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NOTES ON 
 GEOLOGICAL MAP-READING 
 
 BY 
 
 ALFRED MARKER. 
 
 CAMBRIDGE 
 
 W. HEFFER & SONS LTD., 
 1920. 
 
NOTES ON GEOLOGICAL MAP-READING. 
 
NOTES ON 
 GEOLOGICAL MAP-READING 
 
 BY 
 
 ALFRED HARKER. 
 
 CAMBRIDGE : 
 
 W. HEFFER & SONS LTD 
 1920 
 
PREFACE. 
 
 Map-reading, to which the student of Geology should be intro- 
 duced at an early stage of his training, lends itself, by the use of 
 contoured geological maps, to methods of precision. These are 
 here reduced to the simplest terms by the device of reckoning 
 slopes and dips as gradients. Trigonometrical calculation is thus 
 avoided, the protractor becomes unnecessary, and all questions 
 are solved by the scale alone. 
 
 It will be understood that teaching is to be illustrated by 
 examples, and these may be easily constructed on the lines of the 
 following pages. 
 
 September, 1920. 
 
 f i *** /"* /* 
 
 51 / (> b 
 
I. TOPOGRAPHICAL MAPS 
 
 (1) A geological map is an ordinary topographical map with 
 additional indications (boundary-lines, colours, symbols) designed 
 to show the geological constitution and structure of the country. 
 The geological information so presented cannot be adequately 
 realised without constant and instinctive reference to the form of the 
 land-surface on which the various rocks are exposed. It is therefore 
 of the first importance to be able to read a topographical map 
 readily ; i.e. to interpret at a glance the general character and 
 details of the surface-relief as there rendered. We shall accordingly 
 begin with some remarks under this head. 
 
 (2) Since the earth has a curved surface, in the simplest view 
 spherical, while a map is flat, the former or any portion of it cannot 
 be truly depicted on the latter (apart from reduction of scale), 
 but can only be represented according to some particular convention. 
 Numerous different modes of effecting this (different kinds of 
 " projection ") have been devised by map-makers, but they need 
 not be described here. - Either there must be some distortion 
 (i.e. the linear scale is not the same for N.-S. distances as for E.-W.), 
 or if this be avoided, as in Mercator's projection, the areal scale must 
 be different in different parts of the map. The methods in common 
 use are of the nature of compromises, and for a small area the question 
 is not of great importance. Note, however, that on an ordinary 
 map the meridian lines, except the central one, are not parallel to 
 the sides of the rectangular sheet, but (for northern latitudes, 
 converge northward, while the parallels of latitude are curved. 
 The sheets of our Ordnance Survey are not to be regarded as 
 complete maps. The small-scale sheets are parts of a large map of 
 England (or Scotland or Ireland) drawn to the central meridian of 
 the country. The large-scale sheets are each part of a county map 
 drawn to the central meridian of the county. 
 
*': -GEOLOGICAL MAP-READING 
 
 (3) The scale of reduction of a map is expressed either as a 
 fraction or ratio, such as i : 50,000, or more usually in English 
 by the number of inches (or fraction of an inch) representing one 
 mile. The two scales most in use for the Ordnance Survey of this 
 country are : (i.) one inch to a mile, equivalent to i : 63,360, and 
 (ii.) six inches to a mile, equivalent to i : 10,560. The latter scale 
 is that adopted in most of the illustrations given below. Each 
 published sheet is provided with an engraved scale of feet in the 
 margin ; but the student will require a movable scale of ivory or 
 boxwood, which can be applied directly to the map. The usual 
 form is combined with a protractor for measuring and laying off 
 angles. Most convenient is a scale of six inches to the mile graduated 
 in feet. If this is not procurable, a good substitute is a scale, made 
 for use in the Army, at two inches to a mile and graduated in yards ; 
 if the yards be read as feet, this becomes a scale of six inches to the 
 mile. 
 
 (4) A careful draughtsman may construct scales for himself 
 as follows. On a sheet of cardboard, with a fine-pointed pencil, 
 rule twelve horizontal lines at equal intervals, starting from a 
 vertical line on the left. Divide the top line, beginning at the left, 
 into eighths of an inch, each division, therefore, representing no 
 feet. From each dividing point rule a line to the left end of the 
 bottom line. Each horizontal line is now divided into equal parts 
 by the radiating lines. The value of one division, being no feet 
 for the first line, is 100 feet for the second, 90 for the third, and so on. 
 The most useful scales are those for 100, 50, and 20, or a scale of 25 
 can be made by ruling anew line midway between the ninth and 
 tenth. Ink in the desired scales, and cut along the horizontal lines. 
 
 (5) Any land-surface, apart from the curvature of the globe, is 
 diversified by hill and valley, and so is in three dimensions. A map 
 has only two dimensions, and therefore the third (viz. altitude) 
 cannot be directly rendered, but only indicated (and that imperfectly) 
 by some convention. Hill-shading as a mode of rendering surface- 
 
GEOLOGICAL MAP-READING 
 
 relief lacks the element of precision, and we shall use tl\e alternative 
 method of contour-lines. Any horizontal plane intersecting the 
 surface of the ground gives a curve of intersection which is a contour- 
 line of that surface ; and a series of such curves, taken at equal 
 vertical intervals sufficiently close, affords a representation of the 
 shape of the surface. We shall take contour-lines at intervals of 
 
 FIG. I. 
 
 100 feet, beginning from sea-level.* Imagine the sea to flood the 
 land by a rise first of 100 ft., then of 200 ft., 300 ft., etc. (fig. i). 
 The successive shore-lines will be contour-lines on the ground, and 
 the projection of these on a horizontal plane, with reduction on the 
 proper scale, will give the contour-lines on the map. 
 
 (6) A contoured map, then, when rightly interpreted, gives a 
 picture of the shape of the ground represented. The slope at any 
 
 * On the maps of the Ordnance Survey the vertical interval is usually 
 100 ft. up to 1000 ft. altitude and 250 ft. above that. There is also a line for 
 50 ft. altitude. 
 
10 
 
 GEOLOGICAL MAP-READING 
 
 place is in the direction at right angles to the contour-lines, from 
 higher to lower. The relative closeness of the contour-lines indicates 
 the relative steepness of the slope. If as we pass uphill the lines 
 become closer, the slope is becoming steeper, and the curve is a 
 concave one ; if the lines become more open, the slope is becoming 
 
 FIG-. 2. 
 
 FIG. 3. 
 
 j[ 
 
 
 / 
 
 1 
 
 X 
 
 ^ 
 
 /'/: 
 
 X v^ 
 
 / / 1 
 
 ^ V \ 
 
 V / / 
 
 \ \ ^ 
 
 -''''/ 
 
 ^ \ ^^ 
 
 / 
 
 / ' 
 
 \ * 
 
 / ' 
 
 \ 
 
 / ' 
 
 * ^ 
 
 / ' * 
 
 
 '"'//( 
 
 J ' :\ 
 
 \ \ 
 
 \ v 
 
 
 * V v 
 
 t i 
 
 \ \ * 
 
 i ' 
 
 t \ 
 
 / 
 
 i \ 
 
 / ' 
 
 1 V 
 
 / / 
 
 1 \ 
 
 x i 
 
 \' V 
 
 1 I 
 
 \ \ 
 
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 I S 
 
 ' . 
 
 1 V 
 
 I 
 
 V 
 
 S i 
 
 V v 
 
 ''' f 1 
 
 V s ^ 
 
 / / 
 
 V X 
 
 gentler, and the curve is convex. The student should observe on 
 actual contoured maps the characteristic features of land-forms 
 the closed curves about an isolated hill, the indentation of the lines 
 in crossing a valley, the distinctive appearances of a lake-basin, a 
 pass, a cliff, an escarpment, etc. until he can read the map as easily 
 as a raised model. In the figures appended as illustrations the most 
 southerly contour-line is to be taken as the lowest, and the vertical 
 
GEOLOGICAL MAP-READING 
 
 ii 
 
 interval is supposed to be 100 ft. Fig. 2 represents a ridge somewhat 
 over 300 ft. high dying out southward into level ground ; fig. 3 a 
 country sloping southward, intersected by a gorge 250 ft. deep, with 
 a waterfall at its head ; fig. 4 a round hill rising over 800 ft. from a 
 country with a southerly slope (observe the pass on the north side). 
 
 FIG-. +. 
 
 I J 
 
 I ' 
 
 ' / 
 
 N \ ^ 
 
 N S 
 
 \ N^ 
 
 v x N> 
 
 X N 
 
 N 
 
 ^ 
 
 .<"',*' 
 
 (7) The inclination of a slope can be measured and expressed 
 in two ways : (i.) as an angle measured in degrees, viz. the angle 
 which the surface makes with the horizontal, and (ii.) as a gradient. 
 We shall employ by preference the latter method, but the two 
 measures are mutually convertible by means of the table given at 
 the end of the book. If, in fig. 5, ac measures one unit (of any 
 length) and be four units, the gradient along ab is i in 4. Here 4 
 
12 GEOLOGICAL MAP-READING 
 
 is simply the co-tangent of the angle of inclination, which is very 
 closely 14. A gradient i in 4 is the same as 10 in 40, or 100 in 400, 
 and we shall find this last measure most suitable for our purposes. 
 (8) To determine the slope of the ground at any place on a 
 contoured map (fig. 6), apply the scale so as to read off. the distance 
 ab between consecutive contour-lines, say 400 ft. Then the slope 
 has a gradient 100 in 400, equivalent to 14 inclination. Since the 
 slope is probably not constant, this is really the average slope 
 between the two contour-lines at this place. Taken thus, at right 
 angles to the contour-lines, it is the steepest slope there. To 
 determine the slope in any other direction such as ap, we have 
 only to measure ap, say 950 ft., and the slope in that direction is 
 
 F/Gv-6. 
 
 ioo in 950 (about 6 by the table). This holds good whether the 
 contour-lines be straight or curved. 
 
 (9) To draw a profile section along a given line on a contoured 
 map, i.e. the section made by a vertical plane along that line.* Lay 
 down the line on the map (fig. 7), apply the straight edge of a strip 
 of paper to it, and mark there where it crosses the several contour- 
 lines, as also the point where it crosses a stream. The altitude for 
 this point can be roughly fixed by observing where contour-lines 
 cross the stream above and below the point and assuming a steady 
 gradient for the stream-bed. Transfer these data to the ruled base- 
 
 * Styled very laxly by the Geological Survey a " horizontal section. " 
 
GEOLOGICAL MAP-READING 
 
 F/G-.7 
 
 \ 
 
 \ 
 
 W. E 
 
 line (representing sea-level) on the drawing-sheet ; erect perpen- 
 diculars of the proper heights at the marked points ; and draw a 
 
i 4 GEOLOGICAL MAP-READING 
 
 curve through the summits of the perpendiculars. Ink in the curve 
 and erase the pencil marks. Indicate the direction of the section 
 by noting the points of the compass at the opposite ends. It is a 
 convenience to have the drawing-sheet previously ruled with 
 horizontal lines marking altitudes at 100 ft. intervals, as in fig. 26 
 on next page. 
 
II. STRATA WITH CONSTANT STRIKE 
 
 AND DIP 
 
 (10) We can now go on to consider a geological map, assuming 
 it to have as its basis a topographical map with contour-lines. We 
 shall be concerned mainly with stratified rocks, and in particular with 
 those groups of strata which are separately distinguished on the 
 map by colouring or otherwise, to be called for clearness formations. 
 What the map primarily shows is the distribution of these formations 
 as they appear at their outcrop on the ground-surface. The most 
 important geological lines on the map are the boundaries separating 
 the several formations, and each such line represents the outcrop 
 of a particular bedding-surface in the rocks, viz. the summit of one 
 formation and the base of the succeeding one. This surface is 
 intersected by the ground-surface in a certain curved line in space, 
 and the projection of this curve on the horizontal plane is the 
 boundary as shown on the map. It is determined, therefore, by 
 two factors ; the lie of the bedding and the shape of the ground. 
 Given the shape of the ground (say by contour-lines) and the lie of 
 the bedding at every place, the course of the boundary lines could 
 be inferred.* In map-reading it is the converse problem that is 
 presented ; given the shape of the ground and the (projected) 
 boundary-lines, to make out the lie of the bedding, which may vary 
 from place to place. 
 
 (n) In the special case of horizontal bedding the distribution 
 of the several formations depends solely on the shape of the ground, 
 the lowest appearing in the valleys and the highest on the hill-tops. 
 Horizontal bedding is easily detected on the map, since in this case 
 all the boundary-lines are contour-lines, and must conform with the 
 regular system of contour-lines marked on the map. More generally 
 the bedding at any place has a dip in a certain direction and of a 
 
 15 
 
16 GEOLOGICAL MAP-READING 
 
 certain amount. The manner of denoting this on the map is by a 
 small arrow (its point at the spot in question) with a number 
 indicating the inclination in degrees. Horizontal bedding is denoted 
 by the sign + . The strike of inclined strata is often defined as the 
 horizontal direction at right angles to the direction of dip. Strike 
 is, however, a more fundamental conception than dip. We shall 
 accordingly define a strike-line as a horizontal line drawn on a 
 bedding-surface, or (what is the same thing) the line of intersection of 
 a bedding-surface by a horizontal plane. A strike-line on a bedding- 
 surface is therefore precisely analogous to a contour-line on the 
 ground-surface, and this is the best way of realising its significance. 
 The dip of the bedding is analogous to the slope of the ground, and 
 is to be expressed in the same manner, preferably as a gradient (7). 
 The " dip," without qualification, is understood to mean the dip 
 in a direction at right angles to the strike. This is the steepest dip 
 at a given place, and the dip in any other direction is related to it in 
 the manner shown for slopes in 8. 
 
 (12) In reading a geological map the first thing is to determine 
 the strike of the bedding, and only after that the direction and 
 amount of dip, which are much more likely to be variable. A glance 
 at a map with average surface-relief shows whether we have to do 
 with low or with high dips. With low dips the form of the ground 
 is the chief factor in determining the course of an outcrop, and the 
 boundary-lines are accordingly sinuous ; with high dips the lines 
 tend to run in a definite direction, which is that of strike. In any 
 case, if a line declines when followed in one direction (i.e. approaches 
 a lower contour-line), there must be some dip in that direction. If 
 it declines rapidly in one part of its course and only gradually in 
 another part, the former direction must be nearer that of true dip 
 and the latter nearer that of strike. If a boundary-line runs for 
 some distance closely parallel to a contour-line, there is no sensible 
 dip in that direction, or in other words it is approximately the 
 direction of strike. When a boundary-line cuts the same contour- 
 
GEOLOGICAL MAP-READING 17 
 
 line twice, we can lay down the strike with precision by joining 
 the two points of intersection, since a horizontal line on a given 
 bedding-plane is by definition a strike-line. Observe that we are 
 here assuming the bedding-surface to be an inclined plane : folded 
 bedding will be discussed later. 
 
 (13) It is necessary to consider more closely the course of outcrop 
 of a bedding-surface as depending on the strike and dip and on the 
 shape of the ground. We will suppose these constant, so that both 
 bedding-surface and ground-surface are inclined planes. The 
 
 F/G-.8. 
 
 ct 
 
 outcrop, which is the intersection of these two planes, is then a 
 straight line, and so is its projection on the horizontal plane. If 
 the dip is directly up or down hill, or (in other words) if strike and 
 contour-lines are parallel, the outcrop will be parallel to both. If, 
 however, the strike is not parallel to the contour-lines, the outcrop 
 will take a direction different front both. This will be clear from 
 fig. 8, remembering that it is the projection upon the horizontal of 
 a figure in three dimensions. Suppose the slope of the ground to be 
 
i8 GEOLOGICAL MAP-READING 
 
 100 in 1000, and the dip to be obliquely downhill at the steeper 
 gradient of 100 in 600. Let p be a point on the outcrop of a particular 
 bedding-plane at an altitude, say, of 400 ft., and pa the contour-line 
 of the hill at this level. Draw also, 1000 ft. in front of this, the 
 contour-line for the 300 ft. level. In like manner, let pb be the 
 strike-line through p, i.e. at the 400 ft. level, and draw, 600 ft. in 
 front of it, the strike-line at the 300 ft. level. The latter line will 
 meet the contour-line at the same level in a point q, which is clearly 
 a point on the outcrop, and pq is therefore the line of outcrop. In 
 fig. 9, with corresponding lettering, the intersection of the two 
 inclined planes is shown in perspective. Note that p, a, b are in 
 one horizontal plane (at 400 ft.), and q, c, d in another horizontal 
 plane (at 300 ft.). 
 
 (14) We now see how to measure the dip when the strike is known. 
 Suppose a boundary-line (representing a certain bedding-plane) to 
 cross two consecutive contour-lines at p and q (fig. 8). Draw strike- 
 lines through these two points, and measure the distance between 
 them (600 ft.). Then the bedding-plane declines 100 ft. in this 
 distance, and its dip is therefore at 100 in 600 (about 9^) in the 
 direction pd. If in a given part of the map there is no boundary-line 
 which crosses two consecutive contour-lines, we must be content 
 to measure the distance in which there is a decline of, say, 50 ft. or 
 25 ft. This involves interpolating altitudes between the contour- 
 lines, which gives a sufficiently approximate result upon a steady 
 slope. 
 
 (15) The student should construct diagrams like fig. 8 to observe 
 the effect of different slopes and dips, (i.) In the case illustrated 
 the line of strike lies between the contour-line and the outcrop, 
 (ii.) If, however, the slope is steeper than the dip (downhill), the 
 outcrop deviates in the opposite direction, and falls between the 
 contour-line and the strike-line, (iii.) This is likewise true if dip 
 and slope are opposed, (iv.) In all cases the deviation of outcrop 
 from strike is greater according as the dip is lower or the slope is 
 
GEOLOGICAL MAP-READING 
 
 FIG-. 
 
 VALLEY DECLINES lOOtNlOOO 
 A. DIP DOWNSTREAM 
 
 . 100 IN soo _---~~" /.^-^r^-^ c 
 
 1000 
 
 B. DIP DOWNSTREAM 
 100 IN ISOO 
 
 C. DIP UPSTREAM 
 100 IN -50O 
 
 1000 
 
 ZOOO 
 
 3000 
 
20 GEOLOGICAL MAP-READING 
 
 steeper. Note also the extreme cases : for vertical bedding the 
 outcrop is parallel to the strike, for horizontal bedding to the 
 contour-line. On level ground the outcrop is parallel to the strike, 
 whatever be the dip. 
 
 (16) Still supposing the bedding to maintain a constant strike 
 and dip, we can now follow the outcrop of a bedding-plane on 
 ground of varying slope. We see that in general it must curve 
 forward (towards the direction of dip) on falling ground and back- 
 ward on rising ground, the only exception being when the dip 
 (downhill) is less steep than the slope. The effect is clearly seen 
 where an outcrop crosses a pronounced valley, and so makes a 
 sharp deviation and recovery, the indentation being conveniently 
 spoken of as a V. This is illustrated for three different cases in 
 fig. 10, in which contour-lines are shown for vertical intervals of 
 100 ft., and the stream-line of the valley declines at a gradient of 
 100 in 1000. In the first case (A) the dip is downstream at 100 in 
 500. If a bedding-plane crosses a contour-line at the two points 
 a and b, then db is a strike-line. Drawing another strike-line 500 ft. 
 in front of this, we see that the points c and d, in which it intersects 
 the next lower contour-line, must also be points on the outcrop. 
 By proceeding in this way the curve or V is traced out. In B is 
 shown the exceptional case in which the dip, still downstream, is 
 less steep than the gradient of the stream-line, and the V points 
 upstream instead of down. Case C shows the dip upstream and 
 the V pointing upstream accordingly. Observe (comparing C with 
 A) that the V is blunter here than for an equal dip downstream. 
 
 (17) In an area too small to afford a general view of the relations, 
 the following method may be applied with caution for determining 
 strike and dip. A plane is fixed by any three points in it which 
 make a triangle (i.e. are not in one straight line). If, then, strike 
 and dip can be supposed constant, it is sufficient to know the 
 altitudes of three points on one boundary-line. In fig. n three 
 points, a, b, c, are given by the intersections of three contour-lines. 
 
GEOLOGICAL MAP-READING 21 
 
 Since a is at 400 ft. altitude and c at 200, the point d midway between 
 them in the same inclined plane (i.e. a point in midair, not on the 
 ground-surface) is at 300. This is at the same altitude as b, and db 
 
 FIG. II. 
 
 a 
 
 is therefore a line of strike. The dip is in the direction ae, perpen- 
 dicular to db, and is at the rate of 100 in 1200 (1200 being the measure 
 of ae). [Ex. Make a like construction for the other outcrop shown.] 
 This method is of little value for a triangle of narrow shape. 
 
 (18) In general, the outcrops of the several formations follow 
 one another on the map as belts of varying width, younger forma- 
 
FIG. 12. 
 
GEOLOGICAL MAP-READING 23 
 
 tions coming on as we pass in the direction of dip. The order of 
 superposition is therefore easily made out, and should be tabulated 
 at the outset. There may be, however, an isolated area of one 
 formation entirely surrounded by younger (making an inlier) or 
 by older (an outlier). In strata with constant dip an inlier must be 
 very exceptional. It can occur only in the bottom of a valley 
 where the valley-gradient is first steeper and then less steep than 
 the downstream dip. An outlier may occur on any hill, provided 
 that some part of the slope, on the dip side of the hill, is steeper than 
 the dip. This is illustrated in fig. 12, where the dip as shown is 
 eastward at 100 in 750. Beginning with the outcrop ab, we can 
 
 B 
 
 E 
 C 
 
 B 
 
 follow its course eastward and downhill by the method of fig. 10. 
 The line ab is a strike-line at 500 ft. altitude. A parallel line 750 ft. 
 in front of this meets the 400 ft. contour-line in c and d, which are 
 therefore points on the same outcrop ; and so on for successive levels 
 If now we start with the outcrop kl, we obtain in the same way the 
 points m and n, o and p ; but the next strike-line qr does not meet 
 the corresponding (300 ft.) contour-line, and the outcrop must 
 therefore make a closed curve as drawn. Note, however, that the 
 next succeeding strike-line does meet the 200 ft. contour-line in 
 points s and t, which are on the main outcrop to which the outlier 
 belongs. This will be more evident with the aid of fig. 13, which is a 
 W.-E. section through the hill. [This and the following maps, all 
 on the scale of six inches to a mile, should be carefully studied 
 They will be made clearer by adding a thin wash of water-colours.] 
 
2 4 
 
 GEOLOGICAL MAP-READING 
 
 (19) The width of outcrop of any formation is, of course, 
 proportional to its thickness, but depends also upon the dip and 
 upon the slope of the ground. The width as measured on the map 
 is equal to the thickness only when the bedding is vertical. With 
 low dips the width is in general considerably greater than the 
 thickness, unless the beds crop out on a steep escarpment or cliff. 
 If in some place dip and slope coincide, a single bed (of some 
 resistant rock) may be exposed over a wide belt, making a " dip- 
 slope." As regards thickness, we have to distinguish between the 
 true thickness, measured perpendicularly to the bedding (be in fig. 15) 
 and the vertical thickness (bd). To obtain the former from the latter 
 
 FIG. 
 
 FIG. IS. 
 
 we must multiply by the cosine of the angle of dip. The table shows 
 that this correction is negligible for low dips, and does not amount 
 to a difference of 5 per cent, until the dip reaches 18. 
 
 (20) To measure the vertical thickness of a formation from the 
 map. For horizontal bedding (fig. 14) the vertical (and also the 
 true) thickness is merely the difference of altitude between a and b, 
 points on the lower and upper boundaries. For inclined bedding 
 outcropping on level ground (fig. 15) the vertical thickness bd is 
 merely the amount which the base declines in a horizontal distance 
 ab, and can be calculated directly from the known gradient of dip. 
 In the more general case (fig. 16) we have to reckon with both slope 
 and dip. We have to determine therefore both be, the difference of 
 altitude between a and b, and de, the amount by which the base 
 
GEOLOGICAL MAP-READING 25 
 
 declines in the distance ae (measured on the map). The vertical 
 thickness is then the difference or the sum of these in different cases : 
 
 (A) Dip steeper than slope, 
 
 bdde be. 
 
 (B) Slope steeper than dip, 
 
 bdbede. 
 
 (C) Dip opposed to slope, 
 
 bd=be+de. 
 
 FIG. 16 
 
 (21) The sections in fig. 16 were taken in the direction of dip ; 
 but we may suppose them taken in any other direction, provided 
 that the width of outcrop, the slope, and the dip are taken as 
 measured in that particular direction. By taking a section along 
 the strike (fig. 14 will serve) we eliminate the dip, and the vertical 
 thickness is merely the difference of altitude between a and b. This 
 is a ready method for judging roughly by eye. To obtain the best 
 result draw ab so that one end is on a contour-line and the other 
 
'FIG. 17 
 
 D 
 
GEOLOGICAL MAP-READING 27 
 
 on a smooth slope where altitudes can 'be estimated with some 
 accuracy. Alternatively we can eliminate the slope (fig. 15) by 
 taking our -section along a contour-line, or, rather, so that a and b 
 are on the same contour-line (the form of the ground between being of 
 no consequence). Then the vertical thickness is merely the distance 
 through which the base declines in the horizontal distance ab. This 
 method has the advantage of avoiding any judging of altitude 
 between the contour-lines. 
 
 (22) For illustration we will estimate the vertical thickness of 
 the formation D in fig. 17. The dip is seen to be constant at 100 
 in 1200 in a direction nearly E. 30 S. (i.) By the dip method of 
 20. Draw the line ab across the outcrop in the direction of dip, 
 and measure it (1300 ft.). In this distance the base of D declines 
 (at the known rate of dip) 108 ft. Since a is about 25 ft. higher 
 than b, we must subtract this figure, and we obtain 83 ft. for the 
 vertical thickness, (ii.) By the strike method of 21. Draw be 
 across the outcrop in the direction of strike. The altitude of c may 
 be judged at about 315 ft., while b is at 400, and the difference gives 
 the vertical thickness as about 85 ft. (iii.) By the contour-line 
 method. Take the points d and e on the two boundaries of D and on 
 the same contour-line, and draw the strike-line ef and the dip-line df, 
 which measures 1000 ft. In this distance the base of D declines 
 (at the known gradient of dip) 83 ft., which is, therefore, the vertical 
 thickness of the formation. [Ex. Estimate the vertical thicknesses 
 of B, C, and E (50, 67, and 33 ft.).] Note, e.g. how E makes a wide 
 spread in the north, where dip and slope are not very different, but a 
 narrow band in the south, where the dip is into the hill. 
 
 (23) So far we have dealt with a single conformable series of 
 formations, the product of uninterrupted deposition. Consider 
 now the case of two distinct series, each conformable in itself, but 
 with unconformity between the two, marking a discontinuity and a 
 hiatus in the succession. The older strata have been tilted and 
 raised above sea-level ; erosion has followed, tending always towards 
 
FIG-. 18. 
 
GEOLOGICAL MAP-READING 29 
 
 the establishment of " base-level " ; then submergence and the 
 deposition of a new series of strata on the eroded edges of the older. 
 Usually the newer series has also been tilted during its elevation at a 
 later epoch, the inclination of the older being of course modified at 
 the same time. As now exposed there is accordingly an abrupt 
 discordance of dip between the two series where they come together. 
 In the most general case there is a discordance of strike also ; but, 
 unless the dips be very low, the difference of strike is not likely to be 
 considerable. In fig. 18 we have an older series of formations, 
 A to E, and a newer series, K to N. It is easily verified that both 
 have in this case the same strike, and both dip westward, but the 
 older at 100 in 400 and the newer at 100 in 750. The unconformity 
 is most clearly perceived in the exposure of E, the uppermost 
 member (seen) of the older series. Its eastern boundary represents 
 its natural base, and is conformable with D below ; but its western 
 boundary is determined by a surface of erosion, which is also the 
 base of K. Owing to its lower dip, K encroaches obliquely across E, 
 and finally conceals it, coming to rest instead upon the underlying D. 
 (24) The eroded surface of the older strata, on which the 
 newer were laid down, was not necessarily level. If it presented in- 
 equalities, these were filled in by the basal deposits of the newer 
 series. Hence the formation immediately succeeding an uncon- 
 formity often has varying thickness and irregular or inconstant 
 distribution. In fig. 18 K exhibits a common type of such irregularity, 
 involving overlap. This formation has been deposited against a 
 gradually shelving coast to the east, so that successive beds extended 
 farther and farther eastward, overlapping one another. In like 
 manner K as a whole is overlapped by L ; after which the sea-floor 
 seems to have been nearly level, since L maintains a sensibly constant 
 thickness. The effects of unconformity and overlap are to be 
 carefully discriminated. On the map E appears to die out in the 
 N.E. and S.E. directions, because it is covered by newer strata ; 
 but K does really die out, because it was never deposited eastward 
 
30 GEOLOGICAL MAP-READING 
 
 of its present extreme exposures. [Draw sections along the lines 
 ab and cdJ] As a practical point, note that the strike and dip of an 
 unconf ormable series cannot safely be determined from the behaviour 
 of its base, but should be checked from another horizon above. 
 
 (25) We have next to examine the effects of a fault as shown on 
 the geological map. A normal fault is to be conceived as a steeply 
 inclined plane of clean fracture with relative displacement of the 
 rocks on opposite sides of it. Those on one side have been relatively 
 depressed through a certain vertical distance (the " throw " of the 
 fault), but without lateral displacement parallel to the fault. We 
 will further suppose at first that the subsidence is not accompanied 
 by tilting, so that strike and dip are not affected. At the surface 
 of the ground the direct effects of the displacement have been 
 obliterated by erosion, though a fault often figures as a line of weak- 
 ness marked by a depression. Primarily, a fault appears, on the 
 ground and on the map, as a line of discontinuity, against which all 
 boundary-lines (of rocks older than the faulting) are cut off, and 
 are more or less shifted when resumed (if at all) upon the other side. 
 Newer rocks on the downthrow side are brought against older on 
 the upthrow ; so that the direction of downthrow is perceived on 
 simple inspection. The symbol to indicate it is a short line parallel 
 to the fault with a shorter stroke at right angles to it on the down- 
 throw side. A normal fault inclines downward towards that side 
 (" hades towards the downthrow "), but the angle which it makes 
 with the vertical (the " hade ") is usually quite small. Consequently 
 the fault-outcrop experiences no sensible deviation in crossing hill 
 and valley. It may, however, have a somewhat curved course 
 owing to actual curvature of the fault-surface. 
 
 (26) For convenience in describing their effects we may 
 distinguish between strike-faults and dip-faults, according as their 
 course approximates to one or the other direction. A fault making 
 a medium angle with the strike can be regarded from either point of 
 view. If a strike-fault, intersecting a regular succession of strata, 
 
FIG-. 19. 
 
 A 
 
 I 
 
 / 
 I 
 
 *' 
 
 / 
 
 I 
 
 
 \ ,- 
 
 \ 
 
 400 
 
 -*0 
 
 B 
 
 // 
 
 <.o- 
 
 ^ 
 
 c\ 
 
 o 
 
 O' 
 I 
 
32 GEOLOGICAL MAP-READING 
 
 throws down in the direction of dip, a part of the succession will be 
 missing in a traverse of the ground crossing the fault ; while, if the 
 downthrow be in the opposite direction, a part will be repeated. 
 The vertical thickness of the strata cut out or repeated is evidently 
 equal to the throw of the fault. For a dip-fault the most noticeable 
 effect is the shifting of the boundary-lines where they encounter the 
 fault. The amount of lateral shift of any line is proportional to the 
 throw of the fault, but depends also upon the dip and upon the slope 
 of the ground. To realise the relation, turn to figs. 14-16, and 
 suppose ad and bf now to represent one and the same bedding-plane 
 on opposite sides of a dip-fault ; or for a strike-fault imagine the 
 bedding-plane on one side produced across the fault. It is easily 
 seen that the relation between the vertical throw of a fault and 
 the shift which it causes is precisely the same as the relation between 
 the vertical thickness of a formation and the width of its outcrop. 
 The discussion in 20, 21 can therefore be applied, with the 
 necessary substitution of terms, to the case of faulting. Note that 
 the boundary-line on the downthrow side is always shifted backward 
 (i.e. in the direction opposite to that of dip), save only in the 
 exceptional case (fig. 16, B) in which the slope is steeper than the 
 dip. 
 
 (27) For illustration take first the strike-fault shown in fig. 19. 
 Here the dip, 100 in 1125, is seen to be the same on both sides of the 
 fault. The downthrow is north-westward, and so against the dip. 
 Accordingly there is repetition of beds in a traverse downstream, 
 the formation A reappearing as a " faulted inlier." To estimate 
 the throw of the fault, consider the base of C. In the direction of 
 dip it is shifted through a distance ab (1500 ft.), and the fall in this 
 distance at the known rate of dip would be 133 ft. From this we 
 must subtract the difference of altitude between a and b, say 
 50 ft., and the result is 83 ft. Next take points a and c on the same 
 contour-line. The fall from a to c is reckoned by measuring the 
 distance ad in the direction of dip (950 ft.) and using the known 
 
FIG. 20 
 
34 GEOLOGICAL MAP-READING 
 
 gradient of dip, which gives 84 ft. To apply the strike-method, 
 measure ef the shift in the direction of strike (1025 ft.), and also the 
 distance apart of the contour-lines along gh (1200). The slope in 
 that direction being 100 in 1200, 1025 ft. corresponds with a difference 
 of altitude of about 85 ft. This last method is not here very trust- 
 worthy, and would be less so for measurements taken in the northern 
 part of the map, where the boundaries make more acute angles with 
 the strike. 
 
 (28) In general the dip method is better suited to a strike-fault 
 and the strike method to a dip-fault. In fig. 20 we have a dip-fault 
 throwing down to the east. The base of the formation C is about 
 80 ft. higher at c than at a and b on the same strike-line, and this 
 gives therefore the amount of the throw. To apply the contour-line 
 method, take the points d and e, and measure the distance df in the 
 direction of dip (1050 ft.). At the rate of dip (100 in 1325) the fall 
 in this distance is nearly 80 ft., which is therefore the throw of the 
 fault. 
 
Ill STRATA WITH CONSTANT STRIKE 
 BUT VARYING DIP 
 
 (29) We go on to discuss strata which are not merely tilted as 
 inclined planes, but folded. The strike will still be assumed constant. 
 The dip is constant along any one strike-line, but varies in the 
 direction across the strike, so that in a vertical section in such 
 direction any bedding-surface appears as a curve. In any regular 
 system of folding there are in general places where the dip changes 
 from one direction to the opposite, passing through the horizontal. 
 Such a place is a syncline or anticline, according as the opposed dips 
 are towards or away from it. If we regard a particular bedding- 
 surface, folded in wave-like fashion, we note definite lines of strike, 
 in the arches and troughs of the folds, where the bedding is horizontal, 
 and these, as laid down on the map, may be termed anticlinal and 
 synclinal axes. When a conformable series of strata is folded 
 together in an anticline or syncline, these particular strike-lines for 
 the several bedding-surfaces are situated in a certain axial plane, 
 which for symmetrical folding is a vertical plane. As we pass in the 
 direction transverse to the strike, the dip is continually changing, 
 but not always at the same rate. It changes most slowly in the 
 middle limb of a fold, say midway between anticline and syncline, 
 where the curve changes from convex to concave, and it is here that 
 the steepest dip is found. 
 
 (30) It is easy to realise how the course of a boundary-line will 
 be affected by varying dip ( 15). Summarily, for vertical bedding 
 outcrops run parallel to the strike ; for horizontal bedding parallel 
 to the contour-lines ; for inclined bedding in an intermediate 
 direction, nearer the strike for steeper dips and nearer the contour- 
 lines for less steep. Fig. 21 represents an anticline, and shows the 
 outcrop of a particular bedding-surface upon a hill-side of constant 
 
 35 
 
36 GEOLOGICAL MAP-READING 
 
 slope. At , where the dip is steep, the curve makes only a small 
 angle with the strike, and a high angle with the contour-lines. 
 From a to b it turns gradually away from the strike and towards the 
 
 PIG. 21. 
 
 FIG. 22. 
 
 contour-lines as the dip declines ; from b to c and from c to d it 
 turns more rapidly in the same way, as the dip declines more rapidly. 
 At e, on the axis, the bedding is momentarily horizontal, and the 
 outcrop is therefore parallel to the contour-line (touches it in the 
 figure). Then, through fgh, the curve returns towards the strike 
 
FIG-. 23. 
 
38 GEOLOGICAL MAP-READING 
 
 direction as the dip increases in the reverse sense. Fig. 22 is designed 
 to illustrate this case by a view in perspective. The more general 
 case, in which slope varies as well as dip, will present no difficulty. 
 
 (31) An area of folded strata displays its geological structure 
 most clearly when the strike is transverse to the main lines of 
 drainage. Fig. 23 shows an anticline and a syncline intersected 
 transversely by a valley. The V's made by the boundary-lines point 
 away from the anticlinal axis aa and towards the synclinal axis ss. 
 The oldest formation A is exposed in the anticline and in the bottom 
 of the valley, where it makes an inlier. The youngest F is exposed 
 in the syncline and on the higher ground, making two areas which 
 may be portions of two outliers. Note how to lay down on the map 
 the axes of folding. The anticlinal axis must clearly pass between 
 b and c where A rises highest. At some point d we can suppose a 
 contour-line drawn to touch the boundary-line, and this point must 
 be on the axis. So for e on the opposite side, the axis being thus 
 fixed. This affords a good way of determining the strike. Observe 
 that the centre of the inlier is not directly on the axis, but a little 
 downstream ; and similarly an outlier is situated a little upstream 
 in relation to a synclinal axis. 
 
 (32) When the main lines of drainage run approximately with 
 the folds, as in fig. 24, the structure is deciphered without difficulty, 
 if the map is studied both as a whole and in detail. To determine 
 the strike should be the first step. The outcrops in the valley, taken 
 by themselves, recall fig. 10, and might suggest a downstream dip, 
 except that be and de (strike-lines on that supposition) are not 
 parallel. Examined more closely, the lines (as at ef) indicate a 
 strike along the valley, given more accurately by eg, and it then 
 becomes apparent that there is an anticline along the valley (axis aa). 
 So, too, the outcrops on the hill are to be interpreted as showing a 
 syncline with axis ss, parallel to hk and mn, not an anticline with 
 strike hm or kn. The correspondence of a valley with an anticline 
 and a hill with a syncline is a common arrangement, a consequence 
 
F/G. 24, 
 
 D 
 
 a \ 
 
 a 
 
40 GEOLOGICAL MAP-READING 
 
 of the mechanical laws of erosion, by which slope tends to be opposed 
 to dip. It results in a narrowing of outcrops, so that a considerable 
 thickness of strata is exposed in a moderate width of ground. 
 
 (33) At this stage the student should review those preceding 
 articles which deal with strata having a constant dip, and consider 
 how far they are applicable to the more general case now in question. 
 It is clear that all considerations involving strike alone will still 
 hold good so long as the strike is constant. Constructions and 
 calculations involving the rate of dip cannot in theory be applied 
 when the dip is variable, but in practice they can often be used with 
 judgment to obtain results very near to the truth. The general 
 procedure for determining the strike on a map ( 12) is still applicable; 
 and also the particular method by joining the two points of inter- 
 section of a boundary-line and a contour-line, provided that the 
 two points are in the same limb of a fold, not on opposite sides of an 
 axis (e.g. hk in fig. 24, not hm). The method for measuring dip 
 ( 14), when applied to folded strata (in one limb of a fold), gives the 
 average dip between the two strike-lines, or say the dip at midway : 
 if the dip is changing only slowly, the measurement is as good as is 
 needed. In fig. 23, e.g., the dip of the base of D in the middle of the 
 map is estimated at 100 in 300, which is closely the true dip (18). 
 Where the dip is changing rapidly, a vertical interval of 100 ft. is 
 too great, but on a steady slope it is permissible to interpolate 
 between the contour-lines. Thus, to find the dip at m (fig. 23) 
 measure mn in a certain ratio to mo, say one-fourth, and draw through 
 n a portion of a 475 ft. contour-line, meeting the boundary-line in p. 
 Draw strike-lines through m and p, and measure the distance mq 
 between them (150 ft.). Then along mp there is a fall of 25 vertical 
 in 150 horizontal, or 100 in 600 (9^). For estimating the thickness 
 of a formation, or the throw of a fault, the strike method (21) 
 should be used wherever it is practicable. It will give good results 
 if the boundary-lines do not make too acute an angle with the strike 
 (i.e. if the dips are not oo high). 
 
GEOLOGICAL MAP-READING 
 
 (34) In making out the character of the folding from a given 
 geological map, lay down first the position of any anticlinal and 
 
 FIG. 25. 
 
 B 
 
 A 
 
 soo 
 
 95*0 
 
 y 
 
 synclinal axes, and then determine the dip at selected points. 
 Compare the dips at equal distances on opposite sides of an axis, to 
 ascertain whether the folding is of symmetrical type. The most 
 important dip to determine is the steepest one, and this, as we have 
 seen, is the one most easily measured with accuracy. In fig. 25 
 
42 GEOLOGICAL MAP-READING 
 
 we see neither anticlinal nor synclinal axis, but only part of one 
 limb of a fold, since the dip is constantly in one direction. Evidently 
 it is lower in the eastern than in the western part of the map. 
 Measurements give for the average dip between be and d 100 in 950 ; 
 between ef and g 100 in 500 ; between hk and / again 100 in 500. 
 We infer that k is situated at about the middle of the limb, where 
 the dip is steepest, and changes only slowly. 
 
 (35) The student should constantly practice constructing a 
 section from a given geological map ; not as a substitute for studying 
 the map directly, but after such study and as illustrative of its 
 results. The line of section should be chosen to bring out the 
 essential structure, and its direction should be, as a rule, at right 
 angles to the strike. In constructing a profile of the ground ( 9) 
 note at the same time where the line of section crosses each of the 
 geological boundary-lines. At each such place on the profile draw 
 a short line inwards to give approximately the correct inclination 
 of the boundary at its outcrop. For this purpose make use of any 
 dips marked on the map near the line of section, and estimate others 
 as required. The boundary-lines are now to be completed as 
 sweeping curves corresponding with the general character of the 
 folding as already made out, giving each formation its proper 
 thickness. If the dip at outcrop is sufficiently known, the thickness 
 there is given by the section itself. 
 
 (36) If the strike is strictly constant, all parallel sections 
 must be identical, except as more or less of the rocks is cut off 
 above the ground-surface. The section should therefore be taken 
 by preference through the higher ground, so as to utilise the data 
 furnished by the lower ground. This can be done systematically, as 
 in fig. 26, where the several boundary-lines are projected upon 
 the plane of section. The section here is taken along the southern 
 edge of the map, and for clearness it is shown in two stages of con- 
 struction, and with a vertical scale exaggerated twofold. At a on 
 the map the base of E is at 200 ft., and the same at any point along 
 
FIG. 26. 
 
 C 
 
 A 
 
 B 
 
 .,0.' 
 
 C 
 
 . k. 
 
 a 
 
FIG. 27 
 
GEOLOGICAL MAP-READING 
 
 45 
 
 the strike-line through a : this, therefore, is its altitude at the 
 corresponding point in the section. At b on the map the same 
 boundary-line is at 300 ft., which fixes another point in the section ; 
 the line ab in the section being simply the projection (on the vertical 
 plane of section) of the line ab of the map. The point c, at 400 ft., 
 belongs to the same boundary-line, but is in the opposite limb of the 
 anticline. In like manner we can lay down on the section portions 
 
 FIG. 28. FIG*. 29. 
 
 SCL 
 
 of the base of D (de and /), the base of C (gh), and the base of B (ik) ; 
 and from this skeleton the section can be filled in with considerable 
 accuracy. 
 
 (37) In an unsymmetrical fold the axial plane is inclined, and 
 the dips are steeper in one limb than in the other. The type of 
 structure illustrated in fig. 27 is that known as a monoclinal fold. 
 A syncline and an anticline, both unsymmetrical, occur near to- 
 gether. In the middle limb the dip is relatively high, as is apparent 
 from the narrowness of the outcrops and the comparative straight- 
 ness of the lines (indicating the strike at a glance). In the outer 
 limbs the dip is low, and becomes practically constant at a distance 
 
46 GEOLOGICAL MAP-READING 
 
 from the axis. [A section carefully drawn to preserve the thickness 
 of each formation will bring out the inclination of the axial planes.] 
 
 (38) Another type is the overturned fold or over fold, shown in 
 section in fig. 28, in which the strata in one limb are inverted. 
 Except in the core of the fold (b) the dip has the same direction in 
 both limbs, but is steeper in the inverted limb than in the other. In 
 the extreme case this becomes isoclinal folding, in which the opposite 
 limbs are parallel to one another and to the axial plane (fig. 29). 
 The appearance at the outcrop is that of a regular succession of 
 strata with steady dip ; but if the individual members can be 
 identified (by lithological characters or distinctive fossils), the 
 repetition with reversal of order reveals the true relations. Note that 
 in this closely compressed folding there is a stretching and thinning 
 of the middle limbs with correlative thickening in the arches and 
 troughs. 
 
 (39) Fig. 30 illustrates a tract of overfolding, intersected by a 
 transverse valley. The dip is everywhere southerly. The oldest for- 
 mation makes an inlier in the bottom of the valley, as in an ordinary 
 anticline ; but here the dip is in the same direction in both limbs, 
 at a moderate angle in the southern limb and steep in the northern 
 (overturned) limb. [Compare the heart-shaped exposure of A 
 here with the regular shape in fig. 23, a symmetrical anticline, and 
 the intermediate case of fig. 27, an unsymmetrical anticline.] In 
 the northern part of the map we see the correlative infold, analogous 
 to a syncline, but with southerly dips in both limbs, steep on the 
 southern side and moderate on the northern. 
 
 (40) Consider next the case of two series of strata with uncon- 
 formity between them and two periods of folding, the earlier folding 
 affecting only the older series and the later folding affecting both. 
 We suppose the strike still constant, which implies that the axes of 
 the later folding are parallel to those of the earlier. In the double 
 anticline shown in fig. 31 the axes are not merely parallel, but 
 coincident : i.e. there has been elevation along the same axis aa at 
 
FIG. 30. 
 
 D 
 
FIG. 31. 
 
 D 
 
 A 
 
 B 
 
 K 
 
 K 
 
GEOLOGICAL MAP-READING 49 
 
 two periods, one before and the other after the deposition of K and 
 L. On the west side, where the outcrops spread out in accordance 
 with the downstream dip, the discordance is evident from the trans- 
 gression of the base of K across the older formations. On the east, 
 where the dip is into the hill, it is less apparent, but is indicated by a 
 slightly more pronounced curvature and indentation of the lines of 
 the newer as compared with the older series. 
 
 (41) In folded strata the distinction between strike-faults and 
 dip-faults is well marked. Fig. 32 shows an anticline, the arch of 
 which is dropped down between two sir ike- faults (" trough-faulting"). 
 In the sunken strip the boundary-lines are shifted inwards, and 
 higher beds are thrown against lower. To obtain a rough estimate 
 of the amount of throw, observe that the base of D is shifted, in the 
 direction of strike, from m to n (800 ft.), while the slope in the same 
 direction (op) is 100 in 1050, which gives a throw of about 76 ft. 
 Note incidentally that in strata which are not folded (in the ordinary 
 sense) there is sometimes a change of dip at a strike-fault. This is 
 equivalent to folding of a simple type in addition to the faulting, and 
 the throw should be estimated by the strike-method if that is 
 applicable. 
 
 (42) For a dip- fault the shifting of the outcrops is very con- 
 spicuous. In an anticline the opposed outcrops are brought closer 
 together on the downthrow side, and some of the lower beds are 
 dropped out of sight ; in a syncline the outcrops are thrown farther 
 apart, and higher beds come in on the axis. Fig. 33 illustrates the 
 latter case. It shows a sharp syncline, the steepest dip (at fg) being 
 about 50 in 125, or 100 in 250 (nearly 22). Note that in ordinary 
 symmetrical folding (the axial plane being vertical) the axis is not 
 shifted by a fault, and can therefore be laid down as in 31. To 
 measure the throw, draw the strike-line abc, and observe that the 
 base of D is about 150 ft. higher on one side than on the other. Or, 
 again, note that the top of E on one side is thrown against the base 
 of D on the other, and the throw is therefore equal to the vertical 
 
FIG. 32 
 
 CL 
 
 'f> 
 
 ^ .y 
 
 C 
 
FIG. 33 
 
 D 
 
 B 
 
 C 
 
 .-'* 
 C 
 
 5oo 
 
 s .600 
 
 D 
 
 600 
 
 B 
 
 Too 
 
 700 
 
 B 
 
 B 
 
 A 
 
 600 . 
 
52 GEOLOGICAL MAP-READING 
 
 thickness of D + E, which is about 150 ft. as measured at de 
 or mn. 
 
 (43) It is possible for faulting, as well as folding, to be renewed 
 at a later period upon the old lines. Fig. 34 shows two unconform- 
 able series folded in an anticline and intersected by a dip-fault. The 
 elevation of the anticline has been effected partly before and partly 
 after the deposition of the newer series K, L, and the same is true 
 of the movement of the fault. The proof is that the throw amounts 
 to 100 ft. in the older rocks (be), and only about 50 ft. in the newer 
 
 (*) 
 
 (44) While a normal fault is related to lateral tension, and 
 involves merely a relative subsidence as between the severed blocks, a 
 reversed fault is a result of lateral thrust, and involves a piling up of 
 the rocks on one side over those on the other. The hade is accord- 
 ingly towards the upthrow side, and a pronounced hade is much 
 more general here than in normal faulting. Fig. 35 illustrates a 
 common case, in which a reversed fault has arisen as a further 
 development of an overfold, the plane of fracture being in the under 
 limb of the fold. The dip of the strata increases from the N.E. part 
 of the map to the centre, where it culminates in a sharp anticline 
 exposing the lowest beds (A). Just beyond this comes the fault, 
 throwing higher beds on the S. side against lower on the N. Its 
 hade is shown by its making a V (though a very blunt one) in crossing 
 the valley, and the shape of its outcrop shows that it is scarcely 
 steeper than the northern boundary of A. The relations are shown 
 in fig. 36, a section taken along the line mn. It is clear that the 
 " throw " of a fault of this kind, where the strata on the under side 
 are nearly vertical, is not a term of any precise significance. The 
 relative displacement of the rocks on opposite sides is the joint effect 
 of the folding and the faulting, which were essentially successive 
 stages of one process. In some areas of mountain-structure, such as 
 the North- West Highlands of Scotland, there occur great reversed 
 faults (overthrusts) with low inclination, often nearly horizontal. 
 
F/G:34. 
 
 ^ ^ 
 
 -~ s oo 
 
 ~ * 
 
 ^0 
 
 K 
 
 B / \o 
 
 \ K 
 
 A 
 
 B 
 
 B 
 
F/G-.35-. 
 
 D 
 
 >,-, 
 
 A 
 
 D 
 
 B 
 
GEOLOGICAL MAP-READING 
 
 55 
 
 The significant displacement in such a case is that in the horizontal 
 direction, which amounts in some instances to many miles. As a 
 result of such movements a newer formation may come to be overlain 
 for some considerable distance by an older one, either inverted or 
 in its natural posture. Subsequent folding will then affect both in 
 common, the surface of overthrusting itself being thrown into folds. 
 (45) In general the relative displacement at a fault, whether 
 normal or reversed, is wholly in the direction of hade, not hori- 
 zontally along the fault. The observed lateral shifting of outcrops 
 results from the vertical downthrow in conjunction with the dip of 
 
 FIG. 36. 
 
 the strata, and for vertical bedding there is no shifting. As an 
 incident of overthrusting, however, there sometimes occurs a 
 peculiar type of fault with real displacement in the horizontal 
 direction as well as in the vertical (a tear-fault or flaw). Fig. 37 
 shows (diagrammatically on level ground) .an anticline intersected by 
 two such faults. It shows also a vertical dyke (X), the shifting of 
 which affords a measure of the horizontal component of the dis- 
 placement, being unaffected by the vertical throw. Beginning in 
 the north, the first fault has a vertical throw of 100 ft. down to S. 
 and a horizontal slide of 500 ft. to E. By the former the outcrops 
 are shifted inward in accordance with their varying dips ; by the 
 latter they are all shifted through a constant distance to E. The 
 resultant shift is everywhere eastward, but is greater in the western 
 limb, where the two effects are additive, than in the eastern, where 
 
GEOLOGICAL MAP-READING 
 
 they are sub tractive. The second fault has the same vertical throw 
 of 100 ft. to S., but a smaller horizontal slide of 250 ft. to E. This is 
 not enough to counteract in the eastern limb the backward shift 
 
 FIG.. 37. 
 
 D 
 
 .'a 
 
 B 
 
 B 
 
 18 
 
 D 
 
 500 \ 500 
 
 B 
 
 150 
 
 (00 
 
 B 
 
 10 
 
 due to the throw down. Accordingly the resultant shift is inward for 
 both limbs, but is greater in the western than in the eastern. 
 
FIG. 38. 
 
 o 
 V 
 
 B 
 
 D/ 
 
IV. STRATA WITH VARYING STRIKE. 
 
 (46) So far we have been able to assume, at least ideally, a 
 constant strike in the strata, however they may have been tilted or 
 folded. Casual irregularities, affecting strike as well as dip, do not 
 call for notice here ; but we have now to examine certain cases in 
 which varying strike is a necessary consequence of some special type 
 of folding or faulting. Note that in this connection faulting can be 
 regarded as merely an extreme case of folding, changes in dip (and 
 strike) coming on abruptly instead of gradually. A normal fault is 
 the analogue of a monocKnal fold, a reversed fault of an oVerfold 
 (44). Observe firstly that the strike necessarily varies when a fold 
 dies out in the direction of its axis. Recalling that strike-lines are 
 merely contour-lines on a bedding-surface, we can take fig. 2 above as 
 representing the dying out of an anticline in horizontal bedding. 
 The lines in the figure show the changing direction of strike, and 
 their relative closeness corresponds with the relative steepness of 
 dip at different places. On the axis there is a slight dip in the 
 direction of the axis itself. [For a fold dying out in inclined strata 
 there will be a more complex diagram, which the student may 
 construct for himself.] An analogous case is that of a fault which 
 dies out, or gradually changes in the amount of its downthrow. 
 
 (47) Consider next a dome-like elevation, such as is found in 
 connection with certain types of igneous intrusion. For a dome 
 rising from horizontal bedding the strike-diagram will be a system 
 of concentric circles or ovals ; for a dome superposed upon a constant 
 dip we may use fig. 4 above. It will serve as a key to fig. 38, which 
 represents a dome due to a boss of granite G breaking through strata 
 with a steady dip to S. This normal dip is seen in the marginal 
 parts of the map. In the northern part it is opposed to the dip due to 
 the dome, and there results a curved syncline, marked by the outlier 
 
 58 
 
GEOLOGICAL MAP-READING 59 
 
 of D. On the south side of the dome the normal dip is augmented 
 by that due to the elevation, and here the steepest dips are found. 
 [Draw sections N.-S. and E.-W.] 
 
 (48) It is easy to see that folding on certain axes followed by a 
 second folding on axes transverse to the former will give rise to a 
 complicated arrangement of varying strike. For folding of a pro- 
 nounced type strata of any rigidity do not in general behave in the 
 fashion supposed, but yield rather by fracture and faulting. We 
 sometimes find, however, folding with axes which are not horizontal, 
 but have a certain inclination or pitch. [The definition of an axis as 
 a line along which the bedding is horizontal therefore needs revision : 
 it is a line where the dip is a minimum, and is directed along the 
 axis itself.] This occurs not merely incidentally ( 46), but affecting 
 a system of anticlines and synclines. Fig. 39 is a strike-diagram for 
 an anticline between two synclines, the axes having a steady pitch 
 to north. It is to be taken as a key to fig. 40, a map constructed 
 upon the same scheme. The inclination of the axes is manifest upon 
 an examination of the map. Thus, c and d are points in the same 
 bedding-plane and on the same synclinal axis, but from c to d there 
 is a rise of about- 45 ft. in 2450, indicating a pitch to N. of about 100 
 in 5500 (nearly i). There is a similar rise along ab. Note that ef 
 and gh, on opposite sides of an anticlinal axis, converge northward ; 
 while ik and mn, on opposite sides of synclinal axis, converge south- 
 ward. [Compare throughout with fig. 39.] 
 
 (49) Concerning igneous rocks, as they figure on a geological map, 
 a few words will suffice. Interbedded lava-flows, tuffs, etc., have 
 the same relations as ordinary stratified formations, save that the 
 thickness is likely to be more variable than is found in most types of 
 sediments. In particular a coarse bedded agglomerate often thins 
 out very rapidly in any direction away from its source. This 
 behaviour may serve to indicate the situation of the source, and the 
 exact place of the vent may be marked by a " volcanic neck " of 
 agglomerate or a " L plug " of intrusive rock. Among intrusions we 
 
FIG. 39. 
 
F/G.4-0. 
 
62 GEOLOGICAL MAP-READING 
 
 must distinguish two classes : the concordant, which (apart from 
 irregularities) follow the direction of bedding in the strata invaded, 
 and the transgressive, which systematically break across the bedding. 
 Of the former, intrusive sills are often so regular in their habit, and 
 so nearly constant in thickness, that they may be treated like 
 ordinary stratified formations. In the case of a laccolite (or a 
 phacolite among folded strata) the lenticular form is indicated by 
 the outcrop, if erosion has cut through the whole thickness. If 
 only the upper surface is exposed, there will be a doming effect 
 apparent in the overlying strata, as in fig. 38. Note, however, that 
 if G in that figure were a laccolite, its boundary (being really a 
 bedding-surface) would run down into the heads of the small valleys. 
 The smooth oval actually shown, disregarding the form of the 
 ground-surface, implies a nearly vertical boundary. This is the 
 sign of an intrusive boss, as equally of a volcanic plug or a " neck " 
 of cylindrical form. In like manner a vertical dyke pursues its 
 course indifferently across hill and valley. Only when there is a 
 strong hade is the outcrop of a dyke influenced by the form of the 
 ground, showing a blunt V where it crosses a valley. There remain 
 the large subterranean intrusive bodies known as batholites, the 
 true relations of which are only very partially displayed by a map. 
 Such a body, as seen at its upper contact, often cuts obliquely 
 across the strata, an effect indicated on the map by an apparent 
 thinning out of a formation thus invaded. 
 
 (50) In conclusion, the object of these brief notes will have 
 been attained if the student, in the first place, has acquired the habit 
 of constantly visualising a geological map (whether contoured or 
 not) in three dimensions ; and if, secondly, he has effectively realised 
 by practice that the interpretation of the details of structure as 
 presented on a geological map is merely an application of elementary 
 geometry. This is equally true whether the result can be worked 
 out in precise terms or not. It is best illustrated by using, as we 
 have done, large-scale contoured maps ; but, when the point of view 
 
GEOLOGICAL MAP-READING 63 
 
 has become instinctive, it may be brought to bear upon maps on 
 which a larger tract of country is represented on a smaller scale. 
 Above all, it must be realised that whatever information a geological 
 map can convey is displayed on the face of the map, and is there to 
 be read. The use of sections is to illustrate the relations so con- 
 strued, and the student who draws sections in order to make out the 
 geological structure will make little progress in the art of map- 
 reading. 
 
Angle 
 
 Gradient 
 100 in 
 
 Sine 
 
 Cosine 
 
 Tangent 
 
 i 
 
 5728 
 
 017 
 
 I -000 
 
 017 
 
 2 
 
 2864 
 
 035 
 
 '999 
 
 035 
 
 3 
 
 1908 
 
 052 
 
 999 
 
 052 
 
 4 
 
 1430 
 
 070 
 
 998 
 
 070 
 
 5 
 
 H43 
 
 087 
 
 996 
 
 087 
 
 6 
 
 95i 
 
 105 
 
 995 
 
 105 
 
 7 
 
 814 
 
 122 
 
 993 
 
 -123 
 
 8 
 
 712 
 
 139 
 
 990 
 
 141 
 
 9 
 
 631 
 
 156 
 
 988 
 
 -158 
 
 10 
 
 567 
 
 174 
 
 984 
 
 176 
 
 ii 
 
 514 
 
 190 
 
 982 
 
 194 
 
 12 
 
 470 
 
 208 
 
 978 
 
 213 
 
 13 
 
 433 
 
 225 
 
 -974 
 
 231 
 
 14 
 
 401 
 
 242 
 
 970 
 
 249 
 
 15 
 
 373 
 
 259 
 
 966 
 
 268 
 
 16 
 
 349 
 
 276 
 
 961 
 
 287 
 
 17 
 
 327 
 
 292 
 
 956 
 
 -306 
 
 18 
 
 308 
 
 309 
 
 95i 
 
 325 
 
 19 
 
 290 
 
 326 
 
 946 
 
 344 
 
 20 
 
 275 
 
 342 
 
 940 
 
 364 
 
 21 
 
 260 
 
 359 
 
 934 
 
 384 
 
 22 
 
 248 
 
 375 
 
 927 
 
 404 
 
 23 
 
 236 
 
 39i 
 
 920 
 
 424 
 
 24 
 
 225 
 
 407 
 
 914 
 
 445 
 
 25 
 
 214 
 
 423 
 
 906 
 
 466 
 
 26 
 
 205 
 
 438 
 
 899 
 
 488 
 
 27 
 
 196 
 
 '454 
 
 891 
 
 510 
 
 28 
 
 188 
 
 469 
 
 883 
 
 532 
 
 29 
 
 180 
 
 485 
 
 875 
 
 554 
 
 30 
 
 173 
 
 500 
 
 866 
 
 577 
 
 32 
 
 160 
 
 530 
 
 848 
 
 625 
 
 34 
 
 148 
 
 559 
 
 829 
 
 675 
 
 36 
 
 138 
 
 588 
 
 809 
 
 727 
 
 38 
 
 128 
 
 616 
 
 788 
 
 781 
 
 40 
 
 119 
 
 643 
 
 766 
 
 839 
 
Printed by 
 
 W. HEFFEK & SONS LTD. 
 Cambridge, ENG. 
 
UNIVERSITY OF CALIFORNIA. LIBRARY 
 BERKELEY 
 
 Return to desk from which borrowed. 
 This book is DUE on the last date stamped below. 
 
 JW28S5-* 
 ARR2019675 
 
 I '67 -3PM 
 
 LD 21-100m-ll,'49(B7146sl6)476