all one unit, as mapped by Huey,^^
and is involved in major folding in Corral Hollow and in faults in the
Hetcli Hetehy tunnel which have no topographic expression. However,
Harding ^^ separates the Livermore gravel in the southern part of the
Pleasanton quadrangle into two units: the older, "Pliocene A," is
folded with the Briones and older formations, and contains white lime-
stone, volcanic ash, silty shale, and sandstone ; the younger formation,
"Pliocene B," consists of silt and clay interbedded with coarse con-
glomerate containing much Franciscan debris. These gravels dip north
toward Livermore Valley at a low angle, and are warped and faulted,
but not folded with the Pliocene and older rocks. These rocks may con-
ceivably have been deposited during the erosional interval represented
by the Oak Ridge surface.
STRATIGRAPHY
The sedimentary^ rocks of the San Jose-Mount Hamilton area, like
those of most of the central Coast Ranges of California, range in age from
Upper Jurassic to Recent. However, the number of formations of Ter-
tiary age is not as large as in many other areas. With one exception, the
formational names used are those already in use in adjoining areas. The
name Berryessa formation is introduced to denote the Lower ( ?) Creta-
ceous shales which have been called Chico by earlier workers.
The area provides very few opportunities for detailed stratigraphic
study. Because of incomplete exposures and many fault contacts, it has
11 Huey, Arthur S., Geology of the Tesla quadrangle, California : California Div.
Mines Bull. 140, 1948.
1^ Harding, J. W., Geology of the southern part of the Pleasanton quadrangle,
unpublished thesis for master's degree. University of California, Berkeley, 19 43.
popowaaa
wmw iii m t"<^ wwww*n* *1o»vw»o«»f*.^w«nrtj»«w wafcw WT Mw
1951] STRATIGRAPHY 15
not been possible to measure an unbroken section of any of the formations
exposed. Fossils are abundant iu only one formation, the upper ]\Iiocene
Brioues sandstone.
Jurassic System
Franciscan Formation
Disirihxdion and Thidnxess. The Franciscan formation crops out,
or is present beneath the alluvium, in almost two thirds of the area
mapped. East of the Calaveras fault zone it is exposed over more than 100
square miles, and this is only the western edge of the great area of Fran-
ciscan rocks which extends unbroken for 25 miles to the east, and which
forms the core of the Diablo Range. A smaller area of Franciscan crops
out southeast of San Jose, and continues northwest beneath the alluvium
of the Santa Clara Valley to join the Franciscan rocks exposed in the San
Francisco peninsula and the islands of San Francisco Bay. Between the
two areas of Franciscan rocks is a baud of Tertiary and Cretaceous rocks
which is bounded by major faults. A few very small slivers or blocks of
Franciscan occur outside these areas along the bounding faults.
Xo new evidence concerning the thickness of the Franciscan for-
mation has been found in the San Jose-Mount Hamilton area. Neither its
base nor its top is exposed, nor is it possible to delineate the structure
with certainty-. The general scarcity of chert lenses or extensive basalt
flows gives less indication than usual of the general attitude and the posi-
tion within the formation. The thickest continous section in the area
mapped is just east of the Calaveras fault, where at least 6.000 feet of beds
are exposed. There is no evidence of large faults cutting this limb of the
fold, but the lack of stratigraphic control makes even this uncertain. In
any case, 6,000 feet is a Tninimum thickness, and is probably much less
than the total.
Templeton ^^ reported a thickness of 15.000 to 20.000 feet in the San
Jose and Mount Hamilton quadrangles, but did not indicate how or where
the figui-e was reached. Tolman ^^ also reported a thickness of 15,000 feet,
apparently along a line extending from Oak Ridge, in the northern part
of this area, to Corral Hollow in the Tesla quadrangle. He went so far as
to divide the Franciscan into three formations, a sandstone and shale in
Corral Hollow as the lowest unit, shale and chert called "Corral Hollow
shales * ' as the middle unit, and the ' ' Oak-ridge sandstone ' ' as the upper
unit. The writer's observations near Oak Ridge indicate that the struc-
ture there is obscure, and consists of several folds. These structures are
so uncertain that no attempt was made to estimate a thickness. Huey ^^ has
described the section near Corral Hollow, and found no evidence that the
Franciscan formation can be divided successfully there.
The information available concerning the thickness of the Francis-
can was summarized recently by Taliaferro : ^^
"It is impossible at present to give any definite figures for the thick-
ness of the Franciscan . . . however there are excellent and continuous
sections at least 10,000 feet thick . . . (and) the writer does not believe
that an estimate of 25.000 feet ... is excessive."
-^ Templeton, E. C, General geologr of the San Jose and Mount Hamilton quad-
rangles (abstract) : Geol. Soc- America Bull., vol. 24. p. 96, 1913.
1* Tolman, C. F., A geological report on the Coyote dam-site — a report to the Santa
CHara VallevTVater Conser\"ation District (unpublished), 1934.
IS Op. cit.
^« Taliaferro, N. L.., The Franciscan-Knoxville problem : Am. Assoc. Petroleum
Geologists Bull., vol. 27. no. 2, p. 185, 1943.
16
SAN JOSE-MOUNT HAMIIiTON AREA
[Bull. 157
^^
4)
%
«
o
s
Eh
>
9
5
XI
m
c
o
■a
c
a
o
u
o
V,
be
bfl
a
'%
o
x:
tn
-a
a
(i>
bo
-a
P5
O
o
O
MMSaMW
1951] STRATIGRAPHY 17
The Franciscan formation in tliis area consists of sandstone, silt-
stone, shale, conglomerate, radiolarian chert, and altered basaltic and
andesitic lavas and agglomerates. These types are intruded by basalt and
andesite and by serpentinized nltrabasic rocks, diorite, gabbro, and dia-
base. Glaucophane-bearing metamorphic rocks occur locally at the contact
of bodies of serpentine and in isolated areas a few feet to a few hundred
feet in diameter.
Sandstone and Shale. Sandstone, and smaller amounts of inter-
bedded siltstone and shale, comprise more than 90 percent of the Fran-
ciscan formation in this area. The total thickness of these rock types
cannot be estimated, but there is at least 6,000 feet exposed east of the
Calaveras fault, and the total thickness is undoubtedly much greater. The
typical fresh sandstone is a dark-gray medium-grained massive rock, cut
by many tight and almost invisible joints and fractures filled with calcite.
AAHien weathered, it is greenish gray, greenish brown, or buff, and the
joints are so closely spaced that it is often impossible to determine the
attitude except where the sandstone is interbedded with shale or where
the exposures are unusually good. The rock is commonly rather thick
bedded, but thin-bedded sandstone and siltstone occur in many places.
The sandstones of the Franciscan formation were studied and
described in detail by Davis ^'^ and by Taliaf erro.^^ Little to augment these
studies was done in connection with this work. The sandstones can easily
be distinguished from those of other formations by the almost ubiquitous
occurrence of flakes of pearly transparent muscovite. The presence of
this mineral and the absence of brown wrinkled biotite provided a reliable
means of distinguishing Franciscan sandstone from sandstone in the
Berryessa formation, for example. Several other specimens of Franciscan
sandstone from widely separated localities all contained more or less of
this pearly muscovite. An attempt to separate this mineral from crushed
and sized material by centrifuging in heavy media proved unsuccessful.
It was impossible to isolate it from accompanying chlorite in a reasonable
number of separations. In a fresh specimen from the Alameda Creek tun-
nel of the San Francisco Water Department and in a specimen from the
Eel Kiver collected bv C. M. Gilbert, the mica showed Np of 1.600di, and
a 2 Y (negative) of 36°-42°.
A mineral analysis of this same fresh sandstone is contained in
table 1. There are several striking differences between the mineral content
of the specimen from Oak Ridge and the average listed. Some of this
difference is a result of the method of treatment; acid treatment of the
other 17 samples explains the absence there of the chlorite, biotite, musco-
vite, and spliene found in the San Jose sample. Hand magnet separation
of iron from the San Jose sample accounts for the absence of the magne-
tite found elsewhere. The most significant difference is in the quartz-
feldspar ratio. The ratio of the San Jose sample is 0.47, that of the
average for 17 sandstones is 1.51. The only samples listed by Taliaferro,^^
■^dth a ratio approaching this were numbers 2 and 4, whose ratios were 0.64
and 0.55 respectively. The count of the light minerals listed in table 1 was
made by observation of interference figures under high power, and was
later checked with the universal stage.
" Davis, E. F., The Franciscan sandstone : Univ. California, Dept Geol. Sci. Bull.,
vol. 11, pp. 235-432, 1912.
IS Taliaferro, N. L., The Franciscan-Knoxville problem : Am. Assoc. Petroleum
Geologists Bull., vol. 27, no. 2, pp. 109-219, 19 43.
i»Op. cit, p. 134, 1943.
18
SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Table 1. Mineral content of fresh Franciscan sandstone.
Light minerals
Quartz
Feldspar
Rock fragments and composite grains
Heavy minerals
Chlorite
Biotite
Leucoxene
Sphene
Zoisite-epidote
Apatite
Zircon
Tourmaline
Hornblende (green)
Kyanite
Picotite
Composite and indeterminate grains _
Magnetite
Pyrite
Garnet. »_
Sample a
22
47
31
100
32
68
100*
Not acid treated
37
25
17
9
2
2
1
1
1
1
1
3
100
Sample b
56.2
37.1
6.7
100.00
Acid treated
3.5
1.2
9.8
7.7
5.2
5.0
1.8
2.0
51.8
6.1
3.1
3.0
99.2
Sample a: From Alameda tunnel, Oak Kidge, San Jose quadrangle.
Sample b: Average of 17 sandstones from Coast Ranges (Taliaferro, N. L., The Franciscan-Knoxville prob-
lem: Am. Assoc. Petroleum Geologists Bull., vol. 27, p. 135, 1943.
* Quartz and feldspar recalculated to 100%.
A single mineral analysis like that given above is of little significance
by itself, but it does show the range of composition of these rocks to be
considerable, The assemblage of heavy minerals is not unusual with the
exception of kyanite which is rather rare. Three or four grains were seen
in each mount.
Conglomerate. Only a few outcrops of Franciscan conglomerate
were found in the area mapped. The largest of these is on the north side
of Isabel Creek 1^ miles north of Mount Hamilton. It is an outcrop about
100 feet across, consisting of well-rounded smooth cobbles and pebbles an
eighth of- an inch to 10 inches in diameter. The lithologic types observed
include black quartzite or recrystallized chert, coarse-grained quartz and
feldspar porphyries of several types, and a few granitic or dioritic rocks.
A second occurrence is in Los Buellis Hills. This also is coarse cobble
conglomerate containing in approximate order of abundance, dark
quartzite or chert, various porphyries, sandstone, and a few gneissic
granitic rocks. A third occurrence 1.1 miles S. 50° E. of the Ogier Ranch
is of similar character, but with a more quartzitic matrix. Out of 25
pebbles counted, there were 17 of recrystallized chert, mostly brownish
but some black and some white, 5 of quartzite, 2 of sandstone, and 1 of
vein quartz. One other lens of conglomerate was found, in a cut along the
road running along the crest of the San Felipe Hills at a point 0.8 of a
mile south of the road from San Felipe Valley to Horse Valley.
■ IBB— »
1951] STRATIGRAPHY 19
Chert and Contemporaneous Volcanics. Cherts are abundant in
only two relatively small areas, one east of Alum Kock Canyon, the other
southeast of Mount Hamilton. Lenses of chert up to 50 feet thick and a
few hundred feet lon^' occur at a few other places. These rocks are the
rhj'thmically bedded chalcedonic radiolarian cherts characteristic of the
Franciscan, which have been described by Davis -" and Taliaferro.^^ The
most common type is thin-bedded red, brown, green, or white chert with
thin partings of shale. There are many unusual and striking varieties,
however, of purple, yellow, blue, or black chert, which are sought by
amateur rock collectors for polishing. Most of these show brecciation,
recrystallization, or development of new minerals of hydrothermal origin.
The more altered types grade into glaucophane schists. Chert showing
rather unusual alteration of this type was found in place east of Los
Buellis Hills, on the Santa Teresa Hills east of Edenvale, and as float in
many of the larger streams. It is particularly abundant in Isabel Creek,
north of Mount Hamilton.
The restricted occurrence of both chert and altered vokanic rocks
in the area east of the CalaA'eras fault makes the closeness of their
association all the more striking. Both the cherts and the volcanics occur
as lenses enclosed in sandstone and shale, and where one is found, the
other is almost always present.
The Franciscan greenstones of the San Jose-Mount Hamilton area
are mostly rather small bodies, whose character, composition, and origin
is badly obscured b}- alteration. Rocks in which the original minerals can
be recognized are rare. It is for this reason, plus the fact that many types
and varieties occur, that the term ' ' greenstone ' ' is used. It does not imply
that the rocks are schistose. Pillow structure, poorly developed, was
observed in onh' one or two places. i\Iost outcrops show considerable
shearing and internal fracturing, even though they may have been rela-
tively massive when formed.
The typical greenstone is a dark-green to black fine-grained rock so
badly fractured and veined by quartz, calcite, and thin films of man-
ganese and iron, that it is quite difficult to obtain a fresh break. "When
exposed, a fresh surface commonly reveals only a fine-grained dull-
green aphanitic rock in which no minerals can be recognized. In some
specimens scattered phenocrysts, amygdules, or vesicles can be distin-
guished, but considerable search is required to find them.
A somewhat unusual igneous rock, a quartz keratophj^re, is enclosed
in the Franciscan formation near the south edge of the map (pi. 1), just
east of the line of section E-E'. It is not an altered basic igneous rock,
but appears as "greenstone" on the map because its identity was not
recognized in the field. In the hand specimen, the quartz keratophyre
is a greenish to brownish-gray rock with a marked porphyritic texture
except where very badly altered. It weathers yellowish to reddish brown,
and at places along the contact with serpentine can only with difficulty
be distinguished from the typical silica-carbonate rock. Pinkish pheno-
crysts of feldspar up to 2 millimeters in diameter, and epidote-chlorite
masses pseudomorphous after ferromagnesian minerals, are evident.
Most of the exposures are massive ; a few are vesicular, the cavities being
lines with minute euhedral crystals of quartz and magnetite. Two thin
aoOp. cit., pp. 235-432.
21 Taliaferro, N. L,., The relation of volcani.sm to diatomaceous and associated
siliceous sediments : Univ. California Dept. Geol. Sci. Bull., vol. 23, pp. 1-56, 1933.
20 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
sections were examined. They both contain thick complexly twinned
prisms of albite (An 7) about 2 millimeters in length, and a few irregular
grains of quartz, most of them less than 1 millimeter in diameter. The
groundmass is separated into irregular "patches" about .25 of a milli-
meter in diameter by a netAvork structure of dull yellow-green chlorite.
Each patch consists of an irregular area of optically continuous quartz,
or micrographic intergrowth of quartz and feldspar. Rocks of this type
occur in many parts of the world ^^ but so far as known have not been
described from the Franciscan formation.
Manganese Deposits. Several small deposits of manganese have
been found within the San Jose-Mount Hamilton area. Like almost all
the deposits in the Coast Ranges of California, these are closely associated
with Franciscan chert and greenstone. The detailed relations of the
manganese ore to the surrounding chert are not well exposed in any of
the deposits, but the areal association of the manganese with the chert
and greenstone is shoAvn on plate 1. The extent and genetic significance
of this refetionship has been discussed by Taliaferro -^ and by Trask,
Wilson, and Simons.^*
Most of the deposits are small, and one finds little but low grade
oxides exposed now. Here and there, however, fragments of primary
material remain, beneath a coating of fresh black oxide. The "primary
ore" from the Miller Ranch deposit just below Sierra Road on the north
slope of Alum Rock Canyon is a mixture of rhodochrosite and hydrous
silicates. Its specific gravity is 3.8 and it may contain as much as 35 per-
cent manganese. With increasing amounts of silica, such material grades
into manganiferous chert and finally into normal chert. Locally there are
small amounts of a black mineral resembling braunite.
Serjjentine and Its Differentiates. Serpentinized ultrabasic rocks
are of limited extent in the area east of the Calaveras fault. Except for
a small sill near Calaveras Dam, the only large exposure is in Long Branch
north-northwest of Mount Hamilton. This appears to be part of a single
sheet, exposed for a distance of 4 miles, and about 1500 feet thick near
the south end. In general it appears to be concordant with the surround-
ing rocks, though their structure is not entirely clear. The rock is a highly
sheared soapy variety of serpentine in which most traces of original
minerals have been destroyed.
The largest exposures of serpentine in the area mapped are in the
ridge between the Silver Creek fault and Edenvale, and in its extension,
the partly buried ridge whose exposed summit forms the Oak Hills near
Lick. Serpentine is so extensive in much of this area that its relations
to the surrounding rocks are not clear except after mapping a consider-
able area. In the center of the Santa Teresa Hills, there are several dis-
tinct sills of serpentine, ranging from a few feet to a few hundred feet
in thickness, enclosed in sandstone and shale.
22 Gilluly, James, Keratophyres of eastern Oregon and the spilite problem : Am.
Jour. Sci., 5th ser., vol. 29, pp. 22.5-252, 336-352, 1935.
^ Taliaferro, N. L., The Franciscan-Knoxvllle problem : Am. Assoc. Petroleum
Geologists Bull., vol. 27, pp. 109-219, 1943. . . . and Hudson, Frank S., Genesis of the
manganese deposits of the Coast Ranges of California : California Div. Mines Bull. 125,
pp. 277-332, 1943.
^ Trask, P. D., Wilson, I. P., and Simons, F. L., Manganese deposits of California,
a summary report : California Div. Mines Bull. 125, pp. 51-215, 1943.
1951] STRATIGRAPHY 21
In these gently rolling hills, almost all of the serpentine outcrops
form fields of irregular boulders whose rough surface contrasts strongly
with the smooth grassy areas underlain by Franciscan sandstone and
shale. Because of this, the broad pattern of areal distribution is imme-
diately apparent, and can be plotted with ease on aerial photographs
or topographic maps. The outcrops in an area of serpentine weather to
dark-green or brown cavernous masses, whose most prominent structures
are the more resistant quartz veinlets or silicified areas, and which reveal
little about the .structure of the serpentine body as a whole. Fresh expo-
sures in rail or road cuts often reveal innumerable shear zones of which
there is no surface expression. Here and there, however, reef -like masses
of strongly sheared serpentine crop out.
Several distinct varieties of serpentine may be seen in a hand speci-
men. Among the more striking are intensely sheared dark-green or bluish
rocks whose surfaces are almost always slickensides ; dense, massive,
dark, greenish rocks cut by veins of fine-grained jade-green porcelaneous
serpentine ; dense dark-colored rocks cut by veinlets of chrysotile which
may be large and widely spaced, or may form a fine network or mesh
structure ; and dense dull-green rocks with scattered flakes of antigorite
(bastite) pseudomorphous after pyroxene.
An interesting exposure of relatively unaltered basic and ultrabasic
rocks was found in a quarry in the east edge of the Oak Hills, near San
Jose. This occurrence was described by Carey and Miller,-^ and the fol-
lowing description is taken largely from their work. The oldest rock
exposed is serpentine which, with increasing amounts of diallage, grades
into olivine pjTOxenite or locally diallagite; with increasing amounts
of feldspar to olivine gabbro. Locally with an increase in hypersthene,
the rock grades into norite, or with an increase in hornblende, it grades
into hornblende gabbro. Cutting all these rock tj-pes are at least two
sets of diorite dikes, an older set nearly vertical, cut in turn by a younger
set that dips flatly northwest. The borders of the dikes are mostly rather
fine-grained and many of the cores are so coarse as to approach a pegma-
titic texture. Exposures outside the quarry are poor, and reveal little
about the extent of the gabbroic rocks or the diorite dikes.
Many smaller bodies of diorite, diabase, and gabbro crop out within
the areas mapped as serpentine (pi. 1). The usual outcrop is a rounded
knob or a pile of boulders 5 to 20 feet in diameter, projecting several
feet above the .surrounding area. Its contacts with the enclosing serpen-
tine are seldom visible in natural exposures, but the general relations
suggest that these bodies are isolated segregations within the serpentine.
The unaltered rocks of this tyY^e examined microscopically are all horn-
blende diorite or gabbro showing fine-grained textures varying in the
same slide from ''diabasic" to hypidiomorphic granular. The principal
minerals are plagioclase. An 55-60, in euhedral to subhedral laths, and
pale weakly pleochroie hornblende. In one slide the plagioclase is sur-
rounded by rims of clear albite. Minor accessories include small amounts
of skeletal ilmenite, sphene, and magnetite ( ?) . Prehnite occurs in irregu-
lar veins, and in local areas within the rocks sometimes surrounding
euhedral feldspar. Zoisite, chlorite, and saussurite are abundant altera-
tion products. None of the specimens examined contain pyroxenes nor
is there any other evidence to suggest that the amphiboles are secondary.
^ Carey, E. P., and Miller, "W. J., Crystalline rocks of the Oak Hills near San Jose :
Jour. Geology, vol. 15, pp. 161-166, 1907.
22
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
AGE
FORMATION
FEET
SECTION
KIND or ROCK
OS
<
Z
Pi
M
H
<
C
Pi
<
H
oi
u
H
W5
D
O
w
u
<
u
u
y
<
oi
D
Recent alluvium
Santa Clara
formation
Packwood Eravels
Miocene-Pliocene
Orinda formation
Briones sandstone
Miocene
Monterey formation
Temblor formation
Berryessa formation
Oakland
conglomerate ( ? )
Knoxville formation
■.-V.N
v^
Franciscan
formation
'..vv.-c---;;. '//.■
3
a.
o
o
o
■:'^v?1
Uncomolitiated gravel and day
Alofij( u est em slope of range
Unconsolidatt'd gravel and sand
southeast of Evergreen
Loosely consolidated red, maroon,
or greenish conglomerate, sand-
stone and clay
Thick'bedded, massive grey sandstone
with many fossiliferous beds
Upper — fine-grained yelloufsh punky
foraminiferal siltstone
Middle — thin-bedded opaline chert and
cherty shale
Lower — gray or chocolate shale or thin
glauconitic sandstone.
Medium-grained hroun fossiliferous
sandstone
Olive-drah-U'eathering siltstone and
micaceous sandstonetfew limy lenses
Massive cobble and boulder conglomerate
with some sandstone and shale
Dark gray to black silty shale and
thin-bedded sandstone
Thick to thin -bedded gray arkosic sand-
stone, thin- bedded platy shale and silt-
itone. Lenses of ted or green che"' (ch)
and altered igneous rocks (gO
Figure 4. Generalized stratigraphic column of the San Jose-Mount Hamilton area
1951] STRATIGRAPHY 23
An unusual variety of hornblende gabbro crops out in an area of
Franciscan rocks between Los Buellis Hills and Calaveras Creek. This
body is a fine-grained to medium-grained hornblende gabbro at the core,
but" near the edge it becomes increasingly sheared and in places
grades into fine-grained blue-graj* mylonite. Under the microscope the
mylonite shows incipient recrystallization to albite and fine-grained
shreddy glaucophane. The occurrence is of special interest because of
its intrusive relations to the Franciscan sediments, and its incipient
metamorphism. At least two other occurrences of partly sheared gabbro
have been reported in this area. One, in the bed of Calaveras Creek a
short distance below Calaveras Dam, was described by Van Hise ; -^
another, on Oak Ridge, was described by Smith.-" Such an occurrence of
sheared gabbro in which glaucophane was developing apparently led
Van Hise to the conclusion that all the schists are of dynamothermal
origin.
Alteration of Serpentine. Several different products seem to result
from the alteration of serpentine by weak hydrothermal activity, all of
which are distinct from the metamorphic rocks of the glaucophane schist
facies which also may be produced at times from serpentine. In the area
of Calaveras Dam. particularly near and below the high-water line west
of the spillway, a pale gray-green lustrous rock is found which consists
almost entii-ely of talc in interlocking plates up to several millimeters in
diameter, and a few scattered grains of powdery magnetite. This rock
grades laterally into ordinary sheared serpentine.
Another alteration product observed in this area, and in the large
slide of lower Alum Rock Canyon, is a soft friable greenish-gray mass
containing rounded white nodules of hydromagnesite. The nodules are
half a centimeter to -t centimeters in diameter and have a rather botryoi-
dal surface. The mineral is soft and chalky and consists of a cryptocrystal-
line aggregate which cannot be resolved sufficiently to determine its
detailed properties, even under high power. The refractive index and
birefringence are within the range of hydromagnesite.
Alteration to magnesite-quartz rocks is not widespread, but does
occur in the Oak Hills south of San Jose. The common massive porcelane-
ous variety of magnesite veined by quartz, occurs near the now abandoned
San Juan quicksilver mine, and euhedral magnesite from this locality
has been described by Rogers.-^
Silica-Carhonate Bocl'. The most widespread product resulting
from the alteration of serpentine is the distinctive and colorful silica-
carbonate rock. This unusual and characteristic material occurs as lenti-
cular masses or narrow bands along fault zones and around the borders
of some bodies of serpentine. Its bold rugged outcrops consist of yellow-
ish-brown ocherous masses whose texture is diverse. It is most commonly
slaggy or cavernous, but much of it is gnarled, knotted, and breeciated.
"Weathered outcrops are heavily stained -with limonite. The fresh rock,
where found at some depth, usually consists of greenish- or brownish-
white coarse-grained carbonates, commonly dolomite with some ankerite,
veined by quartz, chalcedony, and, rarely, opal.
"Van Hise, C. R., Metamorphism of rocks and rock flowage : Geol. Soc. America
Bull., vol. 9, p. 813, 1S9S.
^ Smith, James Perrin, The paragenesis of the minerals in the glaucophane-
bearing rocks of California : Am. Philos. Soc. Proc, vol. 45, pp. 207-209, 1907
2s Rogers, A. F., Euhedral magnesite crvstals from San Jose, California : Am.
Mineralogist, vol. S, no. 8, pp. 138-140, 1923.
24
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
-eclogite
SJ-e49Q
%SJ-e49
Son Jose
SJ-198
sfe SP^ '
Hilton Gulch
*SJ-246
.5 J -245
*SJ-243, 244
'SJ-241
SJ-24e
\S J- 237,238
. Mt. Homilton
' O
SJ-S35 •
Figure 5. Distribution of glaucophane schist (black) and serpentine (sp) in the
San Jose-Mount Hamilton area. Numbers refer to specimens
1951] • STRATIGRAPHY 25
Small masses a few feet to a hundred feet in length occur along
faults near the edge of the valley from the Silver Creek quicksilver mine
to the mouth of Alum Rock Canyon, and somewhat larger masses occur
along a fault which has localized the deposits at the Hillside mine in Oak
Hill. The most extensive bodies are those at the boundary of the serpen-
tine east of Edenvale, where the silica-carbonate rock occurs as a band
or as interrupted lenses a few feet to a hundred feet thick extending for
a distance of 3 miles along the contact. Throughout most of the distance
it forms a prominent wall or rampart which stands as much as 25 feet
above the surrounding serpentine and Franciscan sediments. Through-
out its length, the silica-carbonate rock shows many textural and struc-
tural features of the serpentine from which it was derived; in fact, it
grades into the serpentine on one side. In places it is cut by sharply
defined quartz veins, whose orientation is remarkably uniform over a
distance of several hundred feet, even where the band of silica-carbonate
rock is sharply curved.
The development of silica-carbonate rock by hydrothermal alteration
of serpentine is no longer questioned, and in the Coast Ranges its asso-
ciation with Tertiary or younger quicksilver deposits has led to the con-
clusion that the hydrothermal solutions are genetically related to
Tertiary or Quaternary volcanism. It is seldom possible, however, to
demonstrate their association with a particular igneous body but such
an association does occur in the Alum Rock area east of San Jose where
a plug of Alum Rock rhyolite has intruded shale and conglomerate of
Cretaceous age. Although the wall rocks at the level now exposed are
all of Cretaceous age, a thin discontinuous wrapping of serpentine has
been carried up around the plug itself. Detailed examination reveals
that this has been altered to silica-carbonate rock at several places along
the contact with the rhyolite. This alteration and the still active thermal
and chalybeate springs nearby, suggest strongly that the silica-carbonate
rock here was formed by solutions which are genetically related to the
Alum Rock rhyolite.
Franciscan Schists. The unusual and distinctive metamorphic
rocks comprising the glaucophane schists are the most remarkable and
characteristic feature of the Franciscan formation. The diversity of their
form and occurrence, their unusual mineralog^^, and the apparent
anomalies among the mineral assemblages have stimulated widespread
interest and study. At the outset, it should be understood that the term
"glaucophane schist" is loosely used, and that it includes an assemblage
of low-grade metamorphic rocks of diverse mineralogical and textural
character, which may or may not contain glaucophane, and which may
be massive rather than schistose.
A large number of the glaucophane schist bodies are small and
isolated and show no visible connection with serpentine or ultrabasie
rocks. They commonly crop out as rugged crags or large bouldery ma.sses
surrounded by smooth grassy areas in which outcrops are poor or lack-
ing; only rarely can one observe a transition from schist to unaltered
source rock.
The most extensive areas of glaueophane-beariug rocks in the San
Jose-Mount Hamilton area are those associated with the long narrow
serpentine sill northwest of Mount Hamilton. Here they occur as narrow
lenses mostly along the western, presumably upper, margin of the sill,
26 SAN JOSE-MOUNT HAMILTON AREA [Bllll. 157
and as irregular areas iiiterfingering witli the serpentine and the sedi-
ments.
The large body at the north end of the sill is a coarse-grained rock
of varied texture and composition. It includes chlorite, muscovite, talc,
actinolite, and glaucophane schists, some of which were derived from
Franciscan sediments, and some of which were formed from the serpen-
tine itself. To the northwest, the intensity of metamorphism dies out,
and the schists grade into sheared semiscliistose sediments. The transi-
tion is gradual here, and any placing of the contact necessarily arbi-
trary.
Glaucophane schists are also associated with serpentine near the
mouth of Berryessa Creek, where a small rounded pillar-shaped body
of chlorite-actinolite rock crops out in an area of soft sheared blue-green
serpentine. The general pipe-like form of this mass and the composition
suggest that it was formed by metasomatic alteration of the serpentine
itself.
Another occurrence of schist on the border of a body of serpentine
was noted in the Oak Hills, near the Hillsdale mine. A strikingly beautiful
glaucophane-muscovite-quartz-albite-lawsonite schist has developed here
from Franciscan chert.
The most unusual rocks of the glaucophane schists are the so-called
eclogites which contain large euhedral garnets, green pyroxene, and
rutile, and smaller amounts of glaucophane, actinolite, chlorite, musco-
vite, and sphene. Such rocks have been described from Hilton Gulch, a
steep tributary to Arroyo Hondo, and from Calaveras Valley.^^
The many small bodies of schist scattered through the Franciscan
formation show a diversity of source rock including chert, sandstone,
and gabbro. Many other rock types have been affected, but only these
could be traced lateralh^ into unaltered rock or could be identified by
relict textures. In several places, the folded and crumpled bedding of
this original rock is preserved, and can be traced through the schists.
The glaucophane schists of the San Jose-Mount Hamilton area are
believed to have formed through metasomatic alteration of Franciscan
sedimentary and igneous rocks (including serpentine, diorite, etc.) by
solutions carrying sodium, calcium, magnesium, iron, alumina, and
genetically connected, probably at depth, with ultrabasic rocks. The
temperature and pressure conditions are believed to be those of the green
schist facies. The eclogites and garnet amphibolites of this area all appear
to be unstable under the conditions which led to the development of
glaucophane and are believed to represent an earlier period and a higher
grade of metamorphism. There is insufficient evidence to determine
whether they were formed at depth and carried up by serpentine, or
were formed in place during an early phase of the metasomatic process.
Knoxville (?) Formation
Distrihution and Thic'kness. Rocks provisionally assigned to the
Knoxville formation crop out in a relatively restricted area southeast of
Evergreen. At the south edge of the map they extend from the Silver
Creek fault to the Calaveras fault, apparently without interruption. The
29Holway, R. S., Bclog-ites in California: Jour. Geology, vol. 12, pp. 344-358, 1904.
Smith, J. P., op. cit.
Pabst, Adolf, The garnets in the glaucophane schi.sts of California: Am. Mineralo-
gist, vol. 16, no. 8, pp. 327-333, 1931.
1951] STRATIGRAPHY 27
western limit, as far as it is visible, appears to be a fault of tlie Silver
Creek zone. To the north thej^ are overlain by the Oakland conglomerate
of Cretaceous ap-e, whose base crosses the ridge west of San Felipe Valley
and disappears beneath tlie alluvium east of Evergreen. The thickness of
these rocks is unknown ; it is impossible even to give a minimum value,
or to recount a continuous section of any kind. This is due almost entirely
to lack of knowledge of the structure, rather than lack of exposure, or
faulting, or other factors. The lack of recognizable stratigraphic or litho-
logic units leaves no basis for even an estimate of the thickness.
Lithology and Structure. Thin-bedded dark-gray shale and silt-
stone are the predominant rocks in the area mapped as Knoxville. Some
sandstone may be interbedded, and it may predominate locally. Thin
lenticular limestone beds and concretions occur here and there, and are
the source of many of the fossils discovered. These are black on fresh
surfaces, but weather to light brown or gray. Conglomerate was found in
several places, and it too was fossiliferous. The prevailing dark-colored
siltstone, presumed to be characteristic of the Knoxville, is particularly
evident along the lower part of San Felipe road, and along the steep
slope west of Dry Creek. At many other places, however, the rocks are
olive drab to light brown, and resemble greatly the rocks of the Berryessa
formation. This is a natural variation, but may also be a result of weather-
ing in more exposed areas. The sandstone is mostly thin bedded, grading
into and interbedded with siltstone and shale. Coarse-grained massive
sandstone is found locally, and may contain the wrinkled biotite which
is so characteristic of the sandstone of the Berryessa formation. Shale
flakes and wood fragments occur in both sandstone and conglomerate.
Most of the conglomerate contains black chert, porphyry, and other
materials of the Franciscan formation, but some also contains black
limestone that is probably derived from the Knoxville itself.
The almost complete lack of information on structure within the
Knoxville has already been pointed out. Some general features are evi-
dent, however. The general trend of the prevailing attitudes and their
relation to the base of the younger Oakland conglomerate indicates that
the Knoxville is crumpled and folded complexly. The numerous north-
east strikes apparently delineate folds trending in that direction, and
suggest the possibility of an unconformitj^ between the Knoxville and
younger rocks.
Andesite. A sill of pale cream, gray, or greenish altered andesite is
enclosed in sandstone and shale of the Knoxville on the slope north of Dry
Creek, southeast of the Richmond Ranch. Where it is exposed along the
road it is only a few feet thick, and is so altered that it is scarcely recog-
nizable. A short distance northwest, it thickens and becomes distinctly
porphyritic. In thin section the rock is vaguely porphyritic, with a
medium-grained pilotaxitic groundmass. What appear to be phenocrysts
in the hand specimen are sharpl}^ outlined areas of chlorite and quartz or
calcite pseudomorphous after hornblende ( ? ) . No feldspar phenocrysts
are evident. The groundmass consists of slender laths of twinned labrador-
ite about 0.2 of a millimeter long. It commonly shows zoning, with a con-
tinuous gradation from about An 70 in the interior to about An 50 in the
rim. The interstices between the laths are filled wath calcite and quartz ;
minute euhedral crystals, probably of magnetite, shreds of brown biotite,
and a few grains of epidote are also present.
28
SAN JOSE-MOUNT HAMILTON AREA
fBull. 157
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Age and Relations to Adjoming Formations. The age and desig-
nation of the unit mapped as Knoxville is based on meager fossil evidence
and general stratigraphic position, and as a result is uncertain and sub-
ject to modification by more detailed study. Moreover, the unit mapped
may not be entirely homogenous ; fossils have been found in only a small
part of the area.
Buchias (Aucellas), a few belemnites, and an ammonite fragment
were found within the San Jose-Mount Hamilton area, and a poorly pre-
served ammonite and a gastropod were found just south of tlie area
mapped. These were identified as :
Buchia piochii (Gabb)
Belemnites sp.
Perisphinctes sp.
Turritella sp.
None of these is sufficiently restricted in range, or well enough pre-
served to define the age of the beds unequivocally. The status of the genus
Buchia is particularly doubtful ; but the occurrence of the narrow-beaked
species (piochii) and the absence of the broad bulbous forms suggests an
Upper Jurassic rather than a Lower Cretaceous age, in spite of the fact
that the two types have been found together at many other places in the
Coast Ranges. Unfortunately, the ammonites found here, and others in
the collections of Stanford University, are too poorly preserved to be
determined specifically.^*'
The base of this formation was not found within the area mapped,
but may crop out in the north edge of the Morgan Hill quadrangle. The
fault contact with serpentine west of Dry Creek is so designated because
it appears from the mapping to dip southwest, and the shale adjoining
appears to dip northeast and to diverge considerably^ in strike. No indura-
tion or baking of the shale was obserA'cd.
Throughout most of its length, the contact between the Knoxville
and the overlying Oakland conglomerate is drawn on a purely lithologic
basis, at the brse of the lowest massive conglomerate ; for most of the dis-
tance this is all that the exposures warrant. However, in one locality 0.85
of a mile N. 10° E. of the Richmond Ranch, a thin fossiliferous conglom-
erate crops out about 200 feet below the base of the massive Oakland
conglomerate. This bed, containing only the broad Buchia crassa, and
overlying the dark Knoxville shales, is believed to be the base of the Cre-
taceous. It was impossible to follow this horizon eastward, and it is likely
that elsewhere along tlie contact beds of Cretaceous age may be included
in the top of the unit mapped as Knoxville. Thus the Jurassic-Cretaceous
time boundary probably falls near the top, but within the unit mapped as
Knoxville, except at the one localit}^ mentioned. This inconsistency in
mapping and usage is unfortunate, particularly in view of the confusion
that already exists concerning the use of the term Knoxville.
Cretaceous System
Overlying the dark shales of the Knoxville, and below the youngest
rocks of the Monterey formation are two lithologic units of Cretaceous
age. The lower unit is mostly coarse cobble and boulder conglomerate
of variable thickness. The upper unit is mostly sandstone and thin-
30 Muller, S. "W., Oral communication.
H^Rff I
1951] STRATIGRAPHY 31
bedded siltstone. Fossils are rare in both units, and the age designation
depends in part on their stratigrapliic position and lithologie similarity
to rocks of Cretaceous age in adjoining areas.
The name Oakland conglomerate is applied to the lower unit of the
Cretaceous system, following the usage of Lawson,^^ who mapped thick
conglomerates of identical character in the Haj-ward and Concord quad-
rangles. From the t^i^e section at Oakland, the unit crops out continuously
to Xiles, in the Pleasanton quandrangle, only 6 miles northwest of the
area mapped. The name as used by Lawson was actually the "Oakland
couglomerate member" of the Chieo formation. This designation is not
adopted here because of the fossil evidence suggesting a Lower Creta-
ceous age and because the term " Chico" has been so broadly applied that
it no longer has a reasonable stratigraphic significance. The lithologie
identity of the thick conglomerates of the San Jose and Oakland areas
seems sufficient to warrant the use of Lawson 's term, whatever the strati-
graphic significance may ultimately prove to be.
A new local name, Berryessa formation, is applied here to the upper
unit of the Cretaceous in the San Jose-Mount Hamilton area. The Berry-
essa formation consists of micaceous sandstone and siltstone called Chico
by Lawson, ''- and included in the KnosA'ille by Templeton.^^ The intro-
duction of a new name for this formation does not imply that its exact age
and stratigraphic position are known, but rather is an attempt to avoid
the confusion associated with the name Chieo, and to provide a simple
name for this unit which eventually can be correlated with other sections
when the information permits. Both the Oakland conglomerate and the
Berryessa formation are Lower Cretaceous in part, and are included in
the Shasta group by Taliaferro ^^ in a section drawn through this area.
Oakland (?) Conglomerate
Distrihiition and Thickness. The largest exposures of the Oakland
conglomerate are east and southeast of Evergreen, where it forms a broad
band extending northwest across the range from San Felipe Valley to
the edge of the Santa Clara Valley. It does not stop there, however, as
indicated by two isolated hills of conglomerate which project through
the alluvium northeast of Evergreen and by wells in the same area which
encountered conglomerate within 500 feet of the surface. It crops out
again in the low hills bordering the valley east of the cross roads labeled
"Alum Kock" on plate 1. Another exposure crops out north of Mount
Hamilton Road and extends northwest across lower Alum Rock Canyon
as far as Berryessa Creek. For much of this distance, it is covered by land-
slides and the Santa Clara formation, so that its relationship to the ser-
pentine against which it is faulted south of Berryessa Creek is obscured.
A single body of similar conglomerate is exposed in the pits and
trenches at the north end of Calaveras Reservoir. This is included in the
Oakland conglomerate with hesitation, and it may actually be an entirely
unrelated conglomerate, though it is undoubtedh' of Cretaceous age.
31 Lawson, A. C, U. S. Geol. Survey Geol. Atlas, San Francisco folio (no. 193)
p. 8, 1914.
3^ Op. cit, p. S, 1914.
^ Op. cit., Geol. Soc America.
s* Taliaferro, N. L., Geologic historv and structure of the Central Coast Ranges
of California: California Div. Mines Bull. 118, pi. 11, p. 162, 1943.
32 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
The thickness appears to be variable, and the lack of visible bedding
makes accurate measurements uncertain. In the one place, near the Kuhn
Ranch southeast of Evergreen, where the base and top are exposed, it
is estimated to be 2,700 feet. Southeast of there, the outcrop expands
greatly, to nearly 7,000 feet. Such a thickness is not impossible, but the
few. dips obtained in the area suggest that there may be folding which
would account for it, at least in part. Nowhere else are both the base
and the top of the formation exposed.
Lithology. The Oakland conglomerate commonly consists of sub-
rounded to well-rounded cobbles with some boulders, in a matrix of
coarse sand. It is generally massive, the bedding being very obscure or
completely lacking. In large exposures, some faint imbrication of the
pebbles is detectable, but most commonly their orientation is completely
haphazard. The lithology is fairly uniform over large areas, pebbles
consisting mostly of the same porphyries and quartzites found in the
conglomerates of the Franciscan formation. A count of 82 pebbles was
made in the cliffs at the mouth of Alum Rock Canyon, showing the
following distribution of rock types:
Black chert 32
Porphyry (dark groundmass) 25
Black quartzite 11
Coarse-grained recrystallized sandstone 3
Gray limey shale 2
Dark-gray fine-grained limestone 2
Granitic rocks 1
Vesicular porphyry 1
Red chert 1
Vein quartz 1
Green porphyry 1
Aphanitic igneous rock 1
Age and Correlation. Fossils were reported from two localities
in the massive conglomerate by Crandall.^^ Near the mouth of Berryessa
Creek, Aucella piochii Gabb was found in both the j)ebbles and the matrix
of the conglomerate. Pliylloeeras knoxvillense Stanton is also reported.
In Alum Rock Canyon, Crandall reports Belemnites sp. and Aucella pio-
chii Gabb from rocks now mapped as Oakland conglomerate. Recent
search failed to disclose additional material from either of these localities,
but Aucella crassicolis was found in a thin bed of pebbly sandstone
enclosed in sandstone and shale in a new road-cut east of the San Jose
Country Club. The beds may be Berryessa formation but are believed to
be interbedded with the Oakland conglomerate.
One collection already mentioned in connection with the Knoxville
(?) was found on the north slope of a small canyon 0.85 of a mile
N, 10° E. of the house shown on the map (pi. 1) as the Richmond Ranch.
The fossils are abundant, and occur in a bed of conglomeratic sandstone
and in limestone concretions just below it. Buchia crassa is the only form
identified. At least 50 feet of conglomerate resembling the Oakland con-
glomerate crops out just below the fossiliferons beds, but the base is
not exposed. It is some distance to the nearest fossiliferons Knoxville
shale. Two or three hundred feet of shale appear to intervene between the
fossiliferons horizon and the main mass of the Oakland conglomerate.
*5 Crandall, Roderic, The Cretaceous statigraphy of the Santa Clara Valley : Am.
Jour. Set., 4th Ser., vol. 24, pp. 33-54, 1907.
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DIVISION OP MINES
BULLETIN 157, PLATE 5
?^^
A, SUBDUED TOPOGRAPHY OF SAN FELIPE HILLS
Mount Hamilton and Lick Observatory in background
B, SMOOTH SUMMIT OF POVERTY RIDGE
View south from point on Poverty Ridge east of junction
of Smith Creek and Isabel Creek
DIVISION OF MINES
BULLETIN 157. PLATE 6
A, VIEW DOWN ARROYO HONDO
From point west of junction of Smith Creek and Isabel Creek.
Oak Ridge in rear, Poverty Ridge at left
B, SUBDUED TOPOGRAPHY
Of Los Buellis Hills and the north slopes of Alum Rock Canyon
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DIVISION OF MINES
BULLETIN 157, PLATE 9
A, NODULE OF BLADED ACTINOLITE AND TALC WRAPPED
BY CHLORITE AND ACTINOLITE
B, CRUMPLED CHERT REPLACED BY
GLAUCOPHANE SCHIST
IRBT-
i 1 1 ■ ^tv^'Wfmwv
DIVISION OF MIXES
BULLETIN 157, PLATE 10
-Vrto-,
OPALINE CHERT, MIDDLE MEMBER OF
MONTEREY FORMATION
Just below Sulphur Springs, Alum Rock Park
DIVISION OF MINES
BULLETIN 157, PLATE 11
BAD LAND EROSION IN SANTA CLARA FORMATION
East of San Felipe Road
■B*
■^rrrfrr^^"?"^?^^^^^^^^^^^^^^^
1951] STRATIGRAPHY 33
The fossil evidence cited above indicates a Lower Cretaceous age
for the beds mapped as Oakland conglomerate in this area. The strike
continuity and lithologic identity with the type locality of the formation
and the lack of fossils at the type locality, suggest that the formation
mapped by Lawsou in Oakland may also be Lower Cretaceous. On this
basis, the Oakland conglomerate is equivalent to parts of the Shasta
group.^^ but the information available is not sufficient to define its range
nor to indicate the stages to which it may be equivalent. In a section
through the area. Taliaferro.^' includes this unit in the Shasta group
or undifferentiated Lower Cretaceous.
Berryessa Formation
Xanie. Occurence, and ThicJiJiess. The name Berryessa formation
is introduced here to designate the sandstone, siltstone, and shale between
the Lower Cretaceous Oakland conglomerate and the Tertiarv Monterev
formation. Berryessa Creek was chosen as the type locality because it
is one of the few places where both the base and top of the formation
are exposed, and because no faults of large displacement are known to
cut the section. However, relations with the overlying Santa Clara
formation suggest the possibility of minor faulting, and exposures are
not complete enough to permit measurement of a detailed section.
Kocks assigned to the Berryessa formation crop out almost continu-
ously along both sides of the Tularcitos s^Ticline. One band extends
along the Calaveras fault from the vicinity- of Calaveras Reservoir to
Halls Valley, except for a 1-mile break at the south end of Los Buellis
Hills, and reappears for a short distance east of San Felipe Valley. The
other band extends from Scott Creek across Alum Rock Canyon, then
is faulted out for a distance of 3 miles except for one small patch. It
reappears west of Masters Hill and extends southeast to Panochita Hill,
faulted segments finally reaching the edge of the mapped area southeast
of San Felipe Valley.
The thickness of the Berryessa formation is variable, and nowhere
has it been determined with great accui-acy. In the western limb of the
Tularcitos syncline. it is about 1.000 feet thick at Berryessa Creek, about
1.600 feet thick at Alum Rock Canyon, and about 2.000 feet thick near
the Hendricks Ranch. It may be considerably thicker south of there,
but the base has not been mapped accurately, and the top is clearly
faulted. At one place, about a mile south of Alum Rock Canyon, the
]\Ionterey formation overlaps the Berryessa formation completely.
Xowhere in the ea.stern limb of the Tularcitos syncline is a complete
section of the Berryessa formation exposed. The only occurrence of a
rock comparable with the Oakland conglomerate is separated from the
rest of the section by obvious faults of the Calaveras system. Conse-
quently, only a minimum thickness can be given, and it is difficult to
be sure that structural complications are eliminated. The field mapping
suggests that the small syncline shown on section A-A' (pi. 2 i dies out
to the south, and it appears that at least 2,000 feet of sandstone, siltstone.
and shale mapped as Berryessa formation crop out below the base of the
ilonterev formation.
* Taliaferro. X. L., The Franciscan-Knox\-ille problem : Am. Assoc. Petroleum
Geologists Bull., vol. 27, pp. 109-219, 1943.
'^ Op. cit., Am. Assoc. Petroleum Geologists, pi. 2, section 9.
3 — 30T79
34 SAN JOSE-MOUNT HAMILTON AREA [BuU. 157
l/ithology. The Berryessa formation includes siltstone and shale
that weather to olive drab, thin-bedded inicaeeous sandstone, and par-
ticularly in the east limb of the Tularcitos syncline, massive, thick-bedded
sandstone. Brown wrinkled biotite occurring in hdr^e conspicuous flakes
in the sandstones and as fine particles throughout the siltstone and shale
is a characteristic feature of most exposures. It is usually oriented parallel
to the bedding and can be used to determine the attitude of small out-
crops of massive sandstone. It is also of assistance in distinguishing these
sandstones from sandstones of the Franciscan formation, which generally
contain smooth pearly flakes of muscovite instead of the wrinkled biotite.
Carbonized wood fragments and plant remains are abundant in many
places, and may be quite large.
The fresh, massive sandstone from a deep cut on the road west of
Calaveras Reservoir is a fine-grained rather poorly sorted arkose, con-
taining many fragments of carbonized wood. The grains are mostly very
angular, and there is a large amount of fine silty interstitial material.
The cement is mostly a bro^^•n carbonate, which is probably ankerite or
ferrian dolomite, since it appears in abundance in the heav.y separate.
Orientation of platy forms is quite marked. The most conspicuous are
large altered biotite flakes, which no longer show the birefringence and
absorption of biotite. Here and there, however, a thicker flake does show
these properties. A grain count of the light minerals in this rock gave
20 percent quartz, 30 percent feldspar, 49 percent rock fragments and
composite grains.
Age and Correlation. Only a few fossils have been found in the
beds designated here as Berryessa formation, none of them during the
present study. However, Crandall ^^ reports fossils from several places
northeast of Milpitas, the best exposures being in the lower port of
Arroyo de los Coches. The forms listed are :
Aucella piochii Gabb
Hoplites (fragment)
Belemnites (fragment)
Pecten complixicosta Gabb
In Alum Rock Canyon, Aucella piochii and Belemnites are reported
from a conglomerate, but the locality given by Templeton ^^ is a quarter
to half a mile west of the hotel, and much of this distance falls within
the Berrj^essa formation. Crandall also reports Aucella piochii Gabb,
Aucella crassicollis Keyserling, and Venus varians Gabb from several
localities in Niles Canyon and from ]\Iission Pass in the southern part of
the Pleasanton (piadrangle in beds which can be traced along the strike
into the Berrj'^essa formation.
Lacking more complete fossil evidence, the accurate correlation of
the Berryessa formation is impossible. It is continuous along the strike
with the Cretaceous shale and sandstone mapped by Harding,'*^' and
Erickson,^^ in the Pleasanton quadrangle, and by Lawson ^^ in the Hay-
ward quadrangle and the southwestern part of the Concord quadrangle.
** Crandall, R., op. cit.
s» Op. cit., Doctor's thesis, 1913.
<" Harding, J. W., Geology of the southern part of the Pleasanton quadrangle,
unpublished Master's thesis, University of California, Berkeley.
"■ Erickson, M. R., Geology of the western portion of the Pleasanton quadrangle,
California, unpublished Master's thesis, University of California, Berkeley.
*2 Lawson, A. C, U. S. Geol. Survey Geol. Atlas, San Francisco folio (no. 193),
1914.
. . ;,.-r-.">r-OOt*'vt'-_>&t-:' '-ii^iS'-' '
1951] STRATIGRAPHY 35
In all these areas, the rocks are mapped as ' ' Chieo ' '. The fossil evidence
summarized above, however, sngsests that much of this so-called Chico
is actually Lower Cretaceous, and should be correlated with the Shasta
group, rather than tlie Chico group of northern California. This does
not apply, of course, to the highly fossiliferous Chico beds mapped by
Lawson in the Franklin anticline on the north edge of the Concord
quadrangle, nor to the beds near Berkeley containing Inoceramus and
Baculites. It is possible that the Berryessa formation may include some
beds of Upper Cretaceous age, but the few fossils found so far do not
indicate that this is the case. It should be pointed out that the rocks
mapped as Berryessa formation in the Avestern limb of the Tularcitos
syncline may not all be the same age as those in the east limb near the
Calaveras Keservoir. Moreover, there is the possibility that part of the
beds mapped as Berryessa near the southeastern corner of the mapped
area may actually be Eocene in age. Eocene shales, indistinguishable
lithologicaily from the Cretaceous, have been mapped by Gilbert ^^ in
the northern part of the ilorgan Hill quadrangle, and their northern
limits are unknown.
Tertiary System
Tertiary rocks occupy much of the area between the Calaveras fault
zone and the Santa Clara Valley, but the sequence of formations is
relatively simple, and the range in age is limited. Although Eocene strata
are known only a short distance south,^^ there is no evidence that they
extend into the area mapped. If present, they are included in the Berry-
essa formation which they resemble closely.
The oldest lithologic unit known to be Tertiary is the middle ( ?)
Miocene sandstone mapped as the Temblor formation. This unit is
limited in extent to a small area north and east of Calaveras Dam, where
it is overlain by a thin cherty shale mapped as Monterey. The Temblor
is quite distinct both lithologicaily and faunally from the basal beds of
the Monterey farther west. West of the Calaveras fault, the oldest rocks
knoAATi to be Tertiary belong to the Monterey formation, and consist
largely of opaline chert and thin-bedded shale overlain by fine-grained
foraminiferal sandstone and siltstone. These strata are overlain with
.slight unconformity by the upper Miocene Briones sandstone which
crops out as prominent reefs at many places in the area. The Miocene-
Pliocene Orinda formation, dominantly maroon-red and greenish clays,
silts, sandstones, and conglomerates, is the yoiuigest unit mapped that
is known to be Tertiary. The aboAe formations all have been strongly
folded, dip steeply, and are overturned at many places. They are sepa-
rated from younger rocks by at least one episode of intense deformation,
the Coast Range orogeny.^^
Temblor Formation
Occurrence and Thickness. Fossiliferous Miocene rocks mapped as
Temblor formation crop out in a small area north and east of Calaveras
Reservoir where thev rest unconformablv on the Franciscan formation
i^' Gilbert, Charles M., Tertiary sediments northeast of Morgan Hill, California:
Am. Assoc. Petroleum Geologists Bull., vol. 27, no. 5, p. 6 44, 19 43.
" Gilbert, C. M., Tertiary sediments northeast of Morgan Hill, California : Am.
Assoc. Petroleum Geologists Bull., vol. 27, no. -5, 1943.
*^ Taliaferro, N. L., The Franciscan-Knoxville problem: Am. Assoc. Petroleum
Geologists Bull., vol. 27, no. 2, pp. 146, 156, 1943.
36 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
and grade upward into silicious Monterey shale. The formation crops
out plainly in a fold trough on the point between the reservoii- and
Arroyo Hondo ; it crops out again in a complexly faulted syncline on the
ridge to the north, where it is overlain by both Monterey and Briones;
it occurs again at the west end of the dam and in the prominent hill to
the north. The nearly flat-lying contact of these beds with the underlying
Franciscan is well exposed north of the quarry bench.
The Temblor formation is limited on the west by a fault of the
Calaveras system which is exposed for less than a mile at the north edge
of Calaveras Reservoir. The most recent movement on the fault was down
on the east, causing the Temblor-Franciscan (serpentine) contact to dip
into the Cretaceous. West of the fault, the Miocene has been considerably
elevated, and the Temblor is absent.
The Temblor formation ranges from 500 to about 1,000 feet in
thickness. A continuous section has not been measured; the thickest
section and best exposures are near the spillway of Calaveras Dam, but
the base is not exposed.
Lithology. The base of the Temblor formation is a conglomerate
or conglomeratic sandstone several feet thick. The pebbles are well
rounded, and average an inch or less in diameter, though large angular
blocks of the underlying Franciscan occur at places. The most abundant
pebbles are pre-Franciscan cherts, porphyries, and quartzites, but Fran-
ciscan chert and Monterey chert are also present. Fossils and especially
fragments of fossils are very abundant; pectens and barnacles were
recognized. Overlying the basal conglomerate are 50 to 100 feet of
interbedded sandstone and pebbly sandstone, most of which is cross-
bedded and filled with shell fragments. The greater part of the formation
consists of fine-grained yellowish sandstone, commonly rather thick
bedded. Fossils, especially pectens, are abundant throughout the section.
The occurrence of Monterey debris in the base of this formation
stratigraphically below beds which are also mapped as Monterey ^^ is
worthy of special mention. Clearly, rocks with Monterey lithology were
deposited over a considerable interval of time,^'^ therefore the possibility
of exposure and re-working does not seem unreasonable. In any case,
however, it would indicate at least local uplift, within the period covered
by the Monterey formation.
Age and Correlation. The age of the beds mapped as Temblor is
made reasonably certain by even the small collection found during this
work. The following were identified bv J. W. Durham.
^to
Bruclarkia oregonensis (Conrad)
Pecten propatulus Conrad
Chione sp.
Mytilus sp.
Cerastoderma sp.
*« Kleinpell, R. M., Miocene stratigraphy of California, fig. 14, Am. Assoc. Petro-
leum Geologists, 1938.
*'' Bramlette, M. N., The Monterey formation of California and the origin of the
siliceous rocks : U. S. Geol. Survey Prof. Paper 212, pi. 2, 1946.
1951] STRATIGRAPHY 37
Monterey Formation
The term "Monterey formation" is here applied, following Bram-
lette ^^ and AVoodring **" to the middle and possibly upper Miocene rocks
characterized by an abundance of siliceous sediments. The fact that they
do not include the same lithologic units mapped by Lawson ^" in the San
Francisco folio has also prompted the use of this term rather than
Lawson 's ' ' Monterey group. ' '
Occurrence and Thickness. The strata of the Monterey formation
crop out on both sides of the Tularcitos syncline throughout most of the
area mapped. In the west limb, the Monterey emerges from beneath the
Santa Clara formation southeast of Warm Springs and extends without
interruption across Alum Rock Canyon. It is faulted off about a mile
south of Alum Rock Canyon, but reappears for a short distance, dipping
east at moderate angles, toward the axis of the syncline. Beginning near
the mouth of Alum Rock Canyon and extending southeast for 4| miles
near the edge of the valley is a down-faulted block of Monterey which
is nearly flat lying and may locally dip either east or west. An isolated
outcrop probably related to this block occurs at the edge of the valley
about If miles northeast of Evergreen.
Between the Hendricks Ranch and the south end of Halls Valley,
an apparently normal band of Monterey, dipping about 40° NE., is
interrupted by two faults along which it is dragged out for a distance
of 5 miles. At the north, it is faulted up against the Briones on the west,
and at the south, it is faulted down into the Cretaceous on both sides.
The Monterey crops out again in the west limb of the syncline south
of Panochita Hill, and southeast of San Felipe Valley.
In the east limb of the Tularcitos syncline, the Monterey is exposed
from the north edge of the map to Los Buellis Hills, where it is overlapped
by the Briones sandstone. On Sierra Road, it is faulted out along the
Calaveras fault but reappears in Alum Rock Canyon and in Arroyo
Aguague where, though poorly exposed, it forms a thin bed between
the Briones and Berryessa formations. The Monterey was not recognized
between the Arroyo Aguague and Halls Valley, where it is again faulted
out by the Calaveras fault zone. At the south end of Halls Valley, and
east of Panochita Hill, it appears as thin lenses faulted against Francis-
can. It is last seen between two faults of the Calaveras system about
three quarters of a mile south of the triangulation station "Bacon,"
east of San Felipe Valley.
The Monterey formation is thickest in Alum Rock Canyon and vicin-
ity, but it is impossible even here to measure a continuous section. The
best exposures are on the steep slopes of Alum Rock Canyon where the
upper foraminiferal member was measured and found to be 350 to 400
feet thick. About 500 feet of the cherty middle member crops out below
this, but the Monterey-Cretaceous contact is not exposed.
The Monterey exposed in the west limb of the Tularcitos syncline is
somewhat thinner to the north of Alum Rock Canyon, averaging 500 to
600 feet between Arroyo de Los Coches and Calera Creek. It thins
« Bramlette, M. N., op. cit., p. 3.
<»Woodring, W. P., Stewart, R., and Richards, R. W., Geology of the Kettleman
Hills oilfield, California : U. S. Geol. Survey Prof. Paper 195, pp. 117-118, 1940.
50 Lawson, A. C, U. S. Geol. Survey Geol. Atlas, San Francisco folio (no. 193), 1914.
38 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
abruptly just south of Alum Eoek Canyon, but this maj^ be partly the
result of faulting-. The Monterey is decidedly thinner everywhere in
the east limb of the syncline, ranging from a thin film to about 350 feet,
but averaging less than 100 feet. East of the Calaveras fault, the
Monterey is only about 350 feet thick.
LitJwlogy. Three distinct lithologie units can be distinguished in
the Monterey formation west of the Calaveras fault. Beginning at the
base, these are: (1) a few feet of light gray to brown sandstone, more
or less glauconitic, which may grade into gra.y or chocolate-colored
shale; (2) thin-bedded opaline chert with very thin shale partings,
usually a few hundred feet thick; (3) pale buff foraminiferal siltstone
with some platy sandstone and massive silty sandstone. The lower sand-
stone unit has been observed in only a few places, where exposures are
unusally good. The typical Monterey chert of the middle member in
general crops out well, and can be recognized readily by float where it
does not actually crop out. The upper member is rather soft, and crops
out well in places, but not at all elsewhere.
In all the work done in this area since 1946, these three divisions
have been recognized, and the Monterey-Briones contact has been drawn
above the foraminiferal silts. However, in the work done before the
war, this unit was included in the Briones at places, as along the lower
part of the Mount Hamilton Road. This inconsistency is most unfortu-
nate, but time has not permitted remapping the contact.
The basal sandstone of the Monterey is best exposed along the road
in Arroj'O de Los Codies, about 100 feet west of the bridge leading to
the ranch house and quarry in the Monterey. Here, shale of the Berryessa
is overlain by medium- to coarse-grained white sandstone containing
glauconite in at least two beds. The sandstone grades upAvard into silty
shale which in turn grades into thin-bedded chert. The transition to
the upper member of the formation is gradual, and occurs within the
quarry on the south side of the canyon. Glauconite-bearing sandstone,
probably from the same horizon, was found in fresh road cuts near the
line of section C-C (pi. 2), just south of the mouth of Alum Rock
Canyon. The base of the Monterey is not exposed higher in Alum Rock
Canyon, but gray to chocolate-colored shale underlying the Monterey
chert is exposed in the lowest switchback of the road to the upper canyon.
The middle member of the Monterey formation is well exposed
around the sulfur and soda springs in Alum Rock Park. It consists of
rhythmically bedded pale cream-colored opaline chert and cherty shale
characteristic of the Monterey formation throughout the Coast Ranges.^^
The bands of chert are from one incli to several inches thick, and most
of them show some thin banding within each bed. The individual chert
beds are separated by partings of shale an eighth to a quarter of an inch
thick. Irregular beds and lenses of dense limestone are common near the
base and the top of this member, and weather out as distinctive yellow
ocherous masses. The cherty part of the Monterey shows strong local
crumpling and may be intricately folded, even though the beds above
and below show only broad, open folds.
The thin-bedded cherts grade upward into the shale, siltstone, and
fine-grained sandstone of the upper member, which is best exposed on
the north rim of Alum Rock Canvon. When fresh, the rocks are dark
61 Bramlette, M. N., op. cit.
■Trr?H!W«??wf?f"
1951] STRATIGRAPHY 39
gray and massive; but in weathering:, the massive beds commonly disin-
tegrate, yielding soft puuky or sandy debris. Foraminifera are abundant
in many parts of the member, and are readily visible in many specimens.
A few small pelecypods fonnd in these beds were identified by J. W. Dur-
ham as Lucina acutilineata Conrad.
Age and Correlation. The age of the lowest beds of the Monterey
formation of the Tularcitos sjaicline has not been determined. Neither
mega fossils nor foraminifera were found in the thin glauconitic sand-
stone at the base. Though foraminifera are very abundant in the upper
member, they are poorly preserved, and the following were identified
by R. M. Kleinpell in a rather small collection from this member.
Bolivina aff. marginata Cushmau rare
Bolivina afE. tumida Cushmau rare
Nonion costiferum (Cushman) few
Nouionella mioceuica (?) Cushmau rare
Virgulina calif oruieusis Cushmau abundant
This assemblage is regarded by Kleinpell ^- as ' ' not higher than
uppermost Luisian. In this area, this would indicate a correlation with
the bulk of the upper Claremont or the lower Tice at San Pablo. It is
definitely older than the bulk of the Tice." On the basis of this data,
it is probable that the Monterey formation as mapped in this area falls
within the upper middle JMiocene.
Briones Sandstone
Occurrence and Thicliness. The Briones sandstone ^^ is here applied
to a thick series of massive coarse-grained fossiliferous sandstones and
sandy shales which unconformably overlie the Monterey formation. The
unit crops out almost without interruption for the entire length of the
Tularcitos syncline. Throughout the west limb it rests on the Monterey
formation, but in the east limb it rests for considerable distances on the
Cretaceous Berryessa formation.
The Briones sandstone varies considerably in thickness. It is 1,650
feet thick near Calera Creek southeast of Warm Springs, 3,400 feet thick
in Alum Rock Canyon, and about 3,500 feet thick on Felter Road at the
north end of Los Buellis Hills. South of section D-D' (pi. 2) the thickness
is unknown because no younger beds are present. Along the line of sec-
tion A- A' (pi. 2), the formation thickens rapidly eastward to 5,000 feet.
Lithology. The Briones sandstone forms the most conspicuous and
distinctive outcrops of any of the Tertiary formations. These are particu-
larly evident on Monument Peak and the Los Buellis Hills where the
steeply dipping beds crop out as narrow "reefs" which resemble rock
walls or fences running straight over hills and valleys. Most of these
reefs are massive sandstone beds 1 foot to 6 feet thick filled with shells
and shell fragments and thoroughly cemented by calcium carbonate.
Between the reefs, the hills are grassy and relatively smooth, and are
underlain by less well cemented parts of the formation.
The sandstone of the Briones is dominantly massive, thick-bedded,
medium- to coarse-grained, light colored when fresh, gray or buff when
weathered. Plant and wood fragments are abundant in certain beds.
=- Oral communication, 1945.
^ Lawson, A. C, op. cit 1914.
i'
40 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Cross-bedding is well developed locally and is particularly well exposed
on the smooth worn outcrops in the bed of Penitencia Creek in upper
Alum Eock Can.yon. Conglomeratic layers noted at several places near
the middle of the formation contain many pebbles and cobbles of yellow-
ish limestone from the Monterey formation, large fragments of oyster
shells, and more rarely some banded Monterey chert and pre-Franciscan
porphyry. Interbedded with the massive, well-cemented sandstones
are beds of friable siltstone and fine-grained sandstone. The typical
fresh, massive Briones sandstone is a medium-grained, well-sorted
arkosic sandstone cemented by carbonate.
Table 2. Mineral Analysis of Typical Briones Sandstone
Light minerals Percent
Quartz 25
Feldspar 28
Carbonate 10
Rock fragments and composite grains 37
100
Heavy minerals
Green hornblende 49
Brown hornblende 11
Leucoxene 9
Magnetite-ilmenite 6
Apatite 5
Hematite? 4
Sphene 3
Zoisite 3
Garnet 3
Carbonate 2
Zircon 1
Chlorite 1
Tourmaline <1
Glaucophane <1
Rutile present
99
Age and Correlation. Although fossils are extremely abundant in
parts of the Briones, only a few poor specimens were obtained by break-
ing the massive sandstones. These include :
Tivela merriami Trask (?)
Spisula selbyensis Packard ( ?)
Pecten propatulus Conrad
Large faunas from this area and adjoining parts of the Pleasanton
quadrangle are listed by Templeton and Trask,^^ and the status of the
unit was summarized in detail by Trask after study of collections at both
the University of California and at Leland Stanford Jr. University.
He concludes that the Briones is upper Miocene, and is more closely
related to the overlying San Pablo group than to the underlying Mon-
terey. This conclusion is borne out by recent work. The progressive over-
lap of the upper members of the Monterey group by the Briones in the
southern part of the Pleasanton quadrangle, ^^ and its ultimate overlap
^ Trask, P. D., The Briones formation of middle California : Univ. California,
Dept. Geol. Sci. Bull., vol. 13, no. 5, pp. 133-174, 1922.
Templeton, E. C, The geology and stratigraphy of the San Jose quadrangle,
unpublished Doctor's thesis, Stanford Univ., 1913.
^ Harding, J. W., op. cit
1951] STRATIGRAPHY 41
onto the Cretaceous in the San Jose-Mount Hamilton area, give ample
evidence of the persistent unconformity between the two. Worn pebbles
of Monterey which appear in the middle of the Briones further emphasize
the degree of uplift and erosion involved.
Orinda Formation
Occurrence and Thickness. Beds mapped as the Orinda formation
are exposed in the center of the Tularcitos syncline from just south of
the line of section D-D' (pi. 2) to the north edge of the map. They are
tightly folded, overturned in part, and rest unconformably on the
Briones. Near the north edge of the quadrangle, the Orinda is about
2,000 feet thick. In sections C-C and D-D', the thickness is estimated at
about 750 feet. Near the Mountain House on the Mount Hamilton Road,
only a hundred feet or so is exposed.
Lithology. The Orinda formation consists of red, maroon, green, or
gray conglomerate, conglomeratic sandstone, silt, and clay. The con-
glomerates are most abundant on the west slopes of Monument Peak near
the northwest corner of the area mapped, where they are commonly
rather well cemented and crop out as broad ledges. In Arroyo de Los
Coches, recent excavations have exposed a large area of maroon siltstone
and mudstone with some greenish streaks. On Sierra Road maroon and
greenish pebble and cobble conglomerate interbedded with pebbly sand-
stone crop out east of the highest Briones, and grade into silty and
clayey beds farther east. The conglomerates are stained by manganese
oxide, and are cut by many sheared surfaces. Recent excavations on the
point between Arroyo Aguague and upper Alum Rock Road have exposed
maroon and greenish clay in a narrow zone in a sharply folded and
faulted part of the Tularcitos syncline.
The southernmost exposures of the Orinda formation are in a
sharply overturned fold along the line of section D-D' (pi. 2). Maroon
and bright-red silt and clay streaked by green crop out in several places.
These exposures are unusual in that they contain interbedded volcanics
which are absent elsewhere in the area. Hornblende andesite lapilli tuff,
and tuffaceous sandstone are interbedded with the usual sediments.
A single slide of hornblende andesite was examined under the micro-
scope. It is a pinkish-gra.y rock containing phenocrysts of feldspar and
pseudomorphs after hornblende. The feldspar phenocrysts are rather
broad laths ranging in size from half a millimeter to 3 millimeters. They
are complexly twinned, and most of them show oscillatory zoning through-
out. The composition ranges from An 48 to.An 57. The hornblende pheno-
crysts are narrow euhedral laths as much as 2 millimeters in length, which
are entirely replaced by magnetite and hematite. The groundmass con-
sists of microlites of basic feldspar in a matrix of cloudy glass with a
refractive index greater than that of canada balsam. Hematite occurs
in fine-grained plates throughout the groundmass ; a few grains of quartz
also occur. The most conspicuous minor accessory is an unusual brown
pleochroic apatite, which occurs as euhedral crystals. The pleochroism is
X — medium to dark brown, Y and Z — pale straw yellow. The refractive
index was determined as E^ 1.634, w= 1.638. Its identity was confirmed
by F. J. Turner, by measuring the interfacial angles on the universal
stage.
42 " SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Age and Correlation. The correlation of the beds mapped in the
San Jose-Mount Hamilton area with the Orinda formation "'" is based
on lithologic similarity and stratigraphie position. From the type locality
at Orinda, the formation extends southeastward through the Concord
and Hay ward quadrangles ; ^" it was mapped by Erickson ^^ to its inter-
section with the Sunol-Calaveras fault at Dublin. It appears again 12
miles south in the Tularcitos syncline near Mission San Jose, and is
continuous from that point through the area mapped in the San Jose
quadrangle. The age of the type Orinda formation was recently dis-
cussed by Durham, •'"''•* and shown to be "in part at least as old as Neroly,"
and possibly "in part still older." Recent discoveries reported by
Savage ^^ indicate that the Orinda as mapped by Lawson includes beds
as young as Blancan (Pliocene). The stratigraphic range of the Orinda
in the San Jose area is unknown, and it can only be tentatively designated
as Mio-Pliocene in age.
The Orinda formation overlies the Briones sandstone with slight
angular unconformity. A discordance in dip of as much as 30 degrees
appears at places. The Orinda is the youngest formation exposed in
the Tularcitos syncline.
Tertiary-Quaternary (?) System
The rocks tenatively regarded as transitional between the Pliocene
and the Pleistocene crop out beneath younger sediments around the
margins of the present basins. In general, they have been tilted and
involved in recent faulting, but they are clearly separated both spatially
and structurally from the youngest folded rocks.
Where first studied, two units, the relatively consolidated Pack-
wood gravels occurring between faults on Silver Creek, and the Santa
Clara formation occurring on the slope east of San Jose, were regarded
as belonging to this system. However, recent study of an extensive fauna
from the Santa Clara formation at Irvington,*^^ indicates that it is
definitely Pleistocene in age. The Packwood gravels are not contiguous
areally, and may be older than Pleistocene.
Packwood Gravels
The term Packwood gravels was applied by Tolman ^^ to a series of
unconsolidated sediments which crop out in the lower part of Packwood
Creek, in the Morgan Hill quadrangle, where they occur in a block
which has been dropped down between the Silver Creek and the Calaveras
fault zones. To the north, they are bounded by a fault which leaves the
Calaveras fault and swings northwest to joint the Silver Creek fault
at the extreme south edge of the San Jose quadrangle. The formation
reappears about a mile north along the Silver Creek fault zone, where
it is dropped down between two faults.
s" Lawson, A. C, and Palache, C, The Berkeley Hills, a detail of Coast Range
geology : Univ. California, Dept. Geol. Sci. Bull., vol. 2, pp. 349-350, 1902.
=' Lawson, A. C, op. cit., 1914.
^ Erickson, M. R., op. cit.
59 Durham, J. W., Relationship of California upper Miocene-lower Pliocene verte-
brate and invertebrate faunas (abstract) : Geol. Soc. America Bull., vol. 57, no. 12, pi. 2,
p. 1386, 1948.
™ Savage, D. E., Blancan horse from near Walnut Creek, California: Geol. Soc.
America Bull., vol. 59, no. 12, 1948.
"1 Savage, D. B., Late Cenozoic vertebrates of the San Francisco Bav region, in
press. University of California, 1950.
«2 Op. cit.
^^glggg«|gg,^
1951] STRATIGRAPHY 43
In the type locality the formation consists of poorly consolidated
•travel, pebbly sandstone, silt, and clay. "Where it is best exposed alon?
section E-E' (pi. 2), it is well consolidated, and contains considerable
white limy cement. It includes medium-grained cross-bedded sandstone
and sandy conglomerate. Some of the pebbles are several inches in
diameter. The most common rock types are pale ivory Monterey chert.
Franciscan chert and sandstone, porphyries probably derived from the
Oakland (?) conglomerate, and sandstone from the Berryessa forma-
tion. Near the west edge of the embayment of Packwood gravel just
north of the line of section E-E' (pi. 2) coarse boulders of diorite
weathered from serpentine are very abundant, and suggest a very local
source for this part of the formation.
The 500-foot thickness of Packwood gravel shown on section E-E'
(pi. 2) is arbitrary, and certainly is a minimum value. In the Morgan
Hill quadrangle, where the outcrop is widest, the Packwood is exposed
across the strike for about 2 miles. Reconnaissance indicates a homoelinal
dip to the east of 10 to 30 degrees, yielding a thickness of at least 3,000
feet with neither base nor top exposed.
The lack of fossils in the Packvrood gravels makes it impossible to
date them or to correlate them accurately. It is possible that they are
the equivalent in age of the Santa Clara formation mapped farther
north, but their structural setting suggests that they are somewhat
older. The magnitude of the faulting and the incipient folding they
have undergone suggest possible correlation with Plio-Pleistocene
deposits such as the Paso Robles and Paicines formations.
Quaternary System
Santa Clara Formation
Distribution and Fades. Unconsolidated sediments of the Santa
Clara formation crop out almost continuously along the east edge of the
Santa Clara Yalle}' from "Warm Springs south to Alum Rock Canyon.
Smaller patches of similar character crop out east of Evergreen. South
of Evergreen is a large area mapped as part of the Santa Clara forma-
tion, but whose identity -with it is questionable.
From Warm Springs to Alum Rock Canyon, the formation shows
reasonably constant character. TVhere exposed in recent cuts, it is
obscurely bedded, poorly sorted, pebbly sandstone, siltstone, or clay. The
strike of the beds throughout this distance is relatively constant, N.
30-40= AY. ; the dip ranges from 10° to 30° E. The formation is incoher-
ent and has been so affected by sliding that little of the original bedding
is visible in natural exposures. In fact, from Arroyo de Los Coches to
Alum Rock Canyon, the outcrops of the formation are virtually outlined
by the features of multiple and continued sliding.
In the small creeks immediately south of Arroyo de Los Coches,
deep narrow gullies have been cut through such slide material. The walls
show contorted and chaotic conglomerate, pebbly sandstone, and silt.
Massive reddish bouldery clay, sticky and plastic in winter, shrunken
and cracked in the summer, occurs everywhere. Curving sliekensided
surfaces cut all the material, no matter how incoherent. The area mapped
as Santa Clara formation near the Hendricks Ranch northeast of Ever-
green is a jumbled slide containing large amounts of angular fragments
of Monterey, some fragments of the Berryessa formation, and much
fine debris.
44 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
The small area of Santa Clara formation near the Kuhn Ranch
consists of poorly sorted, loosely consolidated reddish and yellowish
gravel, sand, and clay. The material is much coarser than average; it
is chiefly cobbles and gravel from the Oakland conglomerate.
The large area of gravels extending from Evergreen to Dry Creek
is included in the Santa Clara formation because it shows a gentle tilt
to the east and crops out above, and is being overlapped by the younger
alluvium. It is well exposed just east of San Felipe road, where small
areas of badland erosion have formed in the steep banks. The gullying
is developed in yellowish and reddish poorly consolidated sand and sandy
gravel. Most of the debris is subrounded to subangular shale and sand-
stone from the Berryessa formation. The bedding strikes N. 60° W., and
dips 30° NE. The difference in lithology and the lack of consolidation
are largely responsible for the separation of these gravels from the area
of gravel on Silver Creek mapped as Packwood.
Farther northwest, and immediately south of Evergreen, the hills
are rolling, and are shown by wells to consist of gravel and clay. The few
outcrops are apparently the tops of buried hills just being exhumed. Most
of these are ordinary silica-carbonate rocks or serpentine. There is one
unusual occurrence, however, 1.75 miles S. 27-|° E. of Evergreen, where
the rock}' knoll consists not of altered serpentine, but of pebbly clay or
silt, cemented and veined by coarse, bladed dolomite. It has the same yel-
low ocherous color and appearance as tj^pical silica-carbonate rock, but
contains rounded to angular pebbles and fragments of sandstone, shale,
tuff, and limestone. This leaves little doubt that solutions capable of pro-
ducing silica-carbonate rock were in action after the deposition of this
part of the Santa Clara formation.
Age and Correlation. The Santa Clara formation has generally
been regarded as Plio-Pleistocene, but no diagnostic or abundant evi-
dence of its age was found in the San Jose area. Recently, a large verte-
brate fauna was discovered in a northward extension of the formation
near Irvington ; this has been designated lower Pleistocene. On the basis
of these recent discoveries, this part of the Santa Clara formation is con-
sidered equivalent to the upper part of the San Benito gravels,*'-^ and
possibly to parts of the Paso Robles formation of the Salinas Valley. It
apparently is younger than intensely folded Pliocene beds exposed near
Concord.
Terrace Gravel
Deposits of coarse alluvium which have been tilted or warped, and
which clearly are older than the present streams, appear in at least two
places along the Calaveras fault zone. The most extensive deposits crop
out in steep, gullied cliffs at the south end of the Calaveras Reservoir.
They consist of poorly sorted gravel, clay, and cobbles dipping about 30°
E. Exposures to the east indicate that deposition was on an irregular
surface, probably much like that found now, but with less relief. There is
very little suggestion of terrace development east of the Calaveras fault,
but west of the fault there is a prominent bench which slopes gently
northward ; this has been deeply dissected by the present streams. It is
developed on coarse unconsolidated cobble gravel and some silty sand;
83 Wilson, Ivan F., Geology of the San Benito quadrangle, California : California
Jour. Mines and Geology, vol. 39, pp. 183-270, 1943.
1951] STRATIGRAPHY 45
the dip is about 20° W. The cobbles are mostly subaugular fragments of
Franciscan sandstone.
The other occurrence of terrace gravel is in the south end of the San
Felipe Valley. Here a -well-developed series of gravels and sands which dip
gently northward are being dissected by San Felipe Creek. Their present
surface is accordant with the long smooth slope to the south. It suggests
a reversal of drainage in this area, which is well supported by physio-
graphic evidence. The gravel here is subangular, and is mostly from the
Berryessa formation.
Landslides
Landslides of sufficient size have been mapped wherever practic-
able ; however, there are many which have been omitted, particularly
where it is difficult to distinguish them from the underlying rock. Large
areas of the Santa Clara formation have undergone repeated sliding, but
it is almost impossible to distinguish them from that formation in place.
In a few places, careful study of the extent and composition of a slide is
imjjortant to the proper interpretation of the geology of the bedrock.
Alum Rock Landslide. A large and important slide occurs on the
north slope of Alum Rock Canyon just west of the Alum Rock rhyolite.
It is a heterogenous mass of soft bluish serpentine muck, intermixed with
silt and gravel of the Santa Clara formation, and a few large boulders
and blocks of Franciscan. It came from well up the north slope, and filled
the canyon to at least the height of the railroad cut on the north end of
the high cliffs of Oakland conglomerate at the mouth of the canyon. The
stream probably cut rapidly through and removed all the finer material,
leaving the larger boulders stranded. Such boulders, up to 15 feet in diam-
eter are exposed along the road and in the railroad cuts at the north
end of the conglomerate cliffs, and were at first mistaken for exposures
of bedrock in place. One such boulder consisted of manganese oxides, and
was for many years believed to be a meteorite. Similar boulders in land-
slides resting on conglomerate can be seen in the road cuts about a quar-
ter of a mile farther up the canyon, just west of the steep narrow canyon
which cuts through the rhyolite. On a narrow point across the creek
northwest of the cliffs of conglomerate, there is a slide mass consisting of
sheared soapy serpentine which rests on cross-bedded gravel and silt, and
broken weathered Oakland conglomerate.
Oak Fidge Landslide. The most spectacular slide in the area is that
which extends from the western edge of Oak Ridge into Arroyo Hondo.
This recent feature has carried great blocks of Franciscan greenstone,
sandstone, and chert more than a mile down the slope.
Alluvium
No attempt has been made to map or dift'erentiate the subdivisions
of the alluvium of the Santa Clara Valley. Data were collected from well
logs, however, in an attempt to ascertain the depth of alluvial fill in the
valley, and the nature of the bedrock surface. The deepest well on record
is that of the San Jose Water Works at Seventeenth and Santa Clara
Streets, which reaches a depth of 1,535 feet. The driUers log of this weU
(table 3) shows in the first 250 feet less than one quarter gravel ; from 250
to 1,000 feet, about one third gravel ; from 1,000 feet to the bottom, only a
little gravel mixed with clay. Although such a log is inadequate geologi-
46
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
o
>>
;h
X
o
o
o
3
llO
a
o
si
M
a
0SS8^JJ3g 01
1951]
STRATIGRAPHV
TaUe 3. Drillers Joy of San Jose Water Works, yell ^
0. 1, nth and Santa Cla
drilled 1909,
1910.
0- 18 soil
500-506 yc
877- 890 be and eem gr
18- 50 yc
506-514 be
890- 895 yc and gr
50- 79 be
514-523 ye
895- 939 be
79- 89 gr
523-535 eem gr
939- 950 ye
89- 97 yc
585-537 ye
950- 952 yc and gr
97-115 gr
537-545 eem gr
952- 957 ye
115-130 be
545-550 yc
957- 971 yc and gr
130-141 gr
550-552 cem gr
971- 980 yc
141-145 ye
552-568 be
980-1005 yc and gr
145-150 y:r
568-583 eem gr
1005-1035 be
150-153 be
583-587 yc
1035-1060 ye
153-189 yc
587-590 eem gr
1060-1072 ye and gr
189-206 gr
590-615 be
1072-1130 be
206-225 ye
615-620 ye
1130-1135 yc
225-255 be
620-&35 cem gr
1135-1145 be
255-262 ye
635-673 ye
1145-1150 ye
262-273 eem gr
673-680 eem gr
1150-1190 be
273-280 yc
680-700 ye
1190-1228 ye
280-287 cem gr
700-705 com gr
1228-1232 yc and gr
287-292 ye
705-712 yc
1232-1240 ye
292-333 cem gr
712-717 cem gr
1240-1330 be
333-355 be
717-725 yc
1330-1300 ye
355-358 ve
725-735 cem gr
1360-1448 be
358-386 be
735-750 yc
1448-1469 yc
386-393 eem gr
750-760 be
1469-1488 be
393-402 be
760-772 yc
1488-1495 ye
402-413 grey clay
772-777 eem gr
1495-1520 yc and gr
413-422 be
777-791 be
1520-1535
422-435 yc
435-440 cem gr
446-460 yc
791-809 vc
809-813 be
813-850 yc & gr
ABBREVIATIONS
yc — yellow clay
be — blue clay
460-465 cem gr
465-475 ge
850-855 be (a boulder here)
855-865 be
gr — gravel
cem gr — cemented gravel
475-495 be
865-870 yc
495-500 eem gr
870-S77 be
47
cally. it indicates a marked decrease in the amount of coarse material at a
depth of about 1,000 feet. What the age of this material maj' be is entirely
coujectural.
One of the most interesting features of the well log, from the stand-
point of both structural history and sedimentation of the Santa Clara
Valley, is that the bottom of the well is more than 1,100 feet below the
present ' " threshold ' ' of the valle}' at the Golden Gate.
Alum Rock Rhyolite
Igneous Rocks
The Alum Rock rliyolite is a body of intrusive igneous rock about
1,000 feet wide and 400 feet long which crops out in the lower Alum
Rock Canyon. The most conspicuous exposures are in cliffs and talus
slopes on the north side of the canyon opposite the Alum Rock Avenue
entrance. It also forms the steep cliffs above the bridge where that road
first crosses Penitencia Creek. It is intruded into the Cretaceous Oak-
land conglomerate and the Berryessa formation but a thin wrapping of
serpentine has been carried up along the contact and locally has been
altered to silica-carbonate rock. Although much of the contact is covered
by landslide and talus, this relationship is visible in several places, and
48 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
is particularly well exposed in the railroad cut at the extreme southern
end of the mass. The rhyolite is bordered by silica-carbonate rock which
grades outward into sheared green serpentine adjoining the conglomer-
rate and shale. Northwest of this exposure the contact continues diago-
nally up the slope, marked by a low rib of silica-carbonate rock which
continues until covered by the talus from the higher cliffs.
Tlie rln-olite is mostly a strongly jointed fine-grained rock more
or less silicified and stained by limonite. Fresh surfaces range in color
from nearly white to brownish red or purple. Phenocrysts of albite up
to 2 millimeters across occur in some phases of the rock. There are no
ferromagnesian minerals. The entire rock when fused yields a glass with
a refractive index of 1.485 dz, indicating a silica content of nearly 80
percent.^^
Quite a different rock is exposed in the extreme northwest end of
the body, where it is cut through by the steep gully, and in a few other
places around the margin. This is a soft yellowish or greenish vesicular
rock which contrasts strongly with the hard cliff-forming rhyolite. The
general appearance suggests that it is a more basic rock than the
rhyolite.
In general appearance and composition, the Alum Rock rhyolite
bears a striking resemblance to the Leona rhyolite of the Oakland area.^^
However, the latter appears to consist of thick flows tilling fault valleys
along the mountain front, rather than of a single plug. In age, the intru-
sion is known only to be post-Cretaceous, but by analogy with the Leona,
it is much more likely that it is post-Orinda and possibly Quaternary.
Andesite
In the fields and rolling hills just east of Silver Creek road 2 miles
southeast of Evergreen is a long, narrow exposure of a fresh black glassy
andesite with a pitchy luster and a porphyritic texture. It intrudes
serpentine and appears to be overlapped by the Santa Clara formation.
Its age is almost entirely a matter of conjecture, but the unusual fresh-
ness suggests that it is probably Quaternary. No other rocks like it are
known within the area. A thin section of the andesite shows large, partly
resorbed phenocrysts of oligoclase-andesine and smaller ones of colorless
augite in a densely felted groundmass of feldspar microlites (andesine)
and rounded grains of augite with interstitial brown glass.
STRUCTURE
The San Jose-Mount Hamilton area lies astride a complicated zone
of folds and faults which form the western margin of the Diablo Range.
The great northwest-trending Hayward and Calaveras fault systems are
major features of the central Coast Ranges, extending many miles beyond
the boundary of the area mapped. Although they are active faults,
characterized by rift-type horizontal displacements, both of them coincide
for some distance with older faults which have had a major part in
delineating the larger structural units of the area.
Northeast of the Calaveras fault is a great area of Franciscan rock
which is part of the core of the Diablo Range. Between the Calaveras
and Hayward faults is the long, narrow block of folded Tertiary and
®* George, "William C, The relation.ship of the physical properties of the natural
glasses to their chemical composition: Jour. Geology, vol. 32, no. 5, pp. 353-372, 1924.
^ Lawson, A. C., op. cit. 1914.
HHMb
1951]
STRUCTTRE
49
Oak Rtdge
Mt Day Black Mountain
FiGTKE &. View of Arroyo Hondo from near its junction wiih Calaveras
Valley. Older valley profile is preserved as benches along the canyon wall
Cretaceous sediments, here designated the Tulareitcs block, which is
the southeastern continuation of ]\Iission Ridge and the Berkeley Hills.
West of the Ha^-wa^d fault, beneath the alluvium of the Santa Clara
Valley, is another area of Franciscan rocks, the Santa Clara Valley
block, which extends northwestward beneath the bay of San Francisco.
The largest and most conspicuous fold of the area is in the Tularcitos
block and is designated the Tularcitos syneline. This structure can be
traced from Xiles in Pleasanton quadrangle at least to the edge of the
]\Iorgan Hill quadrangle. For much of this distance it is OTerturued to
the southwest and is involved in the complex faults of the Calaveras
system. The most recent movements in this zone have offset the fold,
and the amount of offset is the best measure of the horizontal displace-
ment along the fault. Folding within the areas of Franciscan rocks is
not so well known, but undoubtedly there are folds as large as this or
larger. One great syneline appears to extend the length of the area
mapped, just east of the Calaveras fault. East of it, however, the struc-
ture becomes confused and consistent axes could not be traced with
certaintv.
Faults
Calaveras Fault System
The dominant fault of this area is the complex series of fractures
known as the Sunol or Calaveras fault. From Sunol and Calaveras
Valleys it can be traced northward through the Pleasanton quadrangle,
the Concord quadrangle (where it was called the Franklin fault by
Lawson ®^ ). and into the Carquinez quadrangle. On the geologic map of
California ^" it is shown as far north as Port Costa on Carquinez Strait.
Southeast of Calaveras Valley it forms a well-marked trench which con-
tains Halls Valley and San Felipe Valley, and beyond the area mapped,
«Op. cit., 1914.
•^ California Div. Mines, 1938.
50
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
Figure 10. Structural features of the San Jose-Mount Hamilton area
!™«niHi«mi
tmu
1951]
STRUCTURE
5]
Figure 11. Map showing distribution of Cretaceous and Miocene rocks
along the Calaveras fault zone
52 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
the valley of Coyote Creek in the Morgan Hill qnadrangle. It emerges
from the hills just east of Gilroy, and for some distance is traceable along
the east edge of tlie Santa Clara Valley. It disappears beneath alluvium
northwest of Hollister, but N. L. Taliaferro "^ reports that faults aligned
with it continue still farther southeast.
The straightness of the Sunol-Calaveras fault has been commented
upon by Vickery ^^ and others, and in a generalized way the linearity
is indeed remarkable. In detail, however, many irregularities appear,
particularly if the faults required or demonstrated by stratigraphic
evidence be considered ; these are dominantly older planes within the
system. In several places there is physiographic evidence of faults which
appear to take "short cuts" and thus straighten the zone of movement.
Such is the case between Los Buellis Hills and Halls Valley, where the
fault separating the Franciscan from the rocks of Cretaceous and Ter-
tiary age bends westward, and a new fault has developed within the
Franciscan rocks east of the Haskins triangulation station. Earthquake
epicenters which fall within 2 miles of this fault w^ere recently tabulated
for a 4-year period by Louderback '^" and indicate clearly that it is one
of the active faults of the area. There have been no actual displacements
within historic time, however, and the character of the most recent
movements can be determined by interpretation of physiographic fea-
tures along the fault. Within the area mapped, there is no evidence of
systematic vertical displacements, though there are several places where
sliver-like blocks within or parallel to the zone show differential eleva-
tion or tilting. Recent horizontal displacements along the fault zone are
suggested by deflection of stream patterns in a few places, but these
features are not as well developed as they are along the San Andreas
fault.
The most significant systematic recent displacements on the Cala-
veras fault zone are horizontal movements analogous to those which
occurred on the San Andreas fault during the 1906 earthquake, in which
the southwest side of the fault moved toward the northwest. Although
no such movements have occurred on the Calaveras fault within historic
time, the displacements are in this direction, and it is possible to measure
their cumulative effects since the development of the Tularcitos syncline.
The synclinal axis crosses the fault zone at a low angle within the area
mapped, and the total displacement since the folding is the summation
of the individual displacements on faults within the zone. This distance
amounts to about 3 miles.
Throughout most of the distance mapped, the horizontal displace-
ments are concentrated in a fairly narrow zone, but between Halls
Valley and San Felipe Valle3\ the displacements are distributed over
several faults in a zone a mile wide. These faults have affected mostly the
western limb of the Tularcitos syncline, and their continuity and rela-
tion to other faults of the Calaveras zone is uncertain because they could
not be traced through the Briones sandstone west of Halls Valley.
"^ Oral coinmunlcation.
«9 Vickery, F. P., The structural dynamics of the Livermore region: Jour. Geology,
vol. 33, p. GIO, 1925.
™ Louderback, George D., Characteristics of active faults in the central Coast
Ranges of California, with applications to the safety of dams : Seismol. See. America
Bull., vol. 27, pp. 1-27, 1937.
.
i.=iJ..LWi?pj?ci:
1951] STRUCTURE 53
Older Faults of the Calaveras Zone
Evidence for large pre-]Miocene vertical disp'aceiuents ou the Cala-
veras fault zone Avas recognized long ago by C. F. Tolman and F. P.
Viekery.'i Near Calaveras Dam. tlie lowest Pliocene beds (Monterey)
west of the fault rest on at least 2.000 feet of Cretaceous sandstone and
shale (see section A-A'. pi. 3 » of which the base is not exposed; east of
the fault, the Cretaceous is absent and the lowest Miocene beds (Temblor)
rest directly on the Franciscan. The contact of the lowest ^Miocene beds
with the Franciscan formation has been followed northward several
miles into the southeast corner of the Pleasanton quadrangle.^- where
it is finally overlapped by the Livermore gravels. However. 3 miles to
the northeast in the Tesla quadrangle, the Cretaceous is again present,
faulted into the Franciscan between the Williams and Valle faults. Thus
it appears that the Cretaceous was stripped from a narrow block of
Franciscan rocks between the Calaveras and Valle faults before the
Pliocene period. It is uncertain how far to the southeast the Cretaceous
was removed. Where 3iliocene rocks again appear east of the Calaveras
fault in the Morgan Hill quadrangle, they rest on both Eocene and
Cretaceous.'^ Thus if there were pre-3Iiocene post-Cretaceous movements
in this area they must have taken place east of tbe Madrone Springs fault.
In the Hollister quadrangle.'^ and the Quien Sabe quadrangle.*^ ^Miocene
volcanics overlap a similar fault, resting directly on the Franciscan to
the east, and on a thick section of Cretaceous to the west. The same zone
of faulting continues through the San Benito quadrangle.''^ Thus there
is evidence of strong i)ost-Cretaceous pre-^Iiocene faulting along the
west flank of the Diablo Eange for a distance of at least 75 miles. This
older fault coincides with the present Calaveras fault for about 20 miles.
The next movements on the Calaveras fault zone were undoubtedly
post-]\Iiocene. and probably post-Orinda. From Calaveras Reservoir to
San Felipe Valley, the Franciscan was thrust westward over the Tular-
citos s^mcline in a steep reverse fault. Throughout part of this distance
the fault can be mapped rather closely, and a steep east dip is indicated
by deflections as it crosses canyons such as Alum Rock and Arroyo
Aguague. Southeast of San Filipe Valley this fault leaves the Calaveras
zone, and when thus distinct, it is termed the ^tladrone Springs fault. It
has been traced for a distance of 10 miles in the Morgan Hill quadrangle,
and was observed in the Gilroy Hot Springs quadi-angle some 7 miles
farther southeast. Northwest of the Calaveras Reservoir, the post-Mio-
cene uplift east of the fault dies out, and the axes of the folded Tertiary
beds plunge northward beneath the Livermore basin.
Hayward Fault System
The Hayward fault is another of the major features of the San
Francisco Bay area. It is known with certainty to extend from north
Berkeley at least to the north edge of the San Jose quadrangle. Along
the foot of the Berkeley Hills it forms a shallow trench clearly shown
on the San Francisco folio.'' It emerges from the hills near the town of
" Op. cit., p. 611.
" Harding. J. W., op. cit.
"^ GUbert, Charles M.. op. cit
'* Taliaferro, X. K, &eolo&ic map of Hollister qnadransle : California Div. Mines
BulL 143.pl. 1. 1949.
^ Leith, Carlton J.. Geology of the Quien Sabe quadrangle, California : California
Div. Mines BulL 147, pL 1. 1&50.
^ Wilson, Ivan F.. op. cit.. pp. 256-257.
~ Lawson, A. C, op. cit., 1914.
54 SAN JOSE-MOUNT HAMILTON AREA [Bllll. 157
Hayward, and from there south lies along the foot of the hills just under
the edge of the alluvium. It is an active fault whose movement was
responsible for the destructive earthquakes of 1868 ''^ and 1836. "'♦ Accord-
ing to the testimony of residents of the area who were questioned in 1906,
there were fissures in the ground at many places from San Leandro to
Aguague Caliente Creek near Warm Springs. It was agreed by residents
of the area east of San Jose (Berryessa and Milpitas) that cracks did
not extend that far south.
The systematic otfsets of small streams along the Hayward fault
indicate unmistakeably that the Santa Clara Valley block has moved
northwest with respect to the mountain block, and Russell ^" has veri-
fied the existence of such offsets as far south as Niles. There is a well-
developed linear scarjo extending southeast from Irvington into the San
Jose quadrangle near Warm Springs, but at Scott Creek the foot of the
mountain swings eastward away from that line, and the linearity of the
mountain face becomes less apparent. South of Scott Creek, there is no
systematic offset of streams like that observed farther north. Thus both
direct evidence and fissuring and physiographic evidence of horizontal
offset suggests that the active fault dies out or goes beneath the alluvium
of the valley a short distance south of Warm Springs.
Quaternary (post-Pliocene) vertical displacements on or near the
line of the Haj^ward fault are mentioned by Lawson ^^ and Buwalda.^-
Both these writers agree the west face of the Berkeley Hills is a degraded
fault scarp, but Buwalda ^^ has pointed out that the amount of vertical
movement varies greatly along the strike, ranging from several hundreds
of feet near Berkeley to almost nothing near Piedmont. A similar vari-
ation is evident to the southeast. Along the foot of the hills between Hay-
ward and Niles, a low west-facing fault scarp of Recent origin is visible
at the foot of the older degraded scarp. However, an east-facing scarp
20 to 25 feet high crossing the alluvial cone of Alameda Creek between
Niles and Irvington has been described by Clark.^'* Southeast of Irving-
ton there is again a well-formed west-facing scarp as much as 200 feet
high consisting of unconsolidated Santa Clara formation. The recent
origin of this feature is indicated both by the age of the sediments (lowest
Pleistocene) and the relatively small amount of erosion which is evident
even in the unconsolidated material.
Older Faults of the Hayward Zone
Throughout most of the distance shown to be recently active, the
Hayward fault coincides with an older fault of great magnitude which
separates the Santa Clara Valley block from the Tularcitos block. The
evidence for this fault is largely stratigraphic and depends upon the
absence of the Cretaceous and Tertiary rocks of the Tularcitos block
from the Santa Clara Valley block. Buwalda '^^ believes that the sedi-
'8 Lawson, A. C, and others, The California earthquake of April 18, 1906 : Rept.
State Elarthquake Inv. Comm., Carnegie Inst. M^ash. Pub. 187, vol. 1, pt. 2, 1908.
"' Louderback, G. D., Central California earthquakes of the 1830's: Seisniol. Soc.
America Bull., vol. 37, no. 1, 1947.
80 Russell, R. J., Recent offsets along the Hayward fault: Jour. Geology, vol. 34,
pp. 507-511, 1926.
81 Law-son, A. C, op. cit., 1914.
82 Buwalda, J. P., Nature of the late movements on the Havward rift, central Cali-
fornia : Seismol. Soc. America Bull., vol. 19, pp. 187-199, 1929.
83 Op. cit, p. 194.
81 Claris, William O., Ground water in Santa Clara Valley, California : U. S. Geol.
Survey Water-Supply Paper 519, 1924.
85 Op. cit ,
I I'WHWBWWWBBmBWWVi ~ " iTFTTW
1951] STRUCTURE 55
meuts were deposited across or at least on the margin of the block, and
were recoved by erosion resultiug from local uplift. In the San Jose area,
this movement is certainly post-^Mioceue and is probably post ilio-Plio-
cene (Orinda).
Althoiigrh the Recent movements coincide generally with this older
fault line, they do not do so in detail. Lawson ^'^ has noted that in the Hay-
ward quadrangle the horizontal ''rift zone" lies at the foot of the hills,
but the older faults which separate the block underlain by Franciscan
rocks from the block underlain by Cretaceous rocks are farther back in
the hills near Lake Chabot. In the Sau Jose quadrangle, this older fault
is exposed for a short distance near Berryessa. Along the line of section
B-B', plate 2. serpentine crops out west of the fault in contact with Oak-
land conglomerate of Cretaceous age. The old northwest -trending fault
has been offset by several cross faults and is partially covered by the Santa
Clara formation. The southernmost exposures of serpentine are in a small
creek cut through the Santa Clara formation 0.8 of a mile north of the
line of section C-C (pi. 2). Apparently the serpentine is terminated by
another cross fault just south of this exposure. Although the Alum Rock
rhyolite has brought up some serpentine around its margins, it is clearly
intruded into the Cretaceous. The origin of this rectilinear pattern of
offsets is obscure. The cross faults do not offset the base of the Monterey
formation, and consequently must be pre-middle Miocene in age. Their
effect on the Oakland conglomerate-Berry essa formation contact is mostly
obscured by the mantle of Santa Clara formation and landslides, but
the distribution of these Cretaceous rocks suggests that they may be
affected somehow, though not by simple cross faulting. South of Alum
Rock Canyon, there is no consistent physiographic e^'idence of recent
movements nor is there stratigraphic evidence of an ancestral Hayward
fault anywhere along the mountain front east of Evergreen. In this area,
both movements have been effective along another zone of faulting follow-
ing Silver Creek.
Silver Creek Fault
Another major fault zone is exposed along Silver Creek south of
Evergreen. Its course is well marked by both physiographic and strati-
graphic features southeast into the Morgan Hill quandrangle where it
joins the Calaveras fault. This structure has been regarded as a con-
tinuation of the Hayward fault by most writers. However, it is 14 miles
from Scott Creek near "Warm Springs where the Ha^*ward fault is last
known. Its strike where exposed here is about X. 45 ^ "W. ; that of the
Hayward fault is about X. 30' W., and there is no consistent pattern of
stream offsets like those which characterize the Hayward fault. For
these reasons this fault zone is separately designated the Silver Creek
fault. In the Silver Creek quicksilver mining district, are two dominant
faults in the zone, enclosing a sliver-like area of Packwood gravel. The
block to the west is dominantly serpentine and Franciscan ; that to the
east is serpentine and Knoxville. To the south the two faults unite, and
the sliver of Packwood gravel is pinched out. It reappears again at the
south edge of the mapped area, the easternmost fault of the zone swings
sharply eastward, the westernmost fault continues southward, exposing
almost 4.000 feet of the gravel. The attitude of the western fault of this
«Op. cit, 1914, p. 17.
56
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
Fronciscan
Serpentine
Silica -carbonate rock
Knoxville
100 FEET
c —
Pockwood gravels
Adit projected into plane of section
Figure 12. Section through Silver Creek fault zone, along line E-E', plate 2
zone appears to be rather flat from its topographic expression, and two
small accessible adits, though they do not penetrate the fault, indicate
that it must dip 20° or less. Mr. Salvatore, who was foreman during the
mining operations, reports that a 3,500-foot adit was driven west at
about the level of the creek, and was in gravel for 800 feet. Also that
shafts sunk in the serpentine west of the creek penetrated gravel.
Although such a flat fault is most unusual in this area, all the informa-
tion is consistent with its existence. To the south, however, the dip
is much steeper; the west fault dips 65° W., where exposed in a prospect
pit just north of Metcalf road in the Morgan Hill quadrangle. Here also,
it is clear that the serpentine has been thrust eastward over the gravel.
The eastern fault of this zone also appears to be a reverse fault,
though the evidence for an east dip is much less convincing. In fact,
near the Silver Creek mine it is completely obscured by landslides of
serpentine. Only the general relations to the topography suggest a steep
east dip. Just north of the mine, the Packwood gravels are in contact
Avith the serpentine and dip into it. The northwestward extent of this
fault zone is unknown. A possible continuation is suggested by two
earthquake epicenters which are about on the line of the fault beneath
the alluvium of Santa Clara Valley.
other Longitudinal Faults
Other faults parallel to the regional trend of the Coast Ranges
appear near the front of the range south of Alum Rock Canyon. One of
these, which was called the Clayton fault b}^ Vickery,^^ extends from
the mouth of Alum Rock Canyon south about 2 miles to the Mount
Hamilton Road. Throughout this distance, the upper member of the
Montere}^ formation dips eastward into the Cretaceous Oakland con-
glomerate. After an offset to the east about half a mile, the fault con-
tinues southeast mostly within the Briones formation, except for one
small exposure of Monterey and Berryessa. The continuation of this
fault farther south across the foot of Quimby road, connecting the offset
of the Monterey formation north of the road with the small exposure
of Monterey at the foot of the landslide west of the Hendricks Ranch
is uncertain. The displacement on this fault, as mapped, is down on
the west throughout its length.
Displacement on a smaller fault west of the Clayton fault is down
on the east. At several places along these faults, bodies of silica-carbonate
rock appear. These are derived from thin lenticular masses of serpentine
which have been squeezed up in a semi-plastic state, and subsequently
altered bv hvdrothermal solutions.
87 Op. cit, pp. 610, 623.
wmmmm
T!35!5n?W!IC«!!5P!'"W'^
LUlkkUJU'
1951] STRUCTURE 57
Extending both northwest and southeast from Masters Hill, at
the summit of Quimby road, is a long narrow outcrop of the ]\Ionterey
formation which is bounded on the west by the Masters Hill fault for
almost its entire length. The structure along the fault is obscure, but
suggests that a hinge motion must have occurred. At the north end of
the exposure of Monterey, there is a downthrow on the west of about
2.000 feet. At the south end of the strip of Monterey, south of the cross
faults, a downthrow on the east is indicated by both the strike of the
Monterey and the amount of Berryessa formation cut out. Monterey
formation dipping into the Berryessa indicates that there is also a fault
at the east side of this band.
Faults East of Calaveras Dam
The outcrops of Miocene rocks east of the Calaveras Reservoir are
cut by many small faults. Their close spacing and complicated pattern
are characteristic of many Recent faults in the Franciscan formation for
which there is no statigraphic control.
Cross Faults
In several places the sedimentary contacts on the longitudinal
faults are sharply otfset by small faults which strike nearly at right
angles to the regional trend. Several are mapped on Mission Ridge east
of Warm Springs. Those which offset the serpentine may be older than
the cross faults which affect the Tertiary rocks. The Monterey contacts
and the ]\Iasters fault are offset by several such faults.
Folds
The block of Tertiary and Cretaceous sediments between the Hay-
ward and Calaveras faults is dominated by a single fold, the Tularcitos
syncline. This structure extends diagonally across the mapped area
and continues for some distance into the adjoining areas. It is best
exposed on Rancho Tularcitos near the north boundary of the San
Jose quadrangle, where it is overturned; its axis dips about 55° E.
The west limb is simple, but in the vicinity of Weller Road, the east limb
contains several smaller folds. In the vicinity of Alum Rock Canyon,
the axis of the syncline approaches the Calaveras fault zone, and the
east limb has been partly obliterated by the thrust. To the south, the
axis swings south parallel with the fault for about 3 miles, then turns
east and is offset by the Masters Hill fault, and faults of the Calaveras
zone.
The Franciscan formation east of the Calaveras fault is broadly
folded, but only the larger structures were determined because of the lack
of stratigraphic units. The most persistent and well-developed fold in the
area is an open broad syncline whose axis is about parallel with the
Calaveras fault for most of its length. East of this fold, the attitudes
are consistent over small areas, but no continuous folds were traced.
In general, the structural trends of the Franciscan appear to be north to
northwest, as indicated by dips and the few outcrops of chert and serpen-
tine. However, on the slopes of Mount Hamilton, Sulphur Creek, and
the ridge to the south, east, and northeast, strikes are common, and
it seems possible that they may represent a period of compression in a
direction almost normal to that responsible for the most conspicuous
folds. Evidence of this was also observed bj- Wilson ^® in the San Benito
quadrangle.
® Op. cit, pp. 257-258.
58 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Periods of deformation in the San Jose-Mount Hamilton area.
Age
Type of deformation
Area affected
Eocene? (post - Cretaceous,
pre-Miocene)
Block faulting
Western margin of Diablo
Range
Between middle and upper
Gentle folding _ _ _
Tularcitos block
Miocene
Near end of upper Miocene. _
Strong folding
Diablo and Santa Clara Valley
blocks of Franciscan up-
lifted
Between early Pliocene and
earliest Pleistocene
Extreme compression
Tularcitos block strongly de-
formed; folds overturned,
Diablo block thrust west-
ward over trough
Pleistocene
Reverse faulting
Older rocks thrust over Patck-
wood gravels
R«cent- .
Faulting
Uplift of Diablo Range; down-
warp of Santa Clara Valley;
horizontal movement on
Hayward and Calaveras
faults
Structural Dynamics
Structures in the San Jose-Mount Hamilton area range in age from
pre-Miocene to Recent. The Calaveras fault has formed in part along
an ancestral fault which shows pre-Miocene displacements of large mag-
nitude. Locally at least, the post-Miocene movements are in a reverse
direction. Basins of deposition extended across the course of the present
Calaveras rift during the Eocene,^^ and again in the Miocene. These
basins were later compressed into folds whose axes cross the course of
the rift faults and have now been offset by them. There is clear-cut
physiographic evidence that these features are younger than the folding
and the ancestral faults. The rift-type faults are characterized by an
alignment of valleys and saddles of recent consequent development, and
by transverse or tributary streams which are actively adapting to con-
tinued horizontal movements. The older faults cut indiscriminately
across valleys and hills.
The complicated structural history of an area such as this indicates
that the pattern of stresses responsible for its development has varied in
both time and space. There is evidence of periodic compression of the
central Coast Ranges, in a direction varying from N. 40° E. to N. 60° E.,
which began in the early Tertiary, and reached a culmination in the late
Tertiary or early Quaternary. Since that time, a new pattern of regional
stress has developed, in the form of a clockwise couple. The rift-type
faults which have developed in response to this pattern followed older
lines of weakness where available, and thus the Hayward and Calaveras
faults, like the San Andreas, coincide in part with the boundaries of
older structural units.
8B Gilbert, C. M., op. cit, 1943.
. k • , VVA^30rKN>CKU:':
•iy:!ntiKiliOIB
mi
t;i.s^»fyi/-j.ii'.*wS.
1951] GEOLOGIC HISTORY 59
GEOLOGIC HISTORY
Jiirassic. The earliest event recorded in the San Jose-]\Iount Hamil-
ton area is the deposition of the Franciscan formation in the late Upper
Jurassic. This area was apparently near the center of a broad slowly
sinking geos^-neline in which were deposited at least 10,000 feet of arkosic
sandstone and shale, and small amounts of conglomerate and limestone.
Local submarine volcanism produced basaltic and spilitic lavas, and con-
tributed silica, iron, and manganese to form radiolarian cherts which are
interbedded with the clastic sediments. Near the close of the Jurassic,
the rate of deposition decreased, and the fossiliferous shales of the Knox-
ville were deposited. Yolcanism was local and unimportant. Xi about
this time also, large sills and dikes of serpentine and smaller bodies of
gabbro, diabase, and diorite were intruded into the wet fresh sediments.
Diahlan Orogeny. The marked orogeny which affected many parts
of the Coast Ranges at the close of the Jurassic is reflected in the San
Jose-]\Iount Hamilton area by a sudden change in the character of the
sediments rather than hj actual folding.
Cretaceous. The lowest Cretaceous deposits are the massive coarse
Oakland conglomerate, which gives evidence of strong local uplift of
part of the area contributing sediment to the Coast Range geosyncline.
With the wearing down of this source, sediments again became finer, and
the sandstone and siiale of the Berryessa formation were deposited. The
existence of Upper Cretaceous deposits in this area is uncertain.
C retaceons-Miocene Interval. The absence of deposits of Eocene
age makes it impossible to outline in detail the events which took place
during this interval. There was strong uplift of the western margin of
the Diablo Range by faulting; and from evidence in adjoining areas,
one or more periods of folding are known. The western edge of the Diablo
Range may have been exposed during much of this time, because the
entire sequence of Knoxville and Cretaceous rocks w^as stripped off by
middle Miocene time.
Miocene. The middle Miocene seas appear to have transgressed
across the trough from the west, depositing the chert and foraminiferal
siltstones of the jMonterey formation. At their maximum, the seas
extended across the present Calaveras fault onto the edge of the Diablo
Range.
Between middle and upper Miocene, the area was uplifted and the
middle Miocene deposits were exposed and eroded from the Cretaceous
locally.
Upper Miocene seas transgressed farther across the area inundated
earlier, and deposited the Briones sandstone on both the Monterey and
the Berryessa formations. Abundant fossils and conglomeratic sand-
stones indicate a persistent shallow marine environment. Heavy minerals
suggest a possible admixture of Sierran as well as Coast Range sources.
Post-Briones Orogeny. Strong folding, uplift, and erosion of large
areas of Franciscan were initiated at this time. It seems probable that
both the Diablo Range and the San Francisco Bay block may have been
elevated.
60 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
MiO'PUocene. The inarooii, purple, or green conglomerate, sand,
and clay of the Orinda were deposited under flood-plain conditions. Its
rapid variation and diversity suggest that sources were local and erosion
rapid. There is a marked increase in the amount of Franciscan debris.
Post-Orinda Orogeny. After the deposition of the Orinda, the con-
vulsive orogeuic episode began, which destroyed the Pliocene basins, and
resulted in tight or overturned folds, and strong elevation of older
mountain blocks by outward thrusting over adjoining basins. Uncer-
tainty concerning the age of the Orinda, and absence of younger beds
makes it impossible to fix the age of this deformation more closely than
sometime within the Pliocene. Evidence from nearby areas indicates that
it may have had several phases extending over much of that period,
possibly into earliest Pleistocene.
Pleistocene. During the Pleistocene the newly uplifted areas were
rapidly eroded, and the thick heterogenous Packwood gravels were
dumped in orogenic troughs. Faulting continued, and the overthrusting
of these gravels by older rocks. Uplifting of the Diablo Range and
Berkeley Hills continued, and downwarping of the San Francisco Bay
block and the Santa Clara Valley basin. Mountain blocks were eroded to
low relief, and the older alluvium was deposited in Santa Clara Valley
basin. Horizontal displacement began on Hayward and Calaveras faults.
Eenewed uplift of Diablo Range occurred, and ^the Santa Clara for-
mation was deposited in the Santa Clara Valley basin.
Uplift, which initiated the present cycle of erosion, took place.
ECONOMIC GEOLOGY
Quicksilver
Silver Creek Mine. The Silver Creek quicksilver mine is in sec. 33,
T. 7 S., R. 2 E., M. D., 3.6 miles S. 36°E. of Evergreen. It is owned by
James A. Slatore who lives on the property, and it was operated by lessees
from December 1, 1942, to July 15, 1943.
The mine was first listed in the twelfth annual report of the State
Mineralogist for the years 1893-94, and by 1903-04 was reported to have
yielded quicksilver valued at $60,000. Several long adits were driven in
1906-08, but by 1918 operations had apparently been abandoned and no
production was reported by Bradley in 1918.^^ The mine was operated
from December 1, 1942 to July 1943, by Mr. Sam Cohen who produced
25 to 30 flasks of quicksilver. The ore-bearing zones were exposed by open-
pit methods, and the ore was concentrated before burning bj' using a
crusher, washer, and table. Considerable hand-picked ore was burned
direct l3^
The mine is in the Silver Creek fault zone which consists of a narrow
sliver of Packwood gravel, dropped do\\Ti and overthrust from both sides
by Franciscan, Knoxville, and serpentine. The eastern fault of the zone is
a steeply dipping thrust whose contact with the gravel is completely
obscured by landslides. The dip is inferred from the deflection of the
fault as it crosses Silver Creek road, and from adits driven east at the
creek level, which are reported to have been in gravel for 500 feet. The
»o Bradley, Walter W., Quicksilver resources of California: California Min. Bur.
Bull. 78, p. 168, 1918.
^w^nrnf^^^^^""^^'!''
1951]
ECONOMIC GEOLOGY
61
Figure 13. Geologic map of the Silver Creek mine area
62
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
2000 y'»<
OAK HILL QUARRY-*
CTis
LEGEND
Oal A'>'jvb;-.^...-" . . ■.'..,..x^...v.mm
KH
1951] ECONOMIC GEOLOGY 65
and diorite, mixed with serpentine. Much of this coarse material remains,
but its extent appears to be limited to the area surrounding the present
quarry. The rest of the hill consists of soft serpentine or Franciscan
rocks.
Sand and Gravel
Sand and gravel have been obtained from the bed of Coyote Creek
south of San Jose, and at a few other places. Such creek beds appear
to be the best source of this material within the area mapped. The Santa
Clara formation, though containing gravel, would require much more
washing to obtain a clean product.
Ground Water
The ground-water supply of the Santa Clara Valley ^^ is without
doubt its most important natural resource. The alluvium of the basin is
generally regarded as a complex series of alluvial fans which extended
out from the canyon mouths and coalesced in the center of the basin. The
changing pattern of their distributaries has been built into a three-
dimensional network of aquifers extending out from the main stream
canyons. This concept has recently been applied to the problem of
replenishing the underground aquifers, by construction of percolation
beds near the heads of such fans. Springs within the mountains have
an important effect in delaying the runolf of winter rains, though they
do not supply a large part of the water used throughout the valley. The
largest springs are the result of well-defined structural and stratigraphic
controls.
The upper ^Miocene Briones sandstone is the most important aquifer
within the mountain areas. The folded beds of this unit in ^Monument
Peak west of Calaveras Keservoir provide a large flow and make an
important contribution to the Calaveras Reservoir the year round. The
springs which are the largest in this area issue along the eastern slope
of the range, at the Monterey-Briones contact. A similar condition exists
west of Halls Valley, and on the west side of the range south of Alum
Rock, but the flow is not so large. The Orinda and Briones formations
in the Tularcitos syncline on the west slope of Monument Peak also
accumulate large amounts of water which issue as perennial springs
near the base of the slope. A number of small springs are found in the
Oakland conglomerate east of Evergreen.
In the central part of Alum Rock Canyon the thermal and mineral
springs have developed almost entirely within the permeable middle
member of the Monterey formation. This unit is of relatively small
importance as an aquifer, however.
BIBLIOGRAPHY
Bradley, W. W., The quicksilver resources of California : California Min. Bur.
Bull. 78, p. 168, 1918.
Bramlette, M. X., The Monterey formation of California and the origin of the
siliceous rocks : U. S. Geol. Survey Prof. Paper 212. 1946.
Buwalda, J. P., Nature of the late movements on the Haywards rift, central
California : Seismol. Soc. America Bull., vol. 19. pp. 187-199, 1929.
Carey. E. P., and Miller, W. J.. The crystalline rocks of the Oak HUls near
San Jose : Jour. Geology, vol. 15, pp. 161-166, 19<;t7.
^ Clark, "William C, Ground water in Santa Clara Vallev, California : U. S. Geol.
Survey Water-Supply Paper 519, 1924.
4 — 30779
66 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Clark, B. L., Tectouics of the Coast Ranges of middle California ; Geol. Soc.
America Bull., vol. 41, pp. 747-828, 1930.
Clark, William O., Ground water in Santa Clara Valley, California : U. S. Geol.
Survey Water-Supply Paper 519, 1924.
Crandall, Roderic, The Cretaceous stratigraphy of the Santa Clara Valley : Am.
Jour. Sci., 4th ser.. vol. 24, pp. 33-54, 1907.
Davis, E. F., The Franci.scan sandstone : Univ. California Dept. Geol. Sci., Bull.,
vol. 11, no. 1, pp. 235-432, 1918.
Durham, .1. W., Relationship of California upper INIiocene-lower Pliocene verte-
brate and invertebrate faunas (abst) : Geol. Soc. America Bull., vol. 57, no. 12,
p. 1386, 1948. . .
Emmons, R. C, The universal stage : Geol. Soc. America Mem. 8, pp. 115-133,
1943.
X Erickson, M. R., Geology of the western portion of the Pleasanton quadrangle,
California, unpublished masters thesis. University of California, Berkeley, California,
1945.
George, William O., The relationship of the physical properties of the natural
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Gilbert, C. il.. Tertiary sediments northeast of Moi-gan Hill, California: Am.
Assoc. Petroleum Geologists Bull., vol. 27, no. 5, 1943.
Gilluly, James, Keratophyres of eastern Oregon and the spilite problem : Am.
Jour. Sci., 5th ser., vol. 29, pp. 225-252, 336-352, 1935.
Harding, J. W., Geology of the southern part of the Pleasanton quadrangle,
unpublished masters thesis. University of California Library, Berkeley, California,
1943.
Holway, R. S., Eclogites in California : Jour. Geology, vol. 12, pp. 344-358, 1904.
Huey, A. S., Geology of the Tesla quadrangle, California : California Div. IVIines
Bull. 140,*1948.
Jenkins, O. P., Geologic map of California, scale 1 :500,000, California Div.
Mines, 1938.
Kleinpell, R. M., Miocene stratigraphy of California, Tulsa, Oklahoma, Am.
Assoc. Petroleum Geologists, 1938.
Lawson, A. C, and Palache, C, The Berkeley Hills, a detail of Coast Range
geology: Univ. California Dept. Geol. Sci., Bull. 2, pp. 349-350, 1902.
Lawson, A. C, et al. The California earthquake of April 18, 1906, Report of
the State Earthquake Investigation Commission. Carnegie Inst. Washington, Pub.
187, vol. 1, part 2, 1908.
Lawson, A. C, U. S. Geol. Survey Geol. Atlas, San Francisco folio (no. 193)
1914.
Leith, C. J., Geology of the Quien Sabe quadrangle, California : California Div.
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Louderback, G. D., Characteristics of active faults in the central Coast Ranges
of California, with applications to the safety of dams : Seismol. Soc. America Bull.,
vol. 27, pp. 1-27, 1937.
Louderback, G. D., Central California earthquakes of the 1830's : Seismol. Soc.
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Pabst, Adolf, The garnets in the glaucophane schists of California : Am. Mineral-
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California : Am. Jour. Sci., 4th ser., vol. 48, pp. 443-449, 1919.
Rogers, A. F., f]uhedral magnesite ci-ystals from San Jose, California : Am.
Mineralogist, vol. 8, no. 8, pp. 138-140, 1923.
Rogers, A. F., The crystallography of hydromagnesite : Am. Jour. Sci., 5th ser.,
vol. 6, pp. 37-47, 1923.
Rogers, A. F., Kempite, a new manganese mineral from California : Am. Jour.
Sci., 5th ser., vol. 8, pp. 145-150, 1924.
Russell, R. J., Recent offsets along the Hayward fault : Jour. Geology, vol. 34,
pp. 507-511, 1920.
Savage, D. E., Blancan horse from near AValnut Creek, California : Geol. Soc.
America Bull., vol. 59, no. 12, 1948.
Savage, D. E., Late Cenozoic vertebrates of the San Fi-ancisco Bay region, in
press. University of California, 1950.
Sharsmith, Helen K., The flora of the Mount Hamilton Range of California :
Am. Midland Naturalist, vol. 34, no. 2, pp. 289-367, September 1945.
rssm
1951] BIBLIOGRAPHY 67
Smith, James Pcrrin, The paragenesis of the minerals in the glaucophane-bearing
rocks of California : Am. Phil. Soc. Proc, vol. 45, pp. 183-242, 1907.
Taliaferro, X. L., The relation of volcanism to diatomaeeous and associated
siliceous sediments : Univ. California Dept. Geol. Sci., Bull., vol. 23. pp. 1-56, 1933.
Taliaferro, N. L., Geologic history and structure of the central Coast Ranges of
California : California Div. Mines Bull. 118, pp. 119-163, 1941.
Taliaferro, N. L., The Franciscau-Knoxville problem : Am. Assoc. Petroleum
Geologists Bull., vol. 27, no. 2. pp. 109-219, 1943.
Taliaferro, X. L., and Hudson, Franii S., Genesis of the manganese deposits of
the Coast Ranges of California : California Div. Mines Bull. 125, pp. 277-332, 1943.
Taliaferro, X. L., Geology of the Hollister quadrangle, California : California
Div. Mines Bull. 143, [map only].
Templeton, E. C., The geology and stratigraphy of the San Jose quadrangle,
unpublished doctors thesis, Leland Stanford Jr. University, 1913.
Templeton, E. C, General geology of the San Jose and Mount Hamilton quad-
rangles (abst) : Geol. Soc. America Bull., vol. 24, p. 96, 1913.
Tolmau, C. F., A geological report on the Coyote damsite, unpublished report to
the Santa Chira Valley Water Conservation district, 1934.
Trask. P. D., The Briones formation of middle California : Univ. California Dept.
Geol. Sci., Bull., vol. 13, no. 5, pp. 133-174, 1922.
Trask, P. D., Wilson, I. F., and Simons, F. S., Manganese deposits of California :
California Div. Mines Bull. 125, pp. 51-215, 1943.
Trask, Parker D., et al. Geologic description of the manganese deposits of Cali-
fornia : California Div. Mines Bull. 152, 1950.
Van Hise, C. R.. Metamorphism of rocks and rock flowage : Geol. Soc. America
Bull., vol. 9, pp. 269-328, p. 813, 1898.
Vickerv. F. P., The structural dvnamics of the Livermore region : Jour. Geologv,
vol. 33. pp. 608-628. 1925.
Whitney, J. D., Geological survey of California, vol. 1, pp. 47-52, Philadelphia,
Sherman and Co., 1865.
Willis, Robin, Physiography of the California Coast Ranges : Geol. Soc. America
Bull., vol. 36, no. 4. pp. 641-678, 1925.
AVillis, Bailey, and Wood, H. O., Fault map of the State of California compiled
from data assembled by the Seismological Society of America, 1922.
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Hills oilfield, California : U. S. Geol. Survey Prof. Paper 195, 1940.'
i.*oftj^ T~
!./>■ .;■.■. m^
,.. L I ffWffWWWfff^WW^
INDEX
A
Adenostoma, 10
Aguague Caliente Creek, 54
Alameda Creek, S, 9, 11, 12, 54
tunnel 17, 18
Alexander, Douglas C, 8
Alum ^"""^^'^^^^^^^ 9_ ^Q^ ^^ -^3^ -L4 19^ 20, 23, 25, 31, 32, 33, 37, 38, 39, 40, 43, 45, 47,
53, 55, 56, 57, 63, 64, 65
Park, 38
rhvolite. 25, 45, 46, 47, 48, 55, 64
R6ad. 41
Arctostaphylos, 10
Arroyo Aguague, 11, 12, 37, 41, 53
Bavo, 9
de Los Coches. 9, 34, 37, 38, 41, 43
Hondo, 9, 11, 26, 36, 45, 49
Artemisia, 9
Aucella crassicolis, 32
Keyserline, 34
piochii Gabb, 32, 34
B
Baccharis. 10
BacuUtes, 35
Bailev, Edgar, cited, 63
Belemnites. 30, 32, 34:
Berkeley. 8, 35, 53, 54
Hills, 49, 53. 54, 60
Berryessa, 54. 55
i^ 1- Q OA Q"t QO QQ
formation, 7.' 14. ff, 22, 27, 28, 31, 32, 33, 34, 35, 37, 38, 39, 43, 44, 45, 47, 55,
56, 57, 59, 63
Black Mountain, 9, 12
Blancan, 42
Bolivina aff. marginata Cushman, 39
tumida Cushman, 39
Bonita Creek. 11
Bradlev, Walter W., cited. 60. 62
Bramlette, M. X., cited. 36. 37, 38 ^ ^^ ^^ ^„ ^^ ^^ ^^
Briones sandstone. 7, 14, 15, 22, 28, 35, 36, 37, 38, 39, 40, 41, 52, 56, 59, 65,
, post-, orogeny, 59
Bruclarkia oregonensls (Conrad) , 36
Buchia crassa, 30. 32
piochii (Gabb), 30
Buwalda, J. P., cited, 54
C
Calaveras Creek, 23
Dam, 10. 20, 23. 35, 36, 53, 57
fault, 7, 9. 11. 12. 15. 17. 19. 20, 33, 35, 36, 37, 38, 42, 44, 48, 49, 51, 52, 53, 55,
57. 58. 59. 60, 64
Reservoir, 31, 33, 34, 35, 36, 44, 53, 57, 65
rift, 58
Vallev. 9. 11, 26, 49
Calera Creek, 37, 39
Carey, E. P., cited, 10, 21
Carquinez quadrangle, 49
Strait, 49
Cerastoderma sp., 36
Cercocarpus. 10
Chaboya (see Hillsdale) mine, 62
Chapman (see Hillsdale) mine, 62
Chico formation, 14, .31, 35
Chione sp., 36
Claremont shale. 39
Clark. B. L., cited, 11
, William O., cited, 54
5—30779 ( 69 >
70 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
Clayton fault, 56
Coast Range geosyncline, 59
orogeny, 35
Ranges, 7, 10, 11, 14, IS, 21, 25, 30, 38. 48, 56, 58, 59
Cohen, Sam, 60
Concord, 46
quadrangle, 31, 34, 35, 41, 49
Copernicus Peak, 7, 64
Corral Hollow, 14, 15
shales," 15
Coyote Creek, 9, 52, 65
reservoir, 9
Crandell, Roderic, cited, 10, 32, 34
Cretaceous Lower, 7, 14, 30, 31, 33, 35
Miocene interval, 59
period, 7, 15, 22, 25, 27, 30, 31, 34, 36, 37, 39, 41, 47, 49, 51, 52, 53, 54, 55, 56,
57,59
post-, 48, 53
Upper, 35, 59
D
Davis, E. F., cited, 17
Diablan orogeny, 59
Diablo Range, 7, 9, 10, 11, 15, 48, 53, 59, 60
Dry Creek, 10, 11, 27, 30, 44
Dublin, 42
Durham, J. W., 36, 39, 42
E
Eckert, N. A., 8
Edenvale, 9, 12, 19, 20, 25
Eel River, 17
Eocene epoch, 35, 53, 58, 59
Erickson, M. It., cited, 34, 42
Evergreen, 9, 10, 22, 26, 27, 31, 32, 37, 43, 44, 48, 55, 60, 65
F
Felter Road, 39
Franciscan conglomerate, 18
formation, 7, 10, 14, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 28, 32, 34, 35, 36,
37, 43, 45, 48, 49, 52, 53, 55
, pre-, 40
Franklin anticline, 35
fault, 49
G
Geological Survey of California, 10
George, William O., cited, 48
Gilbert, C. M., 17, 35, 53, 58
Gilluly, James, cited, 20
Gilroy, 52
Hot Springs quadrangle, 53
Golden Gate, 47
Grant Ranch, 8
H
Halls Valley, 9, 11, 12, 33, 37, 49, 52, 65
Harding, J. W., cited, 14, 34, 40, 53
Hayward, 54
fault, 7, 11, 48, 49, 53, 54, 55, 57, 58, 60
quadrangle, 31, 34, 41, 55
Hendricks Ranch, 33, 37, 43, 56
Hetch Hetchy tunnel, 14
Hillsdale (Oak Hill, San Juan Bautista, Chapman or Chaboya) mine, 62
Hillside mine, 25, 26
Hilton Gulch, 26
Hollister, 52
quadrangle, 53
Holmes Ranch, 8
, W. S., 8
Holway, R. S., cited, 26
Hoover, Paul C, 8
Hoplites, 34
Horse Valley, 12, 18
Huey, A. S., cited, 14
> n^mmmmim;
wmmmmm
1951]
INDEX 7 1
Inoceramus. 35
IrviiiRton, 42. 44. 54
Isabel Creek. 0. 11. 12, 18. 19
Valley, 10, 12
Jurassic-Cretaceous. 30
l)eriod. 15, 22. 59
, Upper, 7, 14, 30, 59
K
Kartchner. Wayne E.. 8
Kirschner. Mrs. A. F.. 8
Kleinpell. R. M.. cited. 36, 39
Kiioxville formation. 7, 22, 26, 27, 81, 32, 55, 59, 60, 62
Kuhn Ranch, 32, 44
L
Lake Chabot, 55
Lawson. A. C, cited. 31. 34, 37, 39, 42, 48, 53, 54
Leith. Carlton J., cited, 53
Leona rhyolite. 48
Lewis. Manuel, 63
Lick. 20
Livermore basin, 53
sravel, 13. 14, 15, 53
Valley, 13, 14
Lons Branch. 20
Los Biiellis Hills. 9. 12, 18, 19, 23, 33, 37, 39, 52, 64
Los Huecos Rancho. 8
Louderback. Ceorse D.. cited. 52, 64
Lticina aciitilinenta Conrad, 39
Luisian, upper, 39
M
Madrone Sprina; fault, 53
Masters Hill. 9. 33, 57
fault. 57
Mateos Ranch. 04
Mediterranean type climate, 9
Metcalf road, 56
Miller. Jacob. 63
Ranch. 64
deposit. 20
W. J., cited. 10, 21
Milpitas. 10, 34, 54
Mimulus. 9
Miocene epoch. 22. 36, 51, 53, 55, 57, 58
lower. 63
middle. 7, 35. 39, 59
-Pliocene. 22. 35, 42, 55, 60
, pre-, 53. 58
, post-. 53. 58
upper. 7, 15, 32, 40, 59, 65
Mission Pass, 34
Ridse. 49. 57
San Jose. 42
Monterey-Briones. 38, 65
-Cretaceous contact, 37
formation, 7, 22, 30. 33. 35, 36, 37, 38, 39, 40, 41, 53, 55, 56, 57, 59, 64, 65
Rroup. 37
Monument Peak. 9, 12, 39. 41, 65
Morgan Hill, 9
quadrangle, 30, 35, 42, 43, 49, 52, 53, 55, 56
Mountain House. 41
Mount Dav. 9. 10. 12
Hamilton, 7. 9, 10. 18, 19, 20, 25, 57
Range. 10, 11
Road. 31, 38, 41, 56
Isabel, 9
Lewis, 9, 10. 12
Muller, S. W., cited, 30
Mytilus sp., 36
72 SAN JOSE-MOUNT HAMILTON AREA [Bull. 157
N
Neroly, 42
New North Almaden mine, 63
Niles, 31, 49, 54
Canyon, 9, 34
N onion costiferum (Cushman), 39
Nonionella miocenica (?) Cushman, 39
O
Oak Hill, 25
(see Hillsdale) mine. 62
Hills, 7, 9, 20, 21, 26, 62, 64
" Ridffe sandstone," 15
, 12, 13, 14, 15, 16, 17, 18, 23, 45
Oakland, 31, 33, 48
conglomerate, 7, 22, 27, 30, 31, 32, 33, 43, 44, 45, 47, 55, 56, 59, 64, 65
Ogier, Lee, 8
Ranch, 8, 11, 18
Orinda formation, 7, 22, 29, 35, 41, 42, 55, 60, 65
, post-, orogeny, 48, 53, 60
P
Pabst, Adolf, cited, 26
Packard Ridge, 12
Pack wood Creek, 42
gravel, 7, 22, 42, 43, 44, 55, 56, 60
Paicines formation, 43
Panochita Hill, 33, 37
Paradise Ranch, 8
Paso Robles formation, 43, 44
Pecten coniplexicosta Gabb, 34
propatulus Conrad, 36, 40
Penitencia Creek, 9. 13, 14, 40, 47, 63
Perisphinctes sp., 30
Phylloceras knoxvillense Staton, 32
Piedmont, 54
Pleasanton quadrangle. 13. 14, 31, 34, 40, 49, 53
Pleistocene epoch, 7, 42, 00
, lower, 44, 54
"Pliocene A," 14
B," 14
epoch, 7, 14, 42, 34, 54
Plio-Pleistocene, 43, 44
Pliocene, post-, 54
Port Costa, 49
Poverty Ridge, 11
Q
Quaternary period, 22, 25. 43, 48, 58
Quien Sabe quadrangle, 53
Quimbly road, 56, 57
R
Rancho Los Huecos, 8
Tularcitos, .57
Recent epoch, 7, 13, 14, 22, 54, 55, 57, 58
Rhamnus, 10
Richards, R. W., cited, 37
Richmond Ranch, 8, 27, 30, 32
Rogers, A. F., cited, 11, 23, 63
, John, 8
Russell, R. J., cited, 54
S
Salinas VaUey, 44
Salvatore, Mr., 56
San Andreas fault, 52, 58
Antonio Creek, 13
Benito gravels, 44
quadrangle, 53, 57
Felipe Creek, 12, 45
Hills, 10, 11, 12, 18
Road, 44
Valley, 9, 10, 11, 12, 18, 27, 31, 33, 37, 45, 49, 53
■HVP
1951] INDEX 73
San Andreas fault (Continued)
Francisco, 7, 9, 15, 40
Bay, 7, 9, 15
area, 53
block, 59
Water Department, 8, 17
Jose.0. 10, 13, 15, 17, 21,25,31,42,44, 54,55,62,63,64,65
-Mount Hamilton area. 7, 8, 9. 10, 11, 14, 15, 19, 20, 21, 24, 25, 26, 30, 31, 41,
42, 48, 50, 58, 64
quadrangle, 15
quadrangle, 10, 18, 42, 53, 54, 55, 57
State College, 8
Water Works, 45, 47
Juan quicksilver mine, 23
Bautista (see Hillsdale) mine, 62
Leandro, 54
Pablo group, 40 „„ „_
Santa Clara formation, 7, 13, 22, 31, 33, 37, 42, 43, 44, 45, 48, 54, 55, 60, 63, 65
valley, 7, 9, 10, 12, 13, 15, 31, 35, 43, 45, 47, 49, 52, 56, 60, 65
block, 49, 54
Santa Teresa Hills, 19, 20
Savage, D. E., cited, 42
Scott Creek, 33. 54, 55
Seeboy Ridge. 12
Sharsmith, Helen K., cited, 10
Shasta group, 31, 33, 35
Sheldon, Mr., 8
Sierra Road, 9, 12, 20, 37, 41, 64
Silver Creek, 7, 12, 27, 42, 44, 48, 55
fault, 20, 26, 42, 55, 56, 60
Hills, 63
mine, 56, 60, 61
road, 60
Simons, F. L.. cited. 20
Slatore, James A.. 60, 62
Smith Creek, 11, 12
Ranger Station, 11
, James Perrin. cited. 23, 26
Spisiila selbyensis Packard ( ?) , 40
Stanford Geological Survey, 10
University, 36, 40
Stewart, R.. cited, 37
Sulphur Creek, 57
Sunol, 10
-Calaveras fault, 42, 52
fault, 49
valley, 49
T
Taliaferro, N. L., cited. 8, 11, 15, 17, 19, 20, 31, 33, 35, 52, 53
Temblor formation, 7. 22, 35. 36, 53
-Franciscan serpentine, 36
Templeton, E. C, cited, 10, 15, 40
Tesla quadrangle, 15, 53
Tertiary-Quaternary system, 42
' period, 10, 14, 15, 22, 25, 33, 35, 39, 48, 52, 54, 57, 58
Tice shale, 39
Tiernan, John, 8
Tivela merriami Trask ( ?) , 40
Tolman. C. F., cited, 15, 53
Trask. P. D., cited. 20, 40, 94
Tularcitos block, 49, 54
Rancho, 57
syncline. 33, 34, 35, 37, 39, 41, 49, 52, 53, 57, 65
trough, 7
Turner. F. J., 41
Turritella sp., 30
U
University of California, 8, 40
U. S. Army Engineers, 7
Geological Survey, 7, 63
74
SAN JOSE-MOUNT HAMILTON AREA
[Bull. 157
Vallecitos Creek, 13
Valle fault, 53
Valpe Ridge, 12, 13, 15
Van Hise, C. R., cited, 23
Venus-varians Gabb, 34
Vickery. F. P., cited, 10. 52, 53
Virgulina californiensis Cushman, 39
Wallace Ranch, 64
Warm Springs, 9, 37, 39, 43, 54, 55, 57
Weller Road, 57
Whitney, J. D., cited, 10
Williams fault, 53
Willis, Robin, cited, 11
Wilson, I. F., cited 20, 44, 53
Woodring, W. P., cited, 37
Wool, E. O., 7
World War II, 63
Wright, Mr., 8
W
Tount, Walter, 8
printed in California state printing office
30779 8-50 2M
EXPLANATION
Contour Interval BO foet
Datunu main lu Iml ( 1010 J Ittwl (lOtn j