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REMATIKABLE LANDSLIP IN PORTNEUF COUNTY, QUEBEC
BY
GEORGE M. DAWSON
[From Bull. Geol. Soc. Am., Vol. 10, 1808]
b^LOSOVV, O' "^
Ace. No.
Class Mk,
Pup.
Date Rec'd
Agent
Invoice Date
Fund
Notify
Send to
Presented
APR 2 - 19?9
\{. xia^'^^'^^
Exchange
BINDING
Material
Binder
Invoice Date
Cost
McGILL U IVERSITY LIBRARY
ROUTINE SLIP
\
liY (ii-.(>i!<:i': M. n.wvsos
Cdtlllllt!^
Kxiiminiitiim iiwulo
(•h:u-!if.tcr of tlioi'.imitry ■■
Mo.lo au.l extont of tlu- u,ovnu...t
AMumut of maUTiivl involv.Ml
r.xi.liiimtion of ilu> .■i.t.i«lro|.li.'
I'lMtiMHion fruiii siiiiiliir dixisUTs
Si,uil,ivo.Mmn:n.M.sintl,..>anH.n.g,on
I,,,„,.rV AND CoSS.arKN.KS OK TUK LANUSUr
<>.. >^av :, .SOS. . i=^;'-i->;--;;- - .:;;::;:;r :;;;;::'S"'
Saint. AiuumU. lalVnva.", pansl. ..f^uuil lln.unt,
(484)
1X4
IS.-!
is.-.
in.-.
tHT
487
4S'.I
4S9
of lllO
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. IS.-.
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. 487
. 4S<I
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The
BULL. GEOL. SOC. AM.
VOL. 10, 1898, PL. 51
JSi^^B^"^.
Kiiii
IK l,--fm <1.IUS M KVMIIIS I'.M' "I l.\M'-lll' A li 1: V
l-'ii.i 111: J.- I'.IXM hi: \M.i i\ "ii'n>iii- l.wi'-iii', wiiii 1. x \ h^l ii- i M 1 1 i i is lii>iv\o
RIVIERE BLANCHE LANDSLIP
HP
t.AND.SI.n- IN I'<)KT\Ki:i.' COINTY, (iUICnKf!
485
I
Hlip took pliico oil tl„. ,.iVHt Hidi' of the river, at ii di.stivnce of n inilcH from the vil-
liiKf of Saint CiiHimir. Tin- rcHiiits were (lisustroiis to tlio fiirnicrH wliosc property
wiiH iitrectod, one life wan lost, two inliaiiitfil Iioiih.'h. a .hi-IiooI-Iioiim-, two \num,
uikI H.'vcral oiithuiMiii-,'^ were dt'^'troyed or engulfed, and cattle, liorHeH, and other
live Htoek perinlied.
KxAMINAIloN MADK
Oil .May I'i) I visited and t'xaiiiiiie«l tlie locality, takini? Home i)liot()j;rapliH of the
Hceiie, and a few days later, at my recpicst, Mr K. Chalnier.s, of the (ieoloKical Sur-
vey, acconipaiiieil hy Mr ,1. Keele, inaile a doner Htudy of the circnniHtanceH hh
well as an approximate survey of the (ilacis and procur»"tl additional phot()j;raplm.
The following brief tleHcription is Imsed partly on my own ohservations, in part
on those of Mr Chahiiers, and is intended merely to outline the chief facts of in-
terest, from a jrcoloj,'ical st!tnd|)()int. respectinj; a mode of denudation that appears
to have been not uncommon in the day-floored plain of some parts of the Saint
Lawrence valley.
ClIAUACnCK OE- rilK Coi'NTIlV
.\t the place in question, the i.'ivicre liiaiiche, a small stream, occui)ies a valley
rnnnini; fn lorth to south, about I, ODD feet wide, i)etween slopiii",' banks, IV) to
.'!.') feet liijth, and nearly uniform in this re.speet. The surface of the country in
the vicinity is for the most part under tillaije, and is practically level to the eye,
l)eiii>r a terrace-llat or plain composed of the marine Pleistocene deposit known as
Leda clay, the whole thickness of which is not here anywhere shown. The day
is occasionally covered by arenaceous deposits a few feet thick and referable to the
Saxicava sands.
To the north of and adjoininn the wide crater-like depression producei by tlie
landslip here particularly described there is, h)wever, an irrej,'ular depressed
area of nearly the same size, now under tilla<,'e, that evidently represents the site
of a nincli earlier slip of the same character. Still farther to the north, and at a
distance of .■)0 chains from the recent slij), the road, which runs parallel to the river
valley and near it, crosses a low rid<?o of boulder day. This material may be pre-
sumed to underlie the Leda day elsewhere, but the subjacent ro(;k is nowhere seen
in the vicinity.
MoDK AND lv\ri:Nr or run Movk.mknt
A small runnel of water appears to have entered the IJlancihe valley at the point
where the material of the landslip subsei|uently found issue, and 1 was informed
that previous to tlu^ main slip a small slide had been noted to occur at this spot.
•At half i)ast five in the in.-rninj,' the inhal)ilantH were alarmed by the movement
of the soil, which then suddenly began and continued for three or four hours. The
imnuMliate bank of the river valley appears in the first place to have jjiven way
aloiij;; a front of about 'JOO feet in width, and the j;ap thu lade rapi<lly extended
inland, forminj; an openins; throu!j;li whicli a ^rreat body of clay behind rushed
tumultuously out into the l?liinche valley. .\t a short distance from the liank of
the valley the width of the area atl'ected j;reatly enlartreil, the sides of the depres-
sion collajisinj; and falling; into the jjulf, until a crater-like hollow of bottle-sliaped
outline and openinji; on the valley by a narrow neck was produced.
1st;
I'lK" KKIilNfiS (»1' TMK N K\V YOJiK MKKTlNii
The iiiliiil)itiiiit.« oii the Hpol wcivho iinii-li alarmed that they naturally <liil rmt
(iliHiTV;^ tilt' actual proKrcHHof tlic lamlMlip with ^rt'at preciwion.buteyf-witnPHscHde-
HcrilM' till' puHfiiKc nf lildck.M and |iyraiiiids of day tlinxiKli tlii! oridce to tlii' river
valley ax lioiiiir very Mwil't and rcscniMinK' Mtcanu'rH in motion on a river. Tlicoe-
cnrrence, in fact, may lie naiil to have resomlilcd the hurHtinj; out into the valley
of a lake of liipiid nnid, hearinu with it oiitHtandin^t an<l unhroken lilockn of clay
di'tacli(>d from the .ijdesof the c()lla|wini; art'a.
Kiia in: \.SIyi trli-jilnn slinimi./ A,. a ,,i iMndslip (hmiznntal liiu,i;i), I'ml nf I'hiii-tilh.l Hii;-r;-<illiii
(rirtifiil Itiiiiiij). and .i/./i.uriwio'f Ihiumlitrii nf an otn-ient Liiii'lfliii af tin saaif h'iml
On entering the HIanche valley the flood of clay spread upstream for some 500
or 600 feet, ponding hack the river water, lint the greater part, descending the
valley for nearly two miles, tilled it for that distance tii a maxinuim depth of fully
2o feet, causing the destruction of the rich meadows along the valley, besides that
of the agricultural lands immediately ali'ected liy the collapse.
When examined by me, the actual landslip was represented by a depressed area
bordered by clay clitrsfroni 1.'^ tooO feet high, 1,700 feet in maxinmm width, with a
r.ANrtSI.II- IN |'<.|!TM.:( !• (..INTV, (^1 KIJKC
KivutCHt l...mtl, of ;!.()()() f,M.( nn,l :,„ aivii .,f S.i a.T..>
487
X. Tilt" 11, I- of tlii.Mih.pn-HHii.n
w„ .nn...l ,,y nvu, u.. ,, ,. „,,, , ., ,,.,..,, ..^Hay. .{uZ;:. ^Z^
... ..CM, «M,l otlu.r . ..I.ns a,..l sn.all ..f water l.e.va...l tl.... ■ ; alll. , «1. i, i„
tI;:; ' : H-;^ ""'" ""•";. ^^^ ^'■'■" •""'•"^ ""•-"'"• ■•— .;>..• „;,„;;„:
u, kH .,( t ...... ..,„,.,.,,- In.,., a wo„.l.loi, ,.art of whi..). ..ill ,,.„.ains near the
1 .-u.l n •-.••....•. sl,ow...l vc.iy .•l..H,.ly th,. .lhv,-ti..„ .,f ,|.,w of tl.., ,.,aHH. Tl...
••Iwmnel „l ... I'.lam-l.o h.-low tl... o.ill.... of ,1... sli,. wa.- ..n.i,.dv .ill..,!, an.l .
y out into the Hlanche
Amount <.k Maikki ai. involvbi)
Tlie quantity of iiia.oiial whicii .hiiM poured sikI.IcmIv
vail. ■
cnr.
Tl
at wl.i..h tl... ma.l (\,nn..,ly pans.-.l, „..a,- ,|... ,.„,nu,..e to the ..anow ontle.,'wa7
Hcconlmy to har<.i,....,i,. ol.He.va.ioi.s hy M.' ChalM,,.,.., about 10 fo..t only. Tl.J
«I.prov„..ato .1,11..,,.,..... hetween the av..,.aK.. level of the botto... f,.,,.. the l...a.i to
he ,..-e.sen wuer level .n .1... Hhm..h,. vall..y, a. .,11.,.. .o .1... sa,„.. au.ho.i.v. is
bet.veen 2 an.l 2yeet, wh,le .he slop., of that pa... of ,la. H^^
oi.lice of f I... slip .,, .1,,. oxt.en.i.y .,f tl..- (l.,o,l of day is n..t ....i..!, '
I > .s approx,n.at..|y ..H.,n.a...,l at ..:!..m.,.„k, ..„„ie .V...t, with a total wei^rht, m-
; IMK to .1... spe....... ,May.ty .l.....,.,„i,,.d, .,f alaau ^572. U;! t..,,s .,f 2
ll.eslopeof.l.eo,-,K,nalHurfa..elV the l...a.l of thec^ollupse,! a....a to the point
feet.
more than 30
K.\,.|,ANATION OF TIIIC CA.|-Asri{OI'IIK
The l,>r|.t slopes in.li..ale.l by tl... ahov.. Iitr,i,-..s show f;,at H.. :>-,.,» of ehv n.u«t
.ave.m.nl«.e.laH.,„i,l bo.ly when h. n.o.io... M..Chal,;..rH sut^Kesis that a l.,wer
IK.,1 .)r th(. ,.|ay, ,n ..ons,.M„en..e of .ho iinpenn..abiiitv of .he s,il,ia..ent honl.ler
elay,beea,neex....ptioMallysatui.a.e.l, fo,.minKasli,li„i, piano np,j,.whi..l.. he n.oie
.•oh...vnt overlyin- n.ass,.s n.ove.l .low... This wo,.i,l l„. i„ ..o.irn.-.Mii v with ll...
e,v|,lanat.on ns.ially (and ,„.obably in n.os. .'ases ..o.TW.tlv) -iv..,. fo.- lan.'islips ....l
,. se..n.s v..,y hkely .ha. son.e.hinj; ..f .he kind niav have been ..on..e.-n..d i-.' .he
initiati..n .d the slip here described where it hewan on the bank of .he l!lan..he
valley. J. appea.'s .o n.e, however, .l,a. .he ^rea. and sudd.... disdiai-jre .,f Hav in
this ease should .-ather be attrilMi...d to th.. .•ha,.a.t..r ..f .he Nva.....-sa.n.-a.,.,l .nasn
■M a \vh.>le, particularly as no eviden.v was found of anv H|.e..iaiiv permeable or
finentbe<lan.l no .nid..rlyinK si.rfa.... either of boujd,..- ,.|av or ro..k i.s anvwi.ei..
exp...-ed. I. will be n.ted that this landslip dillers verv nia.k...llv in el.'a.a..ter
fr..n. the ordinary form, i.i wl.i.ii .he subsi,l,.iK.,. ufvms alonj,' an ..xtended front.
Thi-ee representative spe<.i,..ensof.hee]ay,..<,lle,.ted by Mr ( 'hah. ....-swhilestill in
a nearly satni'ali.d..onditioii w..|vsMbiiiitt.d to a .■a.vfiil ..xaiiiii.ation in the labora-
.oi'v ..f the .^urv..y un.ler l»..,.l.,r lloirn.a.nrs .-iipervision. A i.i,.an of the resultH
..b.aii....! shows. he speeili,. jr.a-.ityof .be elay as ie(...iv..d to have been l.<)12
e.iuivalent to a wei^rbt of 1 l!i..-) poimds ... .h....|ibi.. foot. Tl... day as reei.ived was
f'.und capable ..f absorbin;; a small a.ldi.ional ainonnt of water, varvinu fr<.ii. 7.0
t.. 0.2 per c.iit by wei;:!... Apart fi'o.., the water, i. ..oiisistt.d of ;;,•)!.-. per cent of
arKillaceouH inatt..r an.l 4;i.:i per ce,i. of silt. When (iilly satiuat...l it ....n.aint.d
on .he avd'ajre, which varied li..lc in the th.ve san.ples, 2;!,5 percent of waterby
wei<,'ht or nearly 50 percent by vol.nne.
■188
l>K()('Ki;i)IN<iS OK TllK M:\V V015K MKKTING
It will lieiioti'il that tlio I-oila chiy hem coiitiiiiis a conHidonvhk' proportion of
HJlt as compaieil with tlie aru'lllacfoiis inattiT proper, hwirinj,' out an ohsfrvatioii
made to thof*anie eli'ect on innpeftioii of the locality. The lariro amount of silty
matter preHont would render the day nnusualiy pernieahle, and it seems, there-
.ore, to he prubalde that the water saturated the mass by descendlnj; directly
through it from the surface, in a manner which would not have been ])()ssilile in
the case of the more purely arj,'illaceous clays of the same au'e usually found.
In Rankine's Civil Engineering it is stated that "the [ire.sence of moisture in
earth to an extent just suliicient to expel air from its crevices seems to increase
its coetlicient of friction slitrhtly ; but any additional moisture acts like an unguent
in diminishing friction and tends to reduce the earth to a semiHuid condition, or
to the state of uuid."' It appears probable that in this i)articular instance the
silty clay, surcharged with water, stood in a condition of unstable e(iuilibrium, re-
taining its soH<lity merely by virtue of its unbroken molecular texture, and that
at the moment in which it became subject to internal movement this texture gave
way and it hqised into a nearly liquivl mass, the particles rearranging themselves
with some freedom in the water previously locked up in its pores.
The fact that many clays when once completely dried and then innuersed in
water lose their plastic character and crumble down into an incoherent nmd, shows
that the natural texture is an important element in their coherence and plasticity,
and one which does not appear to have been fully recognized in connection with
experiments on clays and soils.
The high siH'cilic gravity of the lluent portion of the mass in this case, no doubt
enabled it to carry the unbroken blocks of clay along that were snpi)lied by the
collapsing sides of the crater-like depression which was immediately formed, and
when not subjected to stress these blocks coiitimied to retain their original lirm-
ness and form.
The fact that the great mass of moving material was disciiarged througii a com-
paratively narrow orilice, shows that the bank of the valley through which it
passed was nuich lirmer in character tlian the clay forming the subsoil of tin*
plain beiiiiul. This no doubt arose from the natural drainage of the clay along the
bank preventing its complete saturation. The same -xplanatioii no doubt accounts
for the northern limit of (he collapsed area occurring along the liiK* where the sur-
face begins to slope down toward the hollow of the old landslip already men-
tioned, but the limiting causes on the east ami .-ionth are not clearly ajiparent.
IiKluiries made on the spot showed that no excessive rains had occurred imm.-
diately pn'ceding the slip, but that a great depth of snow lay upon the ground
dm-ing the latter part of the preceding winter. These statements are conlirmed
by the meteiudlogical observations made at (Jnebec, about U) miles distant, which
have been obligingly I'urnished by Mv K, V. Stupart, director of the siieteorological
service. From tlie-^e it ajipears that the total preci[iitation (in rain or melted
snow) for the mouths of Novcmlicrand DecemlK'r, IS'.t?, and in .January, >hin>h,
and .\i)ril, 1S'.)8, was slightly below the normal for the [last 1' I years, but that in
February, 1S98. it was two inches ab<ive the average, in the form of an al)normid
exce.ss of snowfall in thai month of I 7. 1» inches, the total snowfall for rebrmiry,
being 44.2 inches. The ground was thus heavily burdened with snow in the later
winter. During .\pril moat of this melteil and the ■loil itself thawed, permitting
BULL. GEOL. SOC. AM.
VOL. 10, 1898, PL. 52
'■'"■I "> I. - N"iiiii i;i,..r Ml l,\vi,.| I,, \|,Ki. Mi:«
1.11 !■', \M « \i:ii Fi:o>i
\K,Mi I Ml. I Hi i.kt
I' i''i i:i -■ ' '1 I.s\i.-i ir, lis.. i.i« Mill r.i vs. 111. \ VI 1 t;\
RIVIERf BLANCHE LANDSLIP
LANDSLIP IN I'OUTNKUF COUNTY, QUKBEC
489
ti.
V
the absorption of tlic watin- and ivHiiltin<r, early in Mmv, when tlic day IhmIs \m\
thus l)(>conic thoroufjlily .saturated, in llie landslip which has heen described.
I'kotkction iKOM himii.au Disamtkrs
The only way in whicrli the reenrrence of sueli slips in regions of country of the
same character and under similar exceptional conditions of precipitation can be
(.'iiarded against ai)pcars to he the i>rovisioM of effective surface drainage, such as
to carry off the excess of water before 11k> ratlier slow iirocess of absorjjtion by the
siubjacent clays can take place.
ISl.MII,AU OrCl'Klil'.NCKS IN THIO S\MK KkgIOX
In a i)aper entitled " L'Ebouiis do Saint Alhan," '• Monseigneur [.atlanune has
given an e.xcellent account of a landslip tliat occurred on A])ril 27, bS94, on the
Sainte Anne river, (hstant about 7 miles only from that above described ami af-
fecting similar deposits of the saint, plain, altiiough at Saint Alban a laru'e part of
the sli<le consisted of the Saxicava san<ls, there developed in great thickness above
the Leda clav.
The landslip at Saint Alban was also umch larger than that on the Blanche, an
area more than ;{ miles in length along the river and about 7,700 feet in greatest
width having moved bodily down into the valley. Fivi> or six larm-houses were
destroyed or swallowed up, four lives were lost, and the entire mass of the slide is
estimated at from ()00,0l)0,()()0 to 700, 000, 000 cubic feet.
The landslip at Saint Alhan was also different in its cause and character. The
river was first dammed by a comparatively small slide, and when tlie water thus
held l>ack eventually broke through, its undermining action on the high banks
of the valley was such as to precipitate the collapse of the nnich greater area above
noted, t
A brief description of a landslip almost identical in ciiaracter with that of the
Blanche and allectiug a similarly situated part of the same Saint Lawrence plain
has, however, i)reviously been given by Sir William Logan in a pai)er read before
the (ieological Society of London in IS42. :]:
This landslip occurred on the Maskinongc river, about oO miles to the southwest
of the Ulviere Blanche, on April 4, 1S40, and waa examined by Logan in the follow-
ing autumn. Like that on the Hlauche, its outlet through the l)ank of the valley
was narrow, ami its greatest width, about (iOO yards, occurred at some distiince
hack from this bank. The length of the collapsed area was l,;!00 yards, and its
area about .S4 acres. tht> depth of the depression being about ;!0 feet. The nearly
licpiid clay flowed both up and down the valley of the Maskinonge for a distance
of about three-cpiartersof a mile in each direction, hearing with it large blocks and
masses of unbroken clay. The whole movement was effected in about 15 hours,
tlie first nui.ss of clay detacheil being al)out 2t)0 yards in width by 700 in length.
♦Tniiisiii'tions Royal Soniiity of Ciiniiilii, vol. xii, piirt iv, 18'Jt, p. ti:i.
fsiiir-c till' present piipcr wii.s rcml u .short iiutis liy tlic siinie tuKlioi' on tlie HIiuielie liinilslip liiis
lii'eii piililislieil in the IJeiicii't of the Coinniissloner of Coloniziilion iind Mines of (/iieliei. fm- l.siis,
p. l:U.
{ I'roreeili.ii^s of llie (ieoliv^tiiMl •ioi'jety of l.oii'loii, vol. iii, p. 7Ci7 ; iilso liife of Sir \Villi;im
I.OKiin, p. !!.").
„„.„;.,. l,y 0„. .li„ «a» J,.»....K.,1 ,y I-'': - ' ;» f „,: , :," , ,;,„., ,,,,.,,,-,.,1
"■• "'":;:;;:;:t;,;t;. \ ::;;;:::. '^'.'.'""--^
of.ior inat.'nal b>>lovs tlu lay, nm however, mv ii,t(Mpi-etati..ii of
.,,,y have .Krurml on a slopni^- he. ..f .ock. ' ' ^J^J; '^ ^ • „^. .xintenee
U.;nu.t. on the Blanche he eo,-reet . --^ ^ :;;^ ,^:, : ^^ the satuvated
';Zn^Z^:^X^ '-in. heen h.oUen th.oa.h in the first n.t.nce.
ternice-Ha'.
eft hctwceii
LCt ollStTVCll
ic expliiiii'''!
yin^ nick or
' nioMMiiont
■pretation of •
he existence
hesatuniU'd
to till" lower
;tance.