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INTERPRETATION OF

TOPOGRAPHIC AND GEOLOGIC MAPS

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INTERPRETATION OF

TOPOGRAPHIC AND GEOLOGIC MAPS

W ith Special Reference to Determ ination of Structure

BY

C. L. DAKE, P

ii

. D.

Professor o f Geology, M isso u ri Sc/tool o f M in es and M etallurgy, liolla. M o

A N D

J. S. BROWN, P

h

. D.

Associate Geologist, U nited States Geological Survey

M cG RA W -H ILL BOOK CO M PANY, In c . N E W Y O R K A N D L O N D O N

1 9 2 5

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S a

5 SV

S. 05

C o p y r i g h t , 1925, b y t h e M c G r a w - H i l l B o o k C o m p a n y , I n c .

Copyright renewed 1953 by ELLA F . DAICE & J. S. BRO W N

P R IN T E D I N T H E U N I T E D S T A T E S O F A M E R IC A

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PREFACE

The field of m ap interpretation is singularly lacking in anything approaching an adequate textbook, largely as a result of the fact th a t th e numerous m aps considered essential increase the cost of a tex t alm ost prohibitively. The only agency suitably equipped to produce an adequately illustrated treatise on map interpreta­

tion is the Federal Government. Its one essay in this field, Professional Paper 60, “ The Interp retatio n of Topographic M aps,” has never been repeated. This book, though invaluable, contains little information on the reading of structure from contours, and still less on the interpretation of geologic maps.

Lack of a suitable text has tended to lim it the num ber and scope of courses in m ap interpretation, and has in considerable measure deprived the teaching of elem entary geology of one of its strongest assets, adequate m ap illustration. Listing illustrative m aps in th e general texts is admirable, bu t, unfortunately, the beginner w ithout laboratory guidance sees only a small fraction of the significant features. A num ber of small b u t useful manuals on various phases of m ap interpretation have appeared, largely in the form of questions, b u t these m anuals demand a skilful instructor to make them of most value.

M ap interpretation lends concreteness to general and structural geology and has been term ed by one of our ablest teachers an

“ indoor field course” . I t may, in p art, wisely replace field studies, with economy in time and money, and has the very real advantage th a t more of a region or a structure can be visualized a t one tim e on a m ap th an in th e field.

For these reasons the authors feel th a t there is a real place for a treatise of the type herewith presented. The m aterial has been accumulated by the senior author through more th an ten years of active teaching of m ap interpretation. The book was planned

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vi PREFACE

several years ago to include actual m aps; b u t after fruitless conferences with several publishers the plan was abandoned, owing to the expense involved; and it was decided to prepare a text which would refer to the m aps of th e United States Geological Survey, by way of illustration.

I t is sincerely hoped th a t the work will stim ulate th e general interest in m ap interpretation, to th e end th a t m any more courses will be offered in th a t subject th a n now, and th a t it will help to enrich the illustrative teaching of general and structural geology.

The m ajor plan of th e work and m uch of the detail have resulted from the senior au th or’s experience in teaching map interpretation. The junior author has assisted throughout in the assembling and analysis of illustrative m aterial, and is chiefly responsible for certain minor sections of the text.

I t is desired, a t this point, to express the obligation of thé authors to A. K. Lobeck, of the University of Wisconsin; W. A.

Johnston, of the Canadian Geological Survey; W. D. Smith, of the University of Oregon; F rank Leverett, Sidney Paige, N. H.

D arton, and G. R. Mansfield, of the United States Geological Survey; and R. C. Tucker, of the W est Virginia Geological Survey, for information regarding th e geology of certain areas studied by them . T hey are also indebted to Professors Josiah Bridge and G. A. Muilenburg, of the Missouri School of Mines, for m any valuable suggestions; and to Professors E liot Black- welder, of Stanford University, and D. W. Johnson, of Columbia University, for the inspiration of unexcelled courses in map interpretation.

F or the fundam ental facts of geology and physiography, the authors are indebted to m any of our standard texts and reference works. I t is, of course, impossible to give adequate individual credit for facts drawn from so great a variety of sources.

Th e Au t h o r s. Ho l l a, Mo.,

M ay, 1925.

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CONTENTS

Pa o e

Pr e f a c e... v

I INTRODUCTION... ** PA R T I In t e r p r e t a t i o n o f To p o g r a p h i c Ma p s... 1

Introductory s t a t e m e n t... 1

Special features of m ap s... 2

Direction and o r ie n t a tio n ... 2

Scales... 3

Graphic s c a l e s ... 3

Fractional s c a l e s ... 4

Verbal scales... 5

M ethods of representing t o p o g r a p h y ... 6

Shading... 6

H a c h u r e s ... 6

C ontours... 6

Contours and their significance... 8

Contour lines... 8

Contour intervals... 8

Depression contours... 10

Contours and land form ... 11

Numbering of c o n t o u r s ...12

Limits of exactness in reading c o n t o u r s ... 14

Drawing topographic profiles...17

Describing locations on a m a p ... 18

C onventional sym bols (legend)... 20

Land forms, their developm ent and recognition... 20

Topography resulting from th e work of ground w a t e r ...21

Topography resulting from wind work...26

Topography resulting from th e work of glaciers... 30

Erosional features—-Mountain or Alpine g la c ia t io n ...30

Depositional features— C ontinental g la c ia t io n ... 34

Glacial diversion of drainage...38 vii

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Pa g e

Features resulting from the work of w a v es...40

Features of subm ergent shorelines...42

Features of em ergent shorelines... 47

Features of neutral shorelines... 50

Features of compound sh o r e lin e s ... 52

Abandoned s h o r e lin e s ... 53

Features resulting from v u lc a n is m ...60

Cones and c r a t e r s ... 60

D ik e s ... 63

Features resulting chiefly from d ia str o p h ism ... 65

Features resulting from the work of running w a t e r ... 67

D eposits m ade b y running w a t e r ... 67

Fans, cones, and piedm ont alluvial p l a i n s ... 67

Alluvial t e r r a c e s ... 71

Flood-plain f e a t u r e s ... 75

D elta s... 79

Erosion by running w a t e r ...80

The erosion cycle— its sta g es... 81

Criteria for th e erosion stages of a single v a lle y ... 81

Criteria for th e erosion stages of a region...89

Effect of rock hardness— drainage patterns...100

Piracy and a d ju s tm e n t... 106

Evidences of more than one cycle...I l l Relation of land forms to str u c tu r e ... 123

The basis for topographic reflection of s t r u c t u r e ...123

Drainage patterns and s t r u c tu r e ...126

The effect of the stage of th e erosion cy cle... 135

Regional escarpm ents and regional d ip ... 135

The significance of th e asym m etrical ridge... 149

Lack of ridge sym m etry and am ount of d i p ...158

Topographic expression of anticlines... 164

Topographic expression of s y n c l i n c s ...173

Relation of pitching folds to topography... 175

Symm etrical and asym metrical f o l d s ... 180

Topographic expression of f a u l t s ... 185

Topographic expression of unconform ity... 196

Appendix to Part I. L ist of topographic maps used in Part I, arranged b y s t a t e s ... 206

PA R T II I n t e r p r e t a t i o n o f G e o l o g i c M a p s... 215

The nature of a geologic m a p ... 215

Horizontal beds...217

viii CO NTENTS

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Pa g e

Shape of o u t c r o p ... 217

Thickness of b ed s...218

Dipping b ed s... 220

Regional d ip ... 220

Shape of outcrop— Rules for “ V’s ” ...221

Factors determining the w idth of o u t c r o p ... 230

D eterm ination of s t r i k e ... 232

D eterm ination of d i p ...236

Determ ination of thickness... 252

F o l d s ... 259

Anticlines versus synclines...259

Sym m etry and asym m etry in f o l d s ... 264

Overturned f o l d s ...266

Pitch of fold s...267

Effect of erosion on w idth of outcrop of folds... 271

D a te of folding... 271

F au lts...272

Upthrown and downthrown s i d e ... 272

Amount of m ovem ent... 274

Types of fau ltin g... 281

D ip of fault plane...281

R elation of faults to f o l d s ... 283

Effect of faults on outcrop... 285

Effect on nearly horizontal b e d s ... 285

D ip faults and o f f s e t ... 286

Strike f a u l t s ... 289

R epetition of b e d s ... 289

Cutting out of beds... 290

Interm ediate f a u l t s ...292

Offset with o v e r la p ...292

Offset w ith gap ... 294

Faults w ith diminishing t h r o w ...297

Age of fau ltin g... 298

Igneous r o c k s ... 300

I n t r u s iv e s ... 300

Batholiths and b osses... 300

L a c c o lith s ... 302

D ikes and s i l l s ... 303

C ontact m eta m o rp h ism ... 306

E xtrusives...306

Age of igenous r o c k s ... 308

Unconformities... 313

C ONT ENT S ix

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X C ONT E NT S

Paob

Criteria for recognizing unconform ity on a m a p ... 313

D isc o r d a n c e ... 313

Missing b ed s...314

One formation resting on several fo r m a tio n s ... 315

Sedim ents resting on large bodies of igneous rock w ithout m eta­ morphism ... 316

Unmetamorphoscd rocks on highly metamorphosed rocks . . . 317

Truncated f a u l t s ... 318

Truncated d i k e s ... 319

History involved in an unconform ity ...319

The drawing of structure section s...322

Representation of structure by contours...324

Structure contour m a p s ... 324

D epth-line m a p s ...330

Contours on u n c o n fo r m itie s... 331

Hydrologie m a p s ... 333

Miscellaneous adaptations of contours...337

Geologic history of an area... 337 Appendix to Part II. Numerical list of folios and reports used in Part II 342

Index 345

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INTRODUCTION

The m aps utilized as references in this work are, for th e m ost part, those published by the United States Geological Survey.

The topographic maps are all available for purchase a t the rate of 10 cents per single copy, or 6 cents in wholesale lots, except for a few special sheets. Geological maps in the form of Folios range in price from 25 cents to SI. So far as practicable, references have been confined to maps still in print. All the m aps can be consulted in the libraries of the larger colleges and universities.

M any of them are also available in the offices of mining and oil companies.

Inquiries as to availability or price should bo addressed to The Director, United States Geological Survey, Washington, D. C., and remittances, in the form of cash, post office money order, or draft, should be sent to him. M aps reported o ut of stock can sometimes be purchased through the Superintendent of Documents, W ashington, D. C. Inquiries regarding Canadian maps should be addressed to The Director, Canadi'an Geological Survey, Ottawa, Ont.

Preceding each section is a list of maps referred to therein.

These should be a t hand for the study of the section. N ot all the maps in such a list illustrate the topic of the section, since others are inserted by way of specific contrast. R ather long supplem entary lists are added a t the end of each section, so th a t the reader m ay choose from regions of special interest.

The book is divided into two somewhat distinct sections, the first dealing with the reading of topographic, the second with geologic, maps. A knowledge of general geology and plane trigonom etry is assumed, and an understanding of plane surveying is desirable.

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INTERPRETATION OF TOPOGRAPHIC AND

G e o l o g i c M a p s

PART I

INTERPRETATION OF TOPOGRAPHIC MAPS

INTRODUCTORY STATEM ENT

I t is possible to learn m uch about the nature of the rocks and their attitu de from carefully made topographic m aps; and anyone contem plating geologic mapping of any type will find it worth while to study carefully all such available m aps of the area, both before and during field work.

The purpose of P a rt I of this m anual is to discuss, with illustra­

tions chosen from actual topographic maps, the relations which may exist between structure and topography. In order, how­

ever, to gain any adequate conception of this relationship, the observer m ust be reasonably familiar with the common agencies th a t shape land forms, and with the appearance of these forms, on topographic maps, through the various stages of their development.

After a brief preliminary treatm en t of scales, m ethods of indicating relief, and interpretation of contours and contour intervals, some space will, therefore, be devoted to a discussion of the chief land forms and their history. These discussions and illustrations should prove entirely adequate for the use of the average reader of topographic maps, and for general courses in map interpretation, b u t cannot be expected to cover the needs of the specialist in physiography, or to fill the demand for gradu-

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2 THE I N T E R P R E T A T I O N OF MA PS

ate courses in th a t subject. To include th e more specialized phases of physiographic interpretation would enlarge the book beyond suitability for its more immediate purpose, the reading of geologic structure from maps.

Finally, a word of caution m ust be added as to the reliability of available maps. On very small scale maps, with large contour intervals, particularly those made m any years ago, much signifi­

cant detail has been completely om itted, or so generalized as to be badly obscured. For this reason, structural deductions from m any of the older m aps are likely to be unreliable, or, in extreme cases, entirely impossible, in areas where th e more modern detailed m aps would yield a wealth of valuable information.

Specific instances of this will be pointed out in the following discussions.

SPECIAL FEATURES OF MAPS

DIRECTION AND ORIENTATION

M aps

Charleston School sheet, Cal.

Unless otherwise specifically indicated on a map, the top is north, and the right-hand edge, consequently, east. On maps showing m eridian lines, these, of course, represent true or astro­

nomical north. In other cases, n orth is shown by an arrow.

Ordinarily, on published maps, the indicated n orth is astronomical, not magnetic, the difference between the two, known as the magnetic declination, being indicated by the angle, expressed in degrees, between two diverging lines commonly shown on the lower m argin of the m ap (Charleston School sheet, Cal.).

The declination is slowly changing, and on the older m aps the indicated angle m ay not correspond with its present variation, which can always be determined from the isogonic chart published yearly by the U nited States Coast and Geodetic Survey.

O rientation consists in placing the m ap so th a t its directions coincide with those in the field. A simple device for determining

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FEATURES OF MAPS 3 direction if no compass is available is to point the hour hand of a watch a t the sun, the dial being horizontal, in which position a line halfway between the hour hand and twelve o’clock is approxi­

mately south. At night, north is readily determined by the position of the polar star. In cloudy weather, orientation can be easily accomplished if such features as houses, roads, or prominent landm arks can be identified. By standing a t one such known point, and m aking the line on the map between it and another known point coincide with the same line of sight on the ground to the same point, the map m ay be properly oriented.

In the field, the best results are obtained if the m ap is kept continuously oriented, while under observation. If one is driving south along a road, for instance, and trying to follow the map, it is best to keep the bottom of the m ap south, even though it is upside down to the observer. Otherwise, objects on the right side of the road show on the left of the map, and vice versa, which causes confusion in recording data. Extensive observation seems to show th a t fewer errors of location occur when maps are kept properly oriented.

SCALES

Maps

Charleston School, Cal. Ballarat, Cab—N ev.

Lehigh, Iowa Sheep River, Alta.

Lakin, K an. Chu Chua Creek, B. C.

The scale of the m ap is the ratio of a given distance, as represented on the map, to the same distance on the surface of the earth. Scales are of three common types: the graphic, the fractional, and the verbal.

Graphic Scales

A graphic scale consists of a line divided into units representing miles or fractions of a mile (kilometer, etc.) to be applied directly to the m ap in m easuring the distance. Such scales are most commonly employed on m aps designed for public school and general use, because they are most easily interpreted. They have

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4 THE I N T E R P R E T A T I O N OF MA PS

the advantage th at, in case a m ap is enlarged or reduced by photog­

raphy, the scale is proportionately enlarged or reduced, and is still correct.

A ttention should be called to the fact th a t commonly the zero of the scale is 1 mile (kilometer, etc.) from the end, the left-hand unit being finely subdivided (Charleston School sheet, Cal.).

Errors of m easurem ent not infrequently result from failure to realize this fact.

Fractional Scales

A fractional scale is the ratio, expressed in the form of a fraction, between a given distance as represented on the m ap and th a t distance on the e a rth ’s surface. I t is commonly term ed the representative fraction, or R. F., of the map. Since there are 63,360 inches in a mile, an inch to the mile, expressed as a fraction, would read:

Distance represented on the map, in inches 1 Distance on the earth, in inches 63,360

Since this is an awkward figure from which to derive multiples or divisors, the scale adopted as the United States Geological Survey standard is

K2, 500 = approxim ately 1 inch to 1 mile (Lehigh, Iowa).

H2 5,ooo = approxim ately 1 inch to 2 miles (Lakin, Kan.).

K so.ooo = approxim ately 1 inch to 4 miles (Ballarat, C al.-N ev.).

1-5 00, 000 = approxim ately 1 inch to 8 miles (state geological map of Missouri).

H ,ooo,ooo = approxim ately 1 inch to 16 m iles (m illionth map of th e world).

These scales fit the widely adopted “ m illionth” map of the world, and form a convenient series. Some of the maps issued by the Canadian G overnm ent use the above scales (Sheep River, A lta.); others employ the exact scale M3.360 and its m ultiples and divisors (Chu Chua Creek, B. C.).

In the larger scales this approxim ate series departs more widely from the inch-to-the-mile base; th a t is, M2.500 ~ M3,360>

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FEATURES OF MAPS 5

which represents the discrepancy between the approxim ate and the exact inch-to-the-mile scales, is less noticeable th an H i ,25o ~ M l,680) the discrepancy between the approxim ate and the exact 2-inches-to-the-mile scales, the latte r discrepancy being twice as large as the former. Therefore, while M l,250 is exactly twice the scale M2,500> it is commonly replaced by M i,G80> which is exactly 1 inch to H mile (Charleston School sheet, CaL).

The ratio of the map to the area mapped is greater, th a t is, each feature on the m ap is actually larger, with a large than w itt a small scale. Thus, M2)500 is twice as large as K25>ooo the former being a larger fraction with a smaller denominator.

Although in the United States the u n it 1 inch is commonly used in speaking of maps, it is to be remembered th a t the fraction is a ratio equally true w ith any u n it of measure so long as both numer­

ator and denom inator are of the same terms. This fact makes the fractional scale independent of language barriers, one of its most im portant features.

Verbal Scales

Verbally, a scale m ay be expressed as “ an inch to the m ile” ,

" a n inch to 2 m iles” , etc. Commonly, and more correctly, the distance on the m ap is mentioned first, b u t not infrequently the term s are reversed, so th a t we often hear “ a mile to the inch ” , and, as it would be absurd to interpret this as a mile on the m ap to an inch on the e a rth ’s surface, “ inch to the m ile” and

“ mile to the in ch ” are quite commonly used interchangeably, and mean the same thing.

When, however, one hears “ 2 inches to the m ile” and “ 2 miles to the in ch ” , both of which are in common usage, a t least locally, the m atter is likely to be more confusing, as will be readily seen from the following:

2 miles to the inch = 1 inch to 2 miles = 2 x ¿3 360~

2 inches to the mile = 1 inch to H m^e = x 63 360*

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6 THE I N T E R P R E T A T I O N OF MA P S

The second scale is thus seen to be four times as large as the first. This has been found to cause considerable confusion among elementary students of maps.

M ETH O D S OF R EPR ESEN TIN G TOPOGRAPHY

There are a t least three common methods of showing relief on m aps: by shading, by hachures, and by contours.

Shading

Relief m aps designed for use in public schools commonly employ various shades to indicate areas of different elevation.

This device is usually employed on small-scale m aps covering such large areas as states, countries, or whole continents. As commonly applied, it gives a graphic b u t much generalized idea of relief.

H achures

Hachures are a type of shading in which lines are used, usually running in the direction of slope, and therefore normal to con­

tours (Fig. 1). The more closely the lines are spaced the steeper the slope. Hachures are sometimes combined w ith contours to increase the vividness of impression, b u t more often are used alone, as on m any of the French and Swiss maps. Hachured maps are less exact th an those in which the relief is shown by contours, and are seldom used in this country for engineering or geological purposes.

Contours

Commonly in the United States, relief m aps for the use of geologists or engineers are printed with contours, and geological maps are commonly overprinted on such a topographic base.

Since, alm ost w ithout exception, the m aps used in this volume show relief by contours, the m atte r of contouring will be given a somewhat extended discussion in the following section.

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FEATURES OF MAPS 7

Fig. 1.— Hachures as a method of showing relief. Reproduction of part of Verdun sheet, France.

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8 THE I NT E R P R E T A T I O N OF MAPS CONTOURS AND TH EIR SIGNIFICANCE

Contour Lines M aps

Y osem ite Valley, Cal.

A contour line is a line drawn on a m ap (ordinarily in brown) through all points of equal elevation. Every contour, therefore, if viewed broadly enough, is a closed curve, or a group of closed curves. Commonly, however, the entire curve does not show on the area of a single map, in which case it should always be drawn to the edge of the sheet—never left “ hanging” . Contours are merged into one another, or superimposed upon one another, only on vertical or nearly vertical cliffs (Yosemite Valley map, Cal.) and m ay intersect only in the very rare instance of an overhanging cliff, the “ overhang” of which is far enough to be measurable on the scale of the given map. This will happen only on very large scale detailed maps, and the writers know of no single instance on the regular quadrangle sheets of the United States Geological Survey.

Ordinarily, every fourth or fifth contour is accentuated, and numbered (in brown) to facilitate reading the map.

A contour interval is the difference in elevation of two successive contour lines. I t has nothing whatever to do with the distance between contours, which is controlled by the steepness of the slope. Common intervals in use by the Survey are 5, 10, 20, 50,

Contour Intervals M aps

Charleston School, Cal.

M t. W hitney, Cal.

Apishapa, Colo.

Pueblo, Colo.

Monmouth, 111.

Lehigh, Iowa.

Lakin, Kan.

Piedm ont, Md.-W . Va.

Beaverton, Ont.

D eadwood, S. D ., 1S94 and 1901 eds.

Hermosa, S. D ., 1894 and 1901 eds.

Aldino, Tex.

C lintonville, W. Va.

Greenland Gap, W. Va.

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F E AT URE S OF MAPS 9 and 100 feet. In rare cases 1-foot contours have been used (Aldine, Tex.), and on a few m aps the interval is 25 feet (Apis- hapa, Colo.). On some of the earlier maps, 200- and 250-foot intervals have been employed (many of the Arizona and U tah sheets), and maps of an entire state, or of the United States, have been published w ith 500- or 1,000-foot intervals.

The interval is determined by the scale of the map, by the relief of the area, and, to some extent within the limits imposed by these factors, by th e degree of refinement required for the purposes to which the map is expected to be put. W ith the inch scale 0d52,5oo)> 10 (Lehigh, Iowa), 20 (Monmouth, 111.), and 50 feet (Clintonville, W. Va.) are most common, depending on relief; with the half-inch scale ( ^25,000), 20 (Lakin, Kan.), 50 (Pueblo, Colo.), and 100 feet (M t. W hitney, Cal.); with the 2-inch scale ()d}l,68o)> 5 feet (Charleston School, Cal.) is commonly used.

The larger the scale, and the smaller the contour interval, the more detail can be shown, and the more accurately should the map portray the actual relief. This is well shown by the Green­

land Gap (W. Va.) and Piedm ont (Md.-W. Va.) quadrangles.

The former, on the inch scale, with a 50-foot interval, is a resurvey of the southeast one-fourth of the latter, which is on the half­

inch scale w ith a 100-foot interval. The larger scale, the smaller interval, and the b etter grade of topographic sketching being done in more recent years, are all im portant factors in the contrast between the Greenland Gap and the Piedm ont sheets. The better quality of sketching is also well shown by the 1894 and 1901 editions of the Deadwood and Hermosa (S. D.) sheets, both of which employ the same scale and interval. The later edition shows greatly increased detail.

No contours are ever placed on a map except those th a t are a multiple of the contour interval, unless specifically so stated in the legend, or unless definitely numbered on the map, and even then the use of such contours is rare. F or example, w ith a 20-foot interval there will ordinarily be no 10-foot, 30-foot, or other contour not a multiple of 20.

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10 THE I N T E R P R E T A T I O N OF MA P S

On some maps, "interm ediate contours” , halfway between the regular ones, are drawn in fain t dotted lines where detail is to be shown th a t would not “ c a tc h ” a regular contour (Beaverton sheet, Ont.).

On a few maps, in which very rugged areas contrast w ith very flat ones, two intervals m ay be employed on the same map, since an interval th a t would show the relief on the plains area would crowd the m ap too closely in the hilly section; whereas an inter­

val suited to the m ountains would show no detail in the flat parts of the m ap. In such cases, however, both intervals are shown a t the bottom of the map, usually w ith a statem en t of the elevation a t which the change occurs. M aps w ith two intervals are commonly misleading, as they tend to obliterate the contrast between plains and hilly areas. This is particularly well brought out on the Charleston School (Cal.) quadrangle, in which the upland appears to break off suddenly into a very steep escarp­

ment on the 400-foot contour, because of the change from a 25-foot interval above th a t elevation xo one of 5 feet below. If the interval were the same throughout, no such impression of an escarpm ent would be given.

Depression Contours M aps

Ballarat, C al.-N ev. Standingstonc, Tenn.

Depressions w ithout surface outlet are indicated by closed contours on the inside of which are usually drawn hachures (Fig. 3). If such depressions have valleys cut into their sides, as in some of those on the Standingstone (Tenn.) sheet, the valleys become p a rt of the depression. In very large undrained inter- m ontane basins, such as D eath Valley or Saline Valley (B allarat sheet, Cal.-Nev.), the contours m ay be num bered in descending order to indicate the depression, instead of using hachures. A discussion of the various types of depressions is presented on pages 21-26.

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FE ATURE S OF MA PS 11 Contours and Land Form

The significance of a contour line can be best visualized by gradually immersing a small model of a well-dissected region in a tan k of water. If the lowering be done by successive units of inch, and the shore line traced on the model a t each step, it will

Fi g. 2.— Id eal sk etch an d corresponding co n to u r m ap. (A fter U. S . Geol Survey.)

be found on completing the work th a t there is a series of lines, separated by vertical intervals of Y inch, which bend upward toward valley heads and outward around spurs. I t will be possible to cross a valley on a single contour, th a t is, a t a constant level, only by swinging far up toward the valley head.

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12 THE I N T E R P R E T A T I O N OF MA P S

If these lines on the model be now projected downward or upward onto a flat surface, such as a large sheet of paper, a contour map of the model will be the result. I t will a t once be seen th a t the contours are closer together on steep slopes and more widely spaced on gentle slopes. Isolated hills will be shown by small, closed curves. These relations are well brought out in Fig. 2.

N um bering of Contours M aps

Flagstaff, Ariz. M t. Lyell, Cal.

Lakin, Kan.

Difficulties commonly arise in interpreting closed elevation and depression contours on which num bers have n ot been printed. These* are best made clear by the use of a sketch (Fig. 3) illustrating cases which can be duplicated on actual topographic sheets. For similar examples see the Flagstaff (Ariz.) and M t. Lyell (Cal.) sheets, representing volcanic cones with craters, and the Lakin (Kan.) sheet, showing sand dunes and accompanying depressions.

In the sketch (Fig. 3) the contours are lettered for conven­

ience. The student should first try num bering the contours for himself, checking himself afterw ard by the discussion.

I t is obvious th a t A is the 20-foot and B the 40-foot contour.

Since C and I m ark isolated hills next above the 40, they are each 60.

If contour I is 60 feet, the area inside of it, b u t not inside J and M , is above 60 and below 80 feet. From an area above 60 b u t below 80 feet one, therefore, goes up a hill to M , numbered SO, and down into a depression to J , numbered 60 feet. If M is an elevation contour numbered 80, the area within it, b u t not within 0 or N , is above 80 and below 100 feet. From an eleva­

tion above 80 b u t below 100 feet, one goes up a hill to N a t 100 feet, and down into a depression to O a t 80 feet.

If J is a depression contour numbered 60 feet (as already shown), then the area within it, b u t not inside K or L,

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F E AT URE S OF MA PS 13 is below 60 b u t above 40 feet. From an area below 60 b u t above 40 feet, one goes down into a depression to K , which m ust be the 40-foot contour, and up a hill to L, which m ust be th e 60-foot line.

Fi g. 3 .— E x e r c is e i n n u m b e r i n g c o n t o u r s .

Also E is a type frequently m isinterpreted. I t m ust be noted th a t E is not above C. Although within the crescent made byC, E is an independent hill next above B, as m ay be seen by imagining C to be stretched out into a long, narrow ridge, when E will be understood to be a separate hill, contour E having the same elevation as C, th a t is, 60 feet.

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14 THE I N T E R P R E T A T I O N OF MA P S

Contours D and G, which are hachured, represent depressions in an area, the general level of which is above B, 40 feet, b u t below C, 60 feet If one is a t an elevation between 40 and 60 feet, and goes down into a depression, the first contour crossed will be 40 feet; consequently, th a t m ust be the elevation num ber for contours

D and G.

The elevation contour E has already been shown to be the 60-foot line; the area within it, b u t n o t within F, therefore, is above 60 b u t below 80 feet, so th a t the first contour crossed in going into the depression F would necessarily be the 60-foot line.

Since I i is an elevation contour within C, 60 feet, it m ust be the 80-foot line.

Limits of Exactness in Reading Contours M aps

Charleston School, Cal.

Ordinary topographic maps make no claim to m athem atical precision. Critical points, such as the tops of hills, the bottom s of valleys, the base and the crest of cliffs, and points a t which slope changes notably, are usually determined rath er closely, b u t no effort is usually made to have instrum ental " s h o ts ” coincide with contours, these being interpolated by the topographer. An experienced sketcher will m ap an area w ith a few well-selected actual elevations, and produce a more accurate m ap th a n will a beginner with three or four tim es as close control. The topog­

raphers responsible for the work are commonly indicated in a small rectangle on the lower margin of the m ap (Charleston School sheet, Cal.).

The chief effort is centered on producing "expressive” as contrasted with "w ooden” topography, th a t is, on showing all features of the land forms even though elevations are only approximate. Errors of from a quarter to half a contour interval, in placing contours, are permissible, and much greater ones are fairly common, and the user of topographic maps m ust under­

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FE AT URE S OF MA PS 15 stand this fact. A high degree of precision would render the cost of topographic sketching prohibitive for m ost purposes.

Ordinarily, the sketching is most accurate in open countiy and along m ain roads, while in areas of thick woods m any small gullies are often overlooked. Sometimes on the older m aps the head of one valley is even connected with the lower course of another, a situation very confusing to the geologist who uses the minor drainage to locate himself in the field.

The elevation of any point above sea level, were the contours exact, could be told with precision on a contour line, and within an interval between contours. On slopes th a t are fairly regular,

700

000

* • 500 c 400 o

■| 300 c 200 100

0 Fio.

interpolations can be made w ith some confidence, so th a t a point halfway between the 20- and 40-foot contours will be about 30 feet. In rugged country, where slopes change rapidly, such interpolations are less dependable.

A common m isunderstanding arises, in attem p ting to determine the depth of a depression below its rim or the height of a hill above its base, because the depth or the height is not referred to a definite plane, such as sea level, b u t to an elevation which is itself subject to m isinterpretation within the limits of a contour interval; so th a t w ith a variable of an interval a t the top, and another a t the base, the lim it of error is twice the contour interval. This will, perhaps, be made clearer by a study of Fig.

4. In this figure, which represents a profile section, the numbered horizontal lines represent contours with an interval of

lA a _ /T^ X

/— \ / i

4.— Illu s tra tin g th e lim its of ex actness in read in g contours.

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16 THE I N T E R P R E T A T I O N OF MA P S

100 feet, and the irregular line represents the topographic profile across a region with three depressions A , B, and C.

The depression a t A is drawn with both solid and dotted lines, both profiles showing two contours (on a m ap m arked with hachures) within its walls. The solid profile shows a depression a trifle over 100 feet deep; the dotted profile, w ith exactly the same num ber of contours, indicates a hole nearly 300 feet deep.

Similarly, the depression a t C shows only one contour within it, and can be interpreted as only a few feet deep (solid line), or as nearly 200 feet deep (dotted line).

Depression B is so placed th at, even though it is nearly an interval deep, it does not “ catch a contour” . This should be contrasted with the one a t C (solid line), which, though only a few feet in depth, is so placed th a t it “ catches a contour” .

From the above cases, it should be clear th a t, in general, the depth of a depression cannot be positively determined on a topo­

graphic map. Since there is a variable of an interval a t the top and an interval a t the base, the lim it of accuracy is twice the contour interval. The maximum possible depth is always a num ber of intervals greater by one th an the num ber of depression contours showing, and the minimum is always a num ber of inter­

vals less by one th an the num ber of depression contours. To be specific, with three contours showing, the range is between two and four intervals; with four contours, between three and five intervals.

A m oment's reflection will show th a t in reading the height of an isolated hill, not above sea level b u t above its base, the same uncertainty exists, and the principles laid down for depressions in the preceding paragraphs also hold for such isolated elevations.

Thus, in Fig. 4, x m ay be considered to be a hill, showing two contours, the height being somewhere between a maximum (dotted line) of three intervals and a m inimum (solid line) of one interval. Also y illustrates how a hill can be nearly an interval in height and n o t “ catch a co n to u r” , and z shows how a hill only a few feet high may "ca tc h a c o n to u r” even w ith a 100-foot interval.

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F E AT URE S OF MA PS 17 DRAW ING TOPOGRAPHIC PROFILES

The geologist m ost commonly draws topographic profiles for the purpose of inserting structure. Such structure sections will be discussed in the p a rt devoted to geologic maps (pp. 322-324).

I t not infrequently happens, however, th a t the geologist draws a topographic profile for the purpose of showing only topographic relations or contrasts.

One of the common defects is undue exaggeration of the vertical scale. Of course, railroad and road profiles used extensively by civil engineers m ust often have a large vertical scale, since they m ust show small differences of elevation with a m agnitude th a t permits scaling them off the drawing. I t is also true th a t a certain am ount of exaggeration is permissible, in profiles drawn for geologic or physiographic purposes, in order to show m inute detail. On th e other hand, much exaggeration tends to give a false idea of the relative depth and width of valleys, or height and width of m ountains. Also, where structure is to be inserted in the profile, exaggeration of the vertical scale greatly distorts the apparent position of the beds (p. 322). For this reason, students of m ap interpretation should draw m any profiles to natural scale, th a t is, w ith the vertical and horizontal scales the same.

Since this usually gives much trouble to beginners, a single illustration of procedure is given. W ith an inch-to-the-mile scale, 1 inch equals 5,280 feet, or J^foo inch equals essentially 50 feet, both vertically and horizontally. A pair of dividers is useful to locate the desired points along the base line of the section. If the scale on the m ap is to be increased two, three, or four times, for instance, the distances can be easily "stepped off”

with ordinary dividers, or proportional dividers m ay be used.

If the m ap is on an inch-to-the-mile scale, and the pro file is natural scale b u t enlarged twice, 1 inch will, of course, represent 2,640 feet, and 3<loo inch about 26 feet. All other scales are worked out in the same way.

The num ber of points to be chosen in m aking a profile will depend largely on the skill of the worker. Ordinarily, tops of hills, bottom s of valleys, and all points a t which sharp changes in

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18 THE I N T ER P R E T AT I O N OF MA P S

slope occur should be plotted. I t is alm ost never w orth while to plot every contour.

The plotted points are connected by a line which represents the land surface. Here skill is required to avoid "w oo den ” topography. Only rarely do straight lines and sharp angles enter into such profiles, and the better the actual conditions on the ground are visualized the mure nearly will the profile approach those curves really characteristic of topographic slopes.

Commonly, the base line for a profile should be sea level, in order to show the relation of the area and its relief to base level, b u t in some cases the vertical scale is such as to render this impracticable, and an arb itrary base is chosen, such, for example, as 2,000 feet above sea level. Local relief is shown ju st as accurately, b u t the whole relation to perm anent base level is lost sight of.

D ESCRIBING LOCATIONS ON A MAP

M aps

Lakin, Kan. Crystal C ity, Mo.-Hl.

Cadiz, Ohio.

M ost of the quadrangles of the U nited States Geological Survey are divided by the parallels and m eridians into nine smaller rectangles, and these serve as a convenient means of locating points. Various schemes of num bering these rectangles have been used b y different teachers in the classroom. I t is, however, b u t little more trouble to nam e the rectangle th a n to num ber it, and then there can be no confusion. In these pages, the rectangles will be known as follows (given in the abbreviated form ):

N ortheast rectangle... = N E . rect.

North central rectangle... = N . cent. rect.

N orthw est rectangle = N W . rect.

E ast central rectangle... = E. cent. rect.

Central rectangle... = Cent. rect.

W est central rectangle... = W. cent. rect.

Southeast rectangle... = SE. rect.

South central rectangle... = S. cent. rect.

Southw est rectangle... = SW. rect.

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FE A T U R E S OF MA PS 19 In areas which have been sectionized, the standard land divisions constitute the best way to give locations (Fig. 5).

Ranges are num bered a t the north and south margins of the map, townships on the east and west margins. Township 2 N orth, Range 4 West, for example (commonly expressed T. 2 N.,

S t a n d a rd Parallel

O

■<---6 mi.--->- ' K

>

r

T .4 N. ! ' R . 1W. ^

T.3M.

T .2 N . R .4 W .

6 5 4 3 2 I

k CVj 64 7 8 9 10 11 12

18 17 19 15 14 13 19 20 21 22 23 24 30 29 28 27 29 25 31 32 33 34 35 36

R .4 W . R .3 W . R .2 W .

**

s*

R .1W .

CL

R1E;

B a s e Line

F ig . 5.— T h e s ta n d a rd la n d survey.

R. 4 W .), is a body of land G miles each way, divided into th irty - six sections num bered as in Fig. 5. Ordinarily, on a m ap the letters and figures used in m arking townships and ranges are placed opposite the center line, th e units extending 3 miles on each side, the 6-mile square being bounded by slightly heavier

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20 THE I NT E R P R E T A T I O N OF M A P S

lines. Beginners frequently find trouble in counting out the section, when the border of the map splits the township or range, so th a t only p a rt of the sections are shown, for instance, Lakin (Kan.) sheet, T. 29 S., R. 34 W. The sections m ay or m ay not be numbered on the map. Confusion m ay sometimes arise on maps lying in two states using different base lines or prime meridians (Crystal City sheet, M o.-Ill.), or in a state using more th an one prime m eridian or base, as is th e case in U tah. M any of the eastern and southern states are not sectionized, Texas has several local systems, and in portions of Ohio sections are num bered differently from the above scheme (Cadiz sheet,) but the system ju st outlined is the common one in the north central and western states.

CONVENTIONAL SYM BOLS (LEGEND)

The legend of a m ap consists of a list of the conventional symbols used on the map, together w ith their explanation.

Those used on the United States Geological Survey’s topographic m aps are printed in full on the back of each map, together with a brief explanation of the nature of scales, contours, and the basis used in m apping the United States. This descriptive m atter should be studied carefully by anyone desiring to become familiar with the use- of contour maps. For this purpose, one of the newer maps should be utilized, as a few of the symbols have been changed, and a few added, since the earlier m aps were made.

LAND FORM S, T H E IR DEVELOPM ENT AND RECOGNITION

The chief agents active in shaping land forms are ground water, the wind, glaciers, waves, volcanoes, disastrophism, and running water.

Of these, running w ater is responsible for a much larger propor­

tion of the fam iliar topographic forms than any other agent, and it is chiefly in those forms carved by running w ater th a t one finds the clue to structural interpretation.

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DEVELOPMENT OF L A N D FORMS 21 In these pages, therefore, consideration of land forms produced by running water—which in the more fam iliar texts on general geology finds a place after the work of wind and ground water, and before glaciers and waves—will be deferred to the final section on “ Land F o rm s”, imm ediately preceding the section on “ Relations of Land Form s to S tru c tu re ” , to which it is closely related.

TOPOGRAPHY RESULTING FRO M THE W ORK OF GROUND WATER

Maps

Frank, Alta. Lansing, Mich.

Flagstaff, Ariz. B attle Lake, Minn.

Ballarat, Cal.- N ev. Pingree, N . D .

Malaga, Cal. Portland, Ore.-Wash.

San Luis Ranch, Cal. Wind Gap, Pa.

W illiston, Fla. K ingston, Tenn.

Lakin, Kan. Standingstone, Tenn.

Meade, K an. Bristol, Va.-Tenn.

Bowling Green, K y. Quincy, Wash.

Lockport, K y. Clintonville, W. Ya.

B ay C ity, Mich. W hite Sulphur Springs, W. Va.

Although the work of ground w ater is a very vital phase of geology, it plays a relatively subordinate p a rt in shaping topog­

raphy, except in a few regions; its one im portant contribution to the production of surface features being the development of sink holes. The Standingstone (Tenn.), Bristol (Va.-Tenn.), Bowling Green (Ky.), Williston (Fla.), and Clintonville and W hite Sulphur Springs (W. Va.) sheets all show excellent examples of such sinks.

Depressions m ay originate in several other ways. They occur among sand dunes as a result of the scouring of wind or the unequal deposition of sand (Lakin sheet, Kan., p. 28); they are common in glaciated areas as a result of the melting of buried blocks of ice, or of the unequal deposition of drift (Lansing sheet, Mich., p. 34); on very recently emerged coastal plains, original depressions of unequal deposition on the ocean bottom m ay still persist, and pothole action on stream s m ay produce

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22 THE I N T E R P R E T A T I O N OF MA P S

them, though the latte r type is usually no t large enough to be represented by contours on topographic maps. Exceptionally, such depressions, resulting from abandoned waterfalls, are clearly to be seen, as “ The P otholes” , on the Quincy (Wash.) sheet, and elsewhere on the Columbia Plateau.

Depressions m ay also result from various phases of volcanic activity, such as the uneven original surface of lava flows or ash deposits, from the damming of valleys by lava, or from craters.

Of these types, the latte r are the only ones known to show in typical form such th a t they can be identified on topographic sheets (Flagstaff, Ariz., p. 61).

Depressions from unequal alluviation (p. 76), are very common on flood plains, particularly in the form of p artly silted channels of old oxbows (San Luis Ranch sheet, Cal.), and m ay usually be recognized by their form and relation to the present stream.

Long, narrow depressions m ay also occur between beach ridges (p. 46) on a prograding shore line (Bay C ity sheet, Mich.), and m ay closely resemble in shape some of those resulting from partly silted oxbows. Depressions are also common in landslide topography, b u t are usually not large or deep enough to be shown on ordinary contour maps. A particularly good example, how­

ever, is shown on the F rank (Alta.) sheet. Depressions may also result from earthquake subsidence, b u t no good m aps are known which show this type by means of contours.

Artificial depressions are also common. Those resulting from mine subsidence closely resemble sink holes, b u t contour maps of such do not appear to be available. Open-pit mines or quarries are also a common type (Wind Gap sheet, Pa.), as are also those caused by railroad or road fills (Portland sheet, Ore.-Wash.).

Interm ontane basins of large proportions, resulting prim arily from down-warping or down-faulting of considerable areas ip. 67), m ust also be classed among the various types of depressions, though not ordinarily shown by hachured contours

(Ballarat sheet, Cal.-Nev.).

I t m ust be obvious th a t m any isolated depressions occur which cannot, from the map alone, be referred to their proper origin;

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DEV ELOPMENT OF L A N D FORMS 23 and in a few cases there m ay even be some doubt attaching to the origin of whole groups of depressions. B u t there are certain criteria which can be used to good advantage, in discrim inating between these types in map study.

A t Sinking Cane, and also a t M orrison Cove, in the south half of th e central rectangle of the Standingstone (Tenn.) sheet several perm anent stream s of considerable size flow into the depressions and disappear. A similar situation is to be noted on the Bristol (Va.-Tenn.) and Clintonville (W. Va.) sheets. There is no surface outlet, and, since the inflow is greater th an can plausibly be accounted for on the basis of seepage, or of evapor­

ation in a region obviously humid, these disappearing stream s constitute evidence th a t the depressions are sinks, w ith under­

ground outlets.

G reat num bers of depressions will usually indicate some one of the three more common types—those of wind, glacial, or solution origin—though alluviation types are sometimes num er­

ous. The entire absence among such numerous depressions of large num bers of small hills, such as m ark dune and moraine topography, is also indicative of sink holes.

The depth of depressions is sometimes significant; those greater th an 100 feet can usually be referred w ith considerable confidence either to sink-hole origin, or to volcanic craters, the latter ordinarily being easily distinguished because of their position within a definite cone. Wind-scoured depressions of th a t depth are rare indeed, except possibly in tru ly desert areas, in which other evidences of aridity would be present. K ettle holes in moraine topography rarely atta in such depths, and are commonly associated w ith lakes; original depressions on coastal plains are probably rarely more th a n a few feet deep, and are always associated w ith a rath er youthful stage of the erosion cycle, since they soon disappear, w ith the development of a normal drainage system. All depressions of alluviation, landslide, or earthquake origin are usually shallow, and deep artificial depressions are rare and usually isolated. Interm ontane basins of great depth are also usually of large area, and quite commonly of considerable aridity.

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24 THE I N T ER P R E T AT I O N OF MA P S

On the Bristol (Va.-Tenn.) quadrangle, the depressions show a marked tendency to alignment, as a result of the fact th a t the rocks are folded, so th a t the limestones outcrop in long, narrow belts. Such an alignment of depressions is rarely found except in folded regions, and its occurrence is usually indicative of the sink-hole origin of the depressions. Shallow depressions, how­

ever, m ay be aligned along old channels, as on the Malaga, (Cal.), Pingree (N. D .), or B attle Lake (Minn.) sheets.

I t is entirely possible th at, in a dune area, a moraine area, or a newly emerged coastal plain, solution of underlying soluble rock m ay produce sinks among other types of depressions; dunes may occur in areas of glaciation, in close proxim ity to moraines;

or the characteristic features of any type of depression m ay be so poorly developed th a t it is not possible, from the m ap alone, to discriminate between the types. B u t in m any cases such discrimination is easy, from the topographic contour alone.

Since limestone is the m ost widely distributed soluble rock, th e common inference is th a t sinks indicate limestone areas;

b u t they m ay develop in areas where either gypsum or salt beds occur beneath the surface. So far as known, sinks resulting from the solution of salt or gypsum beds do not occur east of the Mississippi River, in the United States, b u t are shown on the Meade (Kan.) sheet, where they are associated w ith obvious dune topography, so th a t it is impossible to say w hat proportion of the depressions are related to the dunes, and w hat are sinks.

Several of the depressions are occupied by tem porary lakes, into which in term itten t stream s em pty. The depression contours are not hachured, and can be recognized only by the numbers, or by the enclosed lakes.

An interesting deduction on the Lockport (Ky.) sheet is th a t, since the sink holes all occur on th e lower ground along the river, the hills are probably capped with an insoluble rock, such as shale or sandstone, and the deeper valleys have cut into and partly uncovered an underlying limestone.

The Kingston (Tenn.) sheet shows a large sink, Grassy Cove (W. cent, rect.), which, a t least on the older maps, is represented

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DEVELOPMENT OF L A N D FORMS 25 not by hachured lines b u t by ordinary contours, with the depth a t the lowest point shown in a brown figure. This sink is particularly interesting, in th a t it occupies a position within an anticline, the central p a rt of which is eroded deeply enough to expose a lower and older limestone, surrounded by an inward- facing escarpm ent (p. 166) of more resistant rock.

The depth of sink holes varies from a few feet to more th a n 200 feet (Sinking Cane, Standingstone sheet, Tenn.). The rim of Sinking Cane is above 1,300 b u t below 1,400 feet, while its lowest point is below 1,100 b u t above 1,000 feet. Assuming th a t the rim is ju st above 1,300 and the base ju st below 1,100, the depth m ay not be much more than 200 feet. On the other hand, if the rim were alm ost up to 1,400 and the base alm ost low enough to “ c atch ” the 1,000-foot line, its depth m ight be as great as nearly 400 feet.

On the other hand, the small depression ju st south of Peekville (SW. rect.), the depression contour for which is not numbered, m ay be very shallow. Since it lies on a plain between the 1,000 and 1,100-foot contours, one first crosses the 1,000-foot line when descending into the sink, hence th a t is the elevation of its depression contour. If the rim, which lies between 1,000 and 1,100 feet, is alm ost 1,100, and its base, which lies below 1,000 b ut above 900, is nearly as low as 900 feet, it m ight reach alm ost 200 feet in depth. On the other hand, if its rim were b u t a little above 1,000 feet, and its base b u t little below th a t line, it m ight be only a few feet deep, b u t be so placed as to “ catch a contour” .

On the Bristol (Va.-Tenn.) sheet are numerous examples of sink holes so situated th a t they do not happen to “ catch a con­

to u r” . They are indicated by little blue dots into which streams are seen to flow. All th a t can be said of their depth is th a t they are less th an a contour interval (100 feet). They m ay actually be deeper than other sinks th a t “ catch a contour” (see pp. 15- 16 and Fig. 4).

I t is of interest to note th a t in limestone regions of extensive caves there are m any cases of the diversion (capture) of surface streams into underground channels (subterranean stream piracy).

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