(au) 

Explanatory Text to Accompany 

The Geologic Map of the 

United States 

By PHILIP B. KING and HELEN M. BEIKMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 901 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON :1974 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog - card No. 74-600169 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 — Price $1.25 (paper cover) 

Stock Number 2401-02573 
CONTENTS 

Introduction 

Page 

1 The Geologic Map of the United States—Continued 

Page 

Previous geologic maps of the United States 1 Contents of the Geologic Map 20 

Sources of information 1 Classification of the rock units 21 

Maps published before 1860 1 Symbolization of rock units 25 

Maps between 1860 and 1880 7 Representation of faults 28 

Maps between 1880 and 1930 7 Representation of contacts 31 

The Geologic Map of the United States of 1932 9 Subcrop geology 31 

The Geologic Map of the United States 10 Northern Interior States 31 

History of the present project 10 Eolian deposits 35 

Sources of the Geologic Map 11 Atlantic Coastal Plain 36 

Uses of the Geologic Map 18 Radiating strikes 36 

Methods of compilation 18 References cited 38 

ILLUSTRATIONS 

Page 

FIGURE 1. Geology of the United States as represented by Maclure (1817) 1 

2. Geology of the United States and adjacent parts of Canada as represented by Lyell (1845) 4 

3. Geology of the United States and adjacent parts of Canada as represented by Marcou (1855) 5 

4. Geology of the United States and adjacent parts of Canada as represented by Edward Hitchcock (1854) 6 

5. Index map showing areas represented geologically on McGee map of 1885, areas added or revised on the McGee map of 1894, 

and additional coverage based on less exact information on the C. H. Hitchcock map of 1887 8 

6. Index map of Texas, showing areas covered by different sources used on the Geologic Map 17 

7. Index map of the United States, showing areas covered by the different sources used for the subsea contours on the Geologic 

map 19 

8. Geologic maps of the Sandia Mountains, New Mexico, to illustrate the process of generalizing data for the Geologic Map of 

the United States 22 

9. Map of the Great Basin in Nevada and Utah, showing regional extent of major low-angle thrust faults that are represented 

on the Geologic Map of the United States as exposed fragments in the mountain areas 29 

10. Circular faults shown on the Geologic Map of the United States 30 

11. Map of eastern South Dakota, to illustrate problems of representing bedrock geology in areas with extensive cover of surficial 

deposits 32 

12. Generalized geologic map of eastern Middle Western States, to show relations of subcrop geology to preglacial river 

systems 33 

13. Geologic map of northern Minnesota, showing the extent of thin Upper Cretaceous deposits (Coleraine Formation) that are 

not represented on the Geologic Map of the United States 34 

14. Map of western Nebraska, showing bedrock geology as represented on the Geologic Map of the United States, superposed on 

which are the areas of Quaternary sand dunes and drifted sand (Sand Hills Formation) as represented on the Geologic 

Map of the United States of 1932 and by Thorp and Smith (1953) 35 

15. Geologic map of the Atlantic Coastal Plain in Maryland, Delaware, and New Jersey, showing the relation of the bedrock 

units that appear on the Geologic Map of the United States to surficial deposits of Quaternary and late Tertiary age 37 

16. Map of the southern Midcontinent region in Oklahoma, Arkansas, and Texas, showing "radiating strikes" in Paleozoic and 

Cretaceous rocks 38 

TABLE 

TABLE 1. Comparison between "American" and "International" systems of coloring stratified rocks on maps 

Page 

27 

HI 

EXPLANATORY TEXT TO ACCOMPANY THE 

GEOLOGIC MAP OF THE UNITED STATES 

By PHILIP B. KING and HELEN M. BEIKMAN 

INTRODUCTION 

The U.S. Geological Survey has published a new 

Geologic Map of the United States (exclusive of Alaska 

and Hawaii) on a scale of 1:2,500,000, which was compiled between 1967 and 1971 by Philip B. King and 

Helen M. Beikman, with geologic cartography by Gertrude J. Edmonston. The map replaces the now outdated 

Geologic Map of the United States on the same scale, 

which was compiled by George W. Stose and Olof A. 

Ljungstedt and was issued by the U.S. Geological Survey in 1932. 

This report is intended to supplement the new map 

and to provide background information to assist its user 

in interpreting it. It describes the historical antecedents 

of the map and the sources from which the map was 

compiled and discusses various general topics related to 

it. Succeeding reports will amplify the necessarily brief 

descriptions of the map units which appear in its legend 

and will deal at length with specific geological problems 

in the United States, insofar as they relate to representation of the features in map form. 

PREVIOUS GEOLOGIC MAPS OF 

THE UNITED STATES 

SOURCES OF INFORMATION 

Source data for previous geologic maps of the United States are plentiful, so we have chosen here to 

present a narrative account, describing the circumstances under which the maps were prepared and commenting on their more interesting features, rather than 

list details which the reader can find in the published 

sources. Maps that appeared before the mid-1880's have 

been listed and annotated by Marcou and Marcou (1884, 

p. 23-32) and have been described at length by C. H. 

Hitchcock (1887); Jillson (1950) has extended the listing to 1946. In our account we have ignored many maps 

that appear in these published lists as being merely 

reprints in the same or slightly different form by a 

single author, or copies of such maps in textbooks and 

other media. Much information on the circumstances of 

geologic maps published by the U.S. Geological Survey 

can be found in the Annual Reports of the Survey. Interesting contemporary reviews of some of the maps are 

cited in "Geologic literature on North America, 

1785-1918" (Nickles, 1923). For our narrative, we have 

obtained background information from Merrill's "Contributions to the history of American Geology" down to 

1880 (1906), and from biographies of later geologists, 

such as the Memorials of the Geological Society of 

America, Darrah's "Powell of the Colorado" (1951), 

Stegner's "Beyond the Hundredth Meridian" (1954), 

and Willis's autobiographical "A Yanqui in Patagonia" 

(1947, especially p. 30-35). Copies of most of the maps 

referred to here are in the files of the Library of the U.S. 

Geological Survey, and we are indebted to Mark Pangborn, curator of these maps, for his generous assistance. 

MAPS PUBLISHED BEFORE 1860 

Efforts to portray on a map the geology of what is now 

the United States extend back more than two centuries. 

The first recorded attempt is a "Mineralogic map, showing the nature of the terrains of Canada and Louisiana" 

("Carte minéralogique où l'on voit la nature des terrains du Canada et de la Louisiane"), by the French 

geologist Jean Etienne Guettard, published in 1752, at 

a time when a large part of the region was still French 

territory. Whether he visited North America is not certain, and most of his information was compiled from 

reports of French officers. A belt of marl and clay is 

shown extending from the Gulf of Mexico to Cape Breton Island, and thence inland toward Quebec. Between 

it and the coast is a sandy belt, and west of it a schistose 

and metalliferous belt. Different signs and annotations 

indicate the places where rocks and minerals were reported between the Atlantic Coast and the Rocky Mountains. 

Aside from this primitive effort, the first geologic map 

of the United States is that published by William Maclure in 1809, of which a revised version appeared in 

1817 (fig. 1). Maclure was a Scotsman who came to 

America as a merchant and after his retirement became 

interested in the sciences; for 22 years he was president 

of the Philadelphia Academy of Natural Sciences. To 
95° 85° 80° 75° 70° 65° 

Primitive Rock 

Transition Rock 

Secondary Rock 

50° 

45" 

35° 

A line to the westward of which 

has been found the greatest 

part of the Salt and Gypsum 

250 

2 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

FIGURE 1.—Geology of the United States as represented by Maclure (1817). Original map in color. Note: To facilitate comparison of figures 

1 through 4, the original geological representations have been replotted on the same projection and base. State and national boundaries 

are retained as they existed at the times of publication; in figures 3 and 4 the geography west of the Mississippi River and within the 

coastlines is retained as on the originals. 
PREVIOUS GEOLOGIC MAPS OF THE UNITED STATES 3 

assemble his map, he traveled widely through what was 

then the United States, and especially the part east of 

the Mississippi River. Both editions of his map were 

accompanied by an explanatory text, including "remarks on the effect produced on the nature and fertility 

of the soils by the decomposition of the different classes 

of rocks." 

In accord with the prevailing thinking of his day, 

Maclure classified the rocks on Wernerian principles, 

dividing them into Primitive, Transition, Secondary or 

Floetz (including a unit of Old Red Sandstone), and 

Alluvial. On the map of 1817, a line is marked along the 

Appalachians "to the westward of which is found the 

greatest part of the Salt and Gypsum." In modern terms, 

his "Primitive Rock" corresponds to the Precambrian 

and other crystalline rocks of the Adirondack Mountains, New England, and the Piedmont Province; his 

"Transitional Rock" to the folded Paleozoic of the Appalachians; his "Secondary Rock" to the flat-lying 

Paleozoic farther west; his "Old Red Sandstone" to the 

Triassic Newark Group; and his "Alluvial Rock" to the 

Cretaceous and Tertiary deposits of the Coastal Plain. 

No significant geologic maps of the whole United 

States appeared for many years after Maclure's publication, but important maps of parts of the region were 

made. The most notable was that by James Hall which 

accompanied his classic Part 4 of "Geology of New York" 

(1843), dealing with the western part of the State and 

establishing the fundamentals of Paleozoic stratigraphy in a large part of the country. The map includes 

not only Hall's survey in New York but also his reconnaissance observations farther west and represents in 

fair detail the Northern States as far south as Virginia 

and as far west as the Mississippi River on a scale of 

1:1,850,000. In addition, geology was also sketched on 

maps showing the routes of some of the exploring expeditions, such as that of Major S. H. Long's expedition to . 

the Rocky Mountains (James, 1823), and David Dale 

Owen's to the northern Middle Western States (1843). 

In 1845, Sir Charles Lyell published an account of his 

epochal travels in North America in 1841 and 1842, 

which was accompanied by a "Geological Map of the 

United States, Canada, etc., compiled from the State 

Surveys of the U.S. and other sources" on a scale of 

1:7,620,000 (fig. 2). (The sources of the map are described at length at the end of the book: v. 2, p. 198-219.) 

Wernerian concepts had by now disappeared, and the 

rocks were divided into conventional systems and series 

(Hypogene, Potsdam, Lower Silurian, Upper Silurian, 

Devonian, Coal Measures, New Red Sandstone, Cretaceous, Eocene, Miocene, and others). These are shown 

in much detail westward as far as the Mississippi River, 

and more vaguely for several hundred miles farther 

west. The map illustrates vividly the improvements 

that had been made in representation since the last 

Maclure map of 1817, as a result of geological mapping 

in the United States during the intervening 38 years. 

Between 1845 and 1853 the territory of the United 

States was extended northward, southward, and westward to its present conterminous limits by various acquisitions, which greatly expanded the field for geological exploration and mapping and also enlarged the 

problem of making a geological map of the United 

States. 

Between 1853 and 1858, Jules Marcou produced a 

succession of geological maps of the United States, the 

later ones extending to the Pacific Coast (fig. 3). Marcou 

was a Frenchman, who came to this country as a protege 

of Louis Agassiz and became a controversial figure. His 

representation of the western country was based in part 

on his service with some of the exploring expeditions for 

the Pacific Railroad, but to an even greater extent on 

freehanded extrapolation and speculation. His maps 

received harsh reviews from his none-too-friendly 

American colleagues (Hall, 1854; Blake, 1856), one of 

whom stated that "there is here a disregard of published 

results and an audacious attempt at generalization that 

has seldom been equalled." Viewed from a distance of 

more than a century, one can deplore Marcou's failure to 

use available data yet commend his bold attempt to 

present the general geological aspect of the western 

country, which his contemporaries had been reluctant 

to do. 

James Hall, one of Marcou's critics, in collaboration 

with J. P. Lesley, compiled a geological map of the 

region west of the Mississippi for the report of the 

United States and Mexican Boundary Survey (Hall and 

Lesley, 1857), based not only on the results of the boundary survey, but also on the Pacific Railroad surveys 

and other expeditions. Their map represented only the 

areas of outcrop that had been identified or reasonably 

inferred and left the remaining areas uncolored. Thus, 

no regional picture emerges, such as the one attempted 

by Marcou. 

Less commendable than these was a contemporary 

map of the United States by Edward Hitchcock, professor of geology at Amherst College, which accompanied 

his "Outlines of the geology of the globe, and of the 

United States in particular" (1854) (fig. 4). This map 

was made by combining Lyell's geologic map of the 

eastern part of the country with the representation of 

the western part from Boué's "Geological Map of the 

World," with a few emendations—with such absurd results that the map would not deserve notice except for 

the eminence of its author. 
35° Coal Measures 

85° 65° 95° 75° 70' 

 EXPLANATION OF 

THE COLOURS 

Alluvium and Post-Pliocene 

Miocene 

Eocene 

Cretaceous 

Coal (Oolite?), Virginia 

New Red Sandstone and Trap 

40° 

Carboniferous Limestone, 

and Gypsum of Nova 

Scotia 

Hudson Rivers, Utica, etc. 

Limestone of Trenton, etc. 

Old Red Sandstone or 

• Devonian 

Hamilton Group 

30' 

0 

0 

 

25' 

Potsdam Sandstone, etc. 

Sandstone of Lake Superior 

(age undetermined) 

500 MILES 

500 KILOMETRES 

N11 

Helderberg Series 

12 

Onondaga Onondaga Salt Group 

Hypogene (Granite, Niagara and Clinton Groups 

Gneiss, etc.) 

45° 

4 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

FIGURE 2.—Geology of the United States and adjacent parts of Canada as represented by Lyell (1845). Original map in color. 
125 120° 115° 1 1 0 ' 105° 100° 95 90° 85° 75° 70° 65° 

EXPLANATION OF 

THE COLOURS 

0 

Modern terrane 

Tertiary terrane 

Cretaceous terrane 

Jurassic terrane 

Silurian terrane Terrane of New Red 

Sandstone 

Eruptive and Metamorphic rocks 

1 

Upper Carboniferous 

Coal Measures 

35 

Copriferous trap 

Volcanic rocks 

500 MILES 

500 KILOMETRES 

Lower Carboniferous 

Mountain Limestone 

Devonian terrane 

45 

FIGURE 3.—Geology of the United States and adjacent parts of Canada as represented by Marcou (1855). Original map in color. 

O 

C) 
Gypsum Formation Upper Silurian System 500 MILES 

110° 05° 100° 95' 90' 85° 125 120° 115' 

75° 70° 65° 

THE COLOURS 

1 

Alluvium and Tertiary 

Miocene Tertiary 

Eocene Tertiary 

Cretaceous Formation 

Oolltk Sandstone 

et* 

Lower Silurian 

(Cambrian) 

New Red Sandstone 

(Trim) 

•—• 1 — 

LOUIS( A NI 

Hyposoic and immnise. 

phi< redo, with granite. 

syenite, and porphyry 

Kf.k 

Igneous mcks of the Tertiary and alluvial periods 

K7NI 

Coal Measures 

Carboniferous 

Limestone 

14 

Trap in Sandstone 

=5V. 

Old Red Sandstone 

(Devonian) 

0 500 KILOMETRES 

FIGURE 4.--Geology of the United States and adjacent parts of Canada as represented by Edward Hitchcock (1854). Original map in color. 

rn 
PREVIOUS GEOLOGIC MAPS OF THE UNITED STATES 7 

MAPS BETWEEN 1860 AND 1880 

After the Civil War period, notable improvements 

were made in geological map publishing, as color 

lithography replaced the former laborious method of 

coloring printed geological maps by hand. Also, representation of the western country passed from the 

realms of fantasy to fact as a result of mapping by the 

Territorial Surveys and other official organizations. 

A noteworthy product of this period is the geologic 

map (scale 1:1,584,000) that accompanied Sir William 

Logan's report on "The Geology of Canada" (Logan and 

others, 1863; the map is dated 1866, but was not issued 

until 1869). It included not only Canadian territory, but 

also the part of the United States north of the fortieth 

parallel and east of the ninety-sixth meridian, based on 

data supplied by James Hall.' 

As a result of the new surveys assembling a reasonably expressive geologic map of the whole country became possible. Compilation of such a map on a scale of 

1:7,000,000 was made by Charles H. Hitchcock and 

William P. Blake and appeared in various official reports, notably in the "Statistical Atlas of the United 

States" that accompanied the report of the Ninth Census of 1870 (1874), a volume which also contains an 

explanation by the compilers of their sources and 

methods. Hitchcock was the son of Edward Hitchcock 

and was himself an eminent New England geologist; 

Blake had had long experience in western exploration 

and was at the time professor at California College (the 

predecessor of the University of California). Aside from 

the many virtues of the map, one can note adversely 

that they assigned the granites and other plutonic rocks 

in the Sierra Nevada and eastward into the Great Basin 

to the "Archean"; this echoed the conclusion of the 

geologists of the Fortieth Parallel Survey and many 

contemporaries, even though a reviewer (Anonymous, 

1873) had requested that those in the Sierra Nevada be 

transferred to the Triassic and Jurassic. More curious is 

the complete omission of the Idaho batholith, or broad 

granitic terrane, of central Idaho; its area is represented 

as being geologically like the Great Basin, consisting of 

half a dozen strips of Cambrian and Archean rocks, 

separated by strips of Cenozoic. 

Hitchcock himself also published privately a geologic 

wall map of the United States (1881) on a scale of 

1:1,226,200, measuring 13 feet long and 8 feet high—

the largest geologic map of the whole country that has 

ever been issued. Although the geographic base of this 

map is much more detailed that that of the smaller 

geologic maps by Hitchcock and Blake, the geologic 

'This map is highly praised by C. H. Hitchcock (1887, p. 478-481), who notes that it was 

omitted from the listing by Marcou and Marcou (1884), and comments that this "must be 

compared to the celebrated performance of Hamlet where, owing to infelicitous circumstances, the part of Hamlet was omitted!" 

representation shows no greater refinement, nor indeed 

was any possible from information available at the time 

(compare Anonymous, 1881). 

MAPS BETWEEN 1880 AND 1930 

In 1882, 3 years after the U.S. Geological Survey was 

organized, it was instructed by Congress "to complete a 

geological map of the United States." This gave the 

Survey authority to conduct geological investigations 

in all parts of the country, and it also obligated the 

Survey to prepare a national geologic map. In the 

summer of 1883, Director J W Powell instructed W J 

McGee to compile such a map in time for Congressional 

hearings the following spring; the map was published in 

the Fifth Annual Report of the Survey (McGee, 1885b) 

on a scale of 1:7,115,000, with the title "Map of the 

United States exhibiting the present status of knowledge relating to the areal distribution of the geological 

groups." Although the published map states that it was 

"compiled by W J McGee," he gives generous credit in 

his administrative report to the assistance of C. H. 

Hitchcock for his "experience and skill in geologic cartography, his extended personal knowledge of American terranes, and his familiarity with American geological literature" (McGee, 1885a, p. 35). 

On McGee's map the two-thirds of the country east of 

the one hundred and third meridian is completely colored, but in the western third only the areas mapped by 

the various Territorial Surveys are colored, the remainder being left blank (fig. 5). As McGee explains 

(1885a, p. 38), 

Much of the western part of the United States remains unexplored 

geologically; repeated efforts were made to gain access to the unpublished material of the now suspended Geological Survey of California, 

and to establish correspondence with the State Geologist of Oregon, 

but without success; the maps prepared by the earliest western explorers can seldom be accurately coordinated with those recently published, either geographically or geologically; and it became necessary 

to leave the following States and Territories either partially or wholly 

uncolored: Arizona, California, Idaho, Montana, Nevada, New Mexico, Oregon, Texas, Utah, Washington. 

On completion of this work for McGee, Hitchcock 

obtained permission from Director Powell to fill in the 

remaining western part of the map from less exact data, 

and the results were published in the Transactions of 

the American Institute of Mining Engineers (Hitchcock, 1887), with an explanatory text. His additions to 

the Survey map closely resemble the representation on 

the earlier maps by Hitchcock and Blake, but there are 

changes and refinements. 

In 1894 the U.S. Geological Survey published a revised version of the official map, again with the authorship of McGee and on the same scale as before, entitled 

"Reconnoissance map of the United States, showing the 

distribution of the geologic systems so far as known." 
125° 120° 115° 110° 10S° 100° 95° 90° 80° 65° 

Area shown on McGee 

map of 1884 (U.S. Geol. 

Survey 5th Ann. Report) 

Additional areas shown on 

McGee map of 1893 (U S 

Geol. Survey 14th Ann. 

Report) 

Approximate areas of major corrections on map 

of 1893 

Areas added from less 

exact information on 

Hitchcock map of 1886 

(Trans. Am. Inst. Min. 

Eng. volume 15) 

E 

Unmapped areas 

25' 

FIGURE 5. — Index map showing areas represented geologically on McGee map of 1885, areas added or revised on the McGee map of 1894, and additional coverage based on 

less exact information on the C. H. Hitchcock map of 1887. 
PREVIOUS GEOLOGIC MAPS OF THE UNITED STATES 9 

Important improvements were made in the previously 

colored area east of the one hundred and third meridian, 

especially in the Great Plains from Kansas to Texas, 

and in the Appalachians. Parts of the area farther west 

that had hitherto been left blank were filled in, especially in the Sierra Nevada and elsewhere in California, 

but based on new mapping by U.S. Geological Survey 

personnel, as the results of the Geological Survey of 

California had never been obtained. Unfortunately, the 

compilers of the revised map chose to group the volcanic 

and plutonic rocks in the Sierra Nevada and elsewhere 

in the west into a single "igneous" unit, thus ignoring 

fundamental distinctions for which many data were 

already available. Representation of the bedrock in the 

northern tier of States and Territories was also obscured by overprinting a pattern of glacial deposits. 

When McGee transferred to the Bureau of American 

Ethnology in 1894, responsibility for national geologic 

maps devolved on Bailey Willis as Map Editor. In 1895 

his staff was augmented by George W. Stose as geologist 

and Olof A. Ljungstedt as cartographer. Shortly afterwards, when Willis became Geological Assistant to Director C. D. Walcott, Stose became Map Editor; 

nevertheless, Willis and Stose continued their collaboration for many years. Willis was part of a Survey 

committee on a Geologic Map of the United States, and 

plans were formulated for a new map which was to be on 

a scale of 1:2,500,000. Stose assembled a manuscript 

copy of such a map which formed part of the Survey 

exhibit at the Louisiana Purchase Exposition in St. 

Louis in 1904, but attempts to put it into more permanent form were hindered because of lack of an adequate 

geographic base and the need for more large-scale 

geologic maps of the States to serve as source material. 

Also, the impending Tenth International Geological 

Congress to be held in Mexico in 1906 indicated the need 

for a Geologic Map of North America, and Willis and his 

assistants quickly produced a preliminary version of 

this map on a scale of 1:5,000,000 with the cooperation 

of the Governments of Canada and Mexico, which was 

published by the Congress as "Carte Géologique de 

l'Amerique du Nord" (Willis, 1906). It then appeared 

more desirable to perfect this preliminary rendering of 

North American geology than to continue on the proposed Geologic Map of the United States. An improved 

version of the Geologic Map of North America was virtually completed by 1910 and published in 1911 under 

the authorship of Willis and Stose; it was also included 

as a companion to Willis' monumental "Index to the 

Stratigraphy of North America" in Professional Paper 

71 (1912). 

On the Geologic Map of North America of 1912 extensive areas north and south of the United States could 

not be adequately represented on account of lack of 

geological knowledge, and some areas in Alaska, northern Canada, and Central America were left uncolored. 

However, the geology of the United States and southern 

Canada were shown in much detail; the part in the 

United States no doubt included the data thus far assembled for the postponed Geologic Map of the United 

States. For the succeeding 20 years the North America 

map was the standard reference work for United States 

geology—including King's student days between 1920 

and 1929. 

THE GEOLOGIC MAP OF THE UNITED STATES OF 1932 

For a considerable period after Willis left the Survey, 

Stose had to devote his efforts to the preparation or 

editing of State Geologic Maps on larger scales, although the eventual objective of a Geologic Map of the 

United States was not forgotten. Actual compilation of 

this map began in 1927 and was accelerated by the 

decision of the Fifteenth International Geological Congress held in South Africa in 1929 to hold its Sixteenth 

Congress in the United States in 1933. Work proceeded 

with sufficient rapidity that printed copies of the map 

were distributed to participants of this Congress in the 

summer of 1933 (but with a publication date of 1932). 

Stose assumed primary responsibility for preparation 

of the map. He compiled the Appalachian part, in which 

he had long been interested,and supervised the compilations of his associates; initial compilations of many 

areas outside the Appalachians were made by 0. A. 

Ljungstedt, who was not a professional geologist but 

who had had long experience as a geologic cartographer 

in the Map Editor's office. Stose traveled widely to obtain manuscript data, especially from State Maps that 

were in process of compilation. Nevertheless, adequate 

source maps were still lacking for much of the northwestern part of the country, so Stose and Ljungstedt, 

with the aid of local specialists, made original compilations of Nevada, Idaho, Oregon, and Washington on 

scales of 1:1,000,000 or larger. In addition, Anna I. 

Jonas (later Mrs. G. W. Stose) was added to the staff to 

complete a reconnaissance of the Piedmont province 

which she had already begun in connection with preparation of a Geologic Map of Virginia. 

The resulting map, attractively printed in many colors, served as a reference work on the geology of the 

United States for the succeeding forty years; it was 

reprinted in 1960 when the stock of the original printing was exhausted. The map represents the best summary that could be made in its time, not only of the areal 

geology of the country, but also of the prevailing geological philosophy. Any apparent imperfections that we 

might now see in the map should be viewed in this 

context. 
10 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

Many geologic features of the country were poorly 

coordinated at the time; consequently greater emphasis 

was given to rock-stratigraphic that to timestratigraphic units. The geology is treated in terms of 

nine geological subdivisions or provinces, shown on an 

index map, for each of which there is a separate legend. 

The sequences in some of the provinces are very 

different—for example, those in the Lake Superior region and the Coastal Plains—but others partly overlap 

in age, and correspondence between these from one 

legend to another is not always clear. 

Some of the stratigraphic classifications have 

changed since 1932, resulting in improvements in representation not possible at the time. Thus, the "Carboniferous System" is now divided into the Mississippian, Pennsylvanian, and Permian Systems, creating 

changes in letter symbols, coloring, and even to some 

extent in geological concepts. Also, separation of the 

Paleocene from the Eocene has clarified relations in the 

northern Rocky Mountains and Great Plains, where the 

two series have different depositional patterns and 

areal distributions; it has also disposed of the so-called 

"Laramie question" that had plagued American geology 

since the days of the Hayden Survey (Merrill, 1906, p. 

647-658), traces of which still lingered in 1932. 

Many improvements have also been made in correlation of the nonfossiliferous crystalline rocks, by means 

of radiometric dating. Classification of the Precambrian 

on the 1932 map was made on the basis of the nowdiscredited "Archean" and Algonkian" Systems, with 

results that are no longer acceptable. The ages of 

Phanerozoic plutons are now known with greater precision. The so-called "Carboniferous" granites shown in 

the Southern Appalachians on the 1932 map are now 

known to be of many Paleozoic ages, mostly preCarboniferous. Similarly, the so-called "Jurassic" granites of the Western States are now known mainly to be 

Cretaceous (for which no provision was made on the 

1932 legend), and to be Jurassic only in small part. 

The crystalline rocks of the Piedmont province were 

poorly known in 1932, and only small parts of them had 

been mapped in detail. By the time of compilation, Arthur Keith's rendering of the province for the North 

America map of 1912 was no longer useful, so Jonas 

undertook a new reconnaissance. Because of the need to 

cover a large area rapidly, her reconnaissance was 

made on the basis of a general theory, outlined in a 

contemporary journal article (Jonas, 1932). The theory 

involved, among other things, correlation of large parts 

of the Piedmont rocks with the Glenarm Series of supposed "Algonkian" age (which had been studied in some 

detail in Maryland and Pennsylvania) and a concept of 

regional belts of retrogressive metamorphism above 

throughgoing low-angle thrusts, in which the alreadyformed crystalline rocks were further altered into 

mylonites and diapthorites. The Piedmont province is 

better known now as a result of extensive field surveys, 

and only parts of these concepts have been substantiated by later work; much greater complexity and 

many more local peculiarities have been discovered. 

Similar problems existed in New England in 1932, 

where the sequences and ages of the crystalline rocks 

were still unresolved over large areas, and where they 

were considered to be largely Precambrian. B. K. Emerson (1917) had indeed made perceptive age assignments 

in Massachusetts, but his rendering of this small area 

had to be suppressed in favor of the overall picture. 

Elsewhere in the country, large areas had already 

been adequately portrayed on State Maps (at least for 

purposes of the 1:2,500,000 scale), and few. differences in 

gross geologic patterns have arisen in the intervening 

years. Differences in detail have resulted from changes 

in stratigraphic classification, from greater precision in 

surface mapping, and from more extensive subcrop data 

in the heavily drift covered region of the Northern Interior States. 

THE GEOLOGIC MAP OF THE UNITED STATES 

HISTORY OF THE PRESENT PROJECT 

By 1955, it had become apparent that the Geologic 

Map of the United States of 1932 had passed its peak of 

usefulness, and plans were made by the U.S. Geological 

Survey for a new and greatly revised map. Philip B. 

King was asked to undertake this project, and facilities 

for the work were set up at the Menlo Park office of the 

Survey. 

A considerable interval elapsed, however, before the 

project could be activated. King had to complete reports 

on other projects, and he contributed much time to reviewing work that was being done by others who were 

revising the Tectonic Map of the United States (Cohee, 

1962) and the Geologic Map of North America (Goddard, 1965). In preparation for the project, however, he 

traveled widely in the United States to visit U.S. 

Geological Survey field parties and to join formal 

geological excursions. 

A further postponement occurred in 1960, during the 

Twenty-first International Geological Congress in 

Copenhagen, when the U.S. Geological Survey accepted 

responsibility for preparing a Tectonic Map of North 

America at the request of the Subcommission for the 

Tectonic Map of the World. King was assigned the task 

of compilation of this map; only after its completion, in 

1967, could actual work on the Geologic Map of the 

United States be started. 

The long delay that followed inception of the project, 

although unfortunate, resulted ultimately in a better 
THE GEOLOGIC MAP OF THE UNITED STATES 11 

product. Acceptable modern geologic data for many 

parts of the country did not become available until the 

mid-1960's and even later. During the delay, new State 

Maps were published covering extensive parts of the 

country, and U.S. Geological Survey personnel completed new mapping of hitherto poorly known territory, 

such as Nevada and eastern Oregon. Many more 

radiometric dates became available, so age assignments 

of the Precambrian rocks, the Phanerozoic plutons, and 

the Cenozoic volcanics could be made with greater 

confidence. 

Also, a competent staff had been assembled. Gertrude 

J. Edmonston, who had assisted in completion of the 

Tectonic Map of North America as geologic cartographer, continued these duties on the United States 

map. Helen M. Beikman was enlisted as geologist and 

fellow-compiler and prepared nearly half of the eventual product. 

A first draft of the compilation was nearly completed 

early in 1970, after which Beikman left the project to 

begin work on a companion Geologic Map of Alaska. 

Several areas, however, were still left in a tentative 

state or uncolored, pending receipt of additional information, or further review of outstanding problems. 

Final decisions on the Piedmont province, the State of 

Texas, the Precambrian of the country, and the 

Cenozoic volcanic rocks of the Western United States 

were thus postponed. 

In the last half of 1970 and during 1971 King and 

Beikman traveled widely to obtain additional information on these matters. Representation of the Precambrian was clarified at a Geological Society of America 

Penrose Conference in Wyoming and during subsequent deliberations of a special panel on the Precambrian of the U.S. Geological Survey under the chairmanship of Max D. Crittenden. A visit to the offices of 

the Texas Bureau of Economic Geology was made to 

complete the compilation for Texas, and several journeys were made to the Southeastern States to obtain 

data on the Piedmont Province. These journeys were 

supplemented, especially for the Piedmont province, by 

extensive correspondence and literature review. Data 

on the volcanic rocks of the West were obtained mainly 

from the Geological Survey staff at Menlo Park. 

Geological plotting of the eastern half of the map was 

completed in July 1971 and of the western half in February 1972, after which each was reviewed by appropriate Survey geologists, whose corrections were incorporated in the final map. The completed map and legend 

were transmitted for publication in midsummer of 

1972, and a hand-colored manuscript copy formed a part 

of the U.S. Geological Survey's exhibit at the Twentyfourth International Geological Congress in Montreal 

in August 1972. 

SOURCES OF THE GEOLOGIC MAP 

During the course of our compilation we consulted all 

pertinent geologic maps and texts, including State 

geologic maps. We also obtained large amounts of unpublished data, revisions, and criticisms from our colleagues on the staffs of the U.S. Geological Survey, the 

State Geological Surveys, universities, and other research institutions. To all these kind friends, collaborators, and contributors we express our deepest 

thanks and appreciation. 

The sources from which the map was compiled are 

summarized below alphabetically by States and are 

cited further at various places in the ensuing text. For 

each State, the first entry is the most recently published 

State Geologic Map, customarily on a scale of 1:250,000 

or smaller. The data taken from all these maps, especially from the older ones, have been somewhat 

modified and revised, those from the older maps the 

most extensively, on the basis of sources listed in the 

following order: (1) Regional maps on scales of 

1:250,000 or smaller. (2) Detailed maps of quadrangles, 

counties, or other small areas on scales of 1:24,000 to 

1:62,500, which are summarized rather than 

specifically cited. (3) Other maps and reports in geological journals and elsewhere, published and unpublished. 

(4) Significant reviews and corrections by U.S. Geological Survey colleagues, and others. 

Alabama.—Geologic Map of Alabama, 1926, by G. I. 

Adams, Charles Butts, L. W. Stephenson, and C. W. 

Cooke: Alabama Geological Survey, scale 1:500,000. 

Northern Alabama Paleozoic area (including Valley and 

Ridge province): Verified, or modified in detail from 

county maps of Alabama Geological Survey published 

after 1960. Piedmont province: Remapped from: R. D. 

Bentley and T. L. Neathery, 1970, Geology of the Breyard zone and related rocks of the Inner Piedmont of 

Alabama: Alabama Geol. Society 8th Ann. Field Trip 

Guidebook; approx. scale 1:500,000. Also manuscript 

map of province furnished through the courtesy of P. E. 

LaMoreaux, State Geologist, Alabama Geol. Survey, 

1970; scale 1:1,000,000. Coastal Plain: Revised from: W. 

H. Monroe, 1945, Geologic map of the Upper Cretaceous 

formations in central Alabama, in C. W. Carlston, 

Ground-water resources of the Cretaceous area in 

Alabama: Alabama Geol. Survey Spec. Rept. 18; scale 

1:5000,000. F. S. MacNeil, 1946, Geologic map of the 

Tertiary formations of Alabama: U.S. Geol. Survey Oil 

and Gas Inv. Prelim. Map 45; scale 1:500,000. Minor 

data from county maps of Alabama Geol. Survey. 

Arizona.—Geologic Map of Arizona, 1969, by E. D. 

Wilson, R. T. Moore, and J. R. Cooper: U.S. Geol. Survey; scale 1:500,000. Radiometric dates of Precambrian 

rocks compiled by Maureen G. Johnson, U.S. Geol. Survey. 
12 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

Arkansas .—Geologic Map of Arkansas, 1929, edited 

by H. D. Miser and G. W. Stose; scale 1:500,000. 

Northwestern Paleozoic area: Manuscript map summarizing data assembled for the new Geologic Map of 

Arkansas, by B. R. Haley and E. R. Glick, U.S. Geol. 

Survey; scale 1:2,500,000. Additional fault data from C. 

G. Stone, Arkansas Geol. and Conserv. Div. 

Southwestern Cretaceous and Tertiary area: Little 

modified from map of 1929. Eastern Cenozoic area (Mississippi Embayment): Map showing Quaternary deposits, in manuscript 1971, by R. T. Saucier, Waterways 

Exp. Sta., Vicksburg, Miss.; scale 1:1,000,000. Geologic 

map [of] alluvial valley floor; sedimentary rocks underlying Recent alluvium, in H. N. Fisk, 1944, Geological 

investigation of the alluvial valley of the lower Mississippi River: Mississippi River Commission, Vicksburg, 

Miss., p1.10, sheet 1; scale 1:500,000; with modifications 

from later data. 

California.—Geologic Map of California, 1958-69, by 

C. W. Jennings and others, California Div. Mines and 

Geol., 2-degree atlas sheets; scale 1:250,000. Revisions 

from: Geologic Map of California, in manuscript 1972, 

by C. W. Jennings and others, California Div. Mines 

and Geol.; scale 1:750,000. Maps and other data in: E. H. 

Bailey, editor, 1966, Geology of northern California: 

California Div. Mines and Geol. Bull. •190; and W. R. 

Dickinson and Arthur Grantz, 1968, Proceedings of conference on geologic problems of San Andreas fault system: Stanford Univ. Pubs. Geol. Sci., v. 11. Also, P. E. 

Hotz, 1971; Geology of lode gold deposits in the Klamath 

Mountains, California and Oregon: U.S. Geol. Survey 

Bull. 1290, p1. 1; scale 1:500,000. J. E. Evernden and R. 

W. Kistler, 1970, Chronology of emplacement of 

Mesozoic batholithic complexes in California and western Nevada: U.S. Geol. Survey Prof. Paper 623 (for 

radiometric ages on plutonic rocks). Maps and other 

data, partly unpublished, from P. E. Hotz, E. H. Bailey, 

W. P. Irwin, L. D. Clark, P. C. Bateman, J. G. Vedder, 

and T. W. Dibblee, Jr., of U.S. Geol. Survey, and B. M. 

Page of Stanford University. 

Colorado.—Geologic Map of Colorado, 1935, compiled 

by W. S. Burbank, T. S. Lovering, E. N. Goddard, and E. 

B. Eckel: U.S. Geol. Survey; scale 1:500,000. Geologic 

maps of 2-degree quadrangles, scale 1:250,000, issued 

as U.S. Geol. Survey Misc. Inv. Maps, as follows: Moab, 

1964, by P. L. Williams, Map 1-360; La Junta, 1968, by 

G. R. Scott, Map 1-629; Trinidad, 1969, by R. B. Johnson, Map 1-558; Cortez, 1972, by D. D. Haynes and 

others, Map 1-629. Great Plains, eastern Colorado: Few 

changes, except for revised age assignments. Rocky 

Mountains and Colorado Plateau, western Colorado: 

Compiled by Helen M. Beikman and Philip B. King 

from published and unpublished data of U.S. Geol. Survey geologists. Radiometric ages of Precambrian rocks 

and of Cretaceous-Tertiary, instrusives- from: Z. E. 

Peterman and C. E. Hedge, 1968, Chronology of Precambrian events in the Front Range, Colorado: Canadian Jour. Earth Sci., v. 5, no. 3, pt. 2, p. 749-756; and 

written communications by Ogden Tweto. Final review 

and correction of compilation by Ogden Tweto, U.S. 

Geol. Survey, December 1971. 

Connecticut.—See New England. 

Delaware .—See Maryland. 

Florida.—Geologic Map of Florida, 1964, scale approx. 1:2,000,000, in H. S. Puri and R. 0. Vernon, Summary of the geology of Florida and a guidebook to the 

classic exposures: Florida Geol. Survey Spec. Pub. 5, pl. 

2. Supplemented by Geologic Map of Florida, 1945, scale 

1:1,000,000, in C. W. Cooke, Geology of Florida: Florida 

Geol. Survey Bull. 29, pl. 1. Quaternary of northeastern 

Florida adjusted from F. S. MacNeil, 1950, Pleistocene 

shorelines of Florida and Georgia: U.S. Geol. Survey 

Prof. Paper 221-F, pl. 1. Pliocene age assignment of 

Caloosahatchee Formation of southern Florida from J. 

E. Hazel, U. S. Geol. Survey, September 1971. 

Georgia.—Geologic Map of Georgia, 1939, compiled 

by C. W. Cooke, G. W. Crickmay, and Charles Butts: 

Georgia Div. Mines, Mining, and Geol.; scale 1:500,000. 

Valley and Ridge province, northwestern Georgia: Little 

revision, but verified from county geologic maps of 

Georgia Geol. Survey published after 1960. Blue Ridge 

and Piedmont provinces: Compiled by Philip B. King 

and Michael W. Higgins from large-scale published 

maps of Georgia Geol. Survey, U.S. Geol. Survey, Coosa 

Valley Planning and Devel. Comm., and Central 

Savannah River Planning and Devel. Comm.; also 

manuscript maps furnished through courtesy of Georgia Geol. Survey; with extrapolations in intervening 

areas. Published maps include: Western Piedmont by J. 

S. Clarke, 1952; R. D. Bentley and T. L. Neathery, 1970 

(see under Alabama); V. J. Hurst and T. L. Crawford, 

1969; T. L. Crawford and J. H. Medlin, 1970. Central 

Piedmont by L. A. Hermann, 1954; and M.W. Higgins, 

1968. Eastern Piedmont by W. H. Grant, 1958; and T. L. 

Crawford, 1968. Northeastern Blue Ridge by R. D. 

Hatcher, Jr., 1971. Coastal Plain: F. S. MacNeil, 1947, 

Geologic map of the Tertiary and Quaternary formations of Georgia: U.S. Geol. Survey Oil and Gas Inv. 

Prelim. Map 72; scale 1:500,000. D. H. Eargle, 1955, 

Stratigraphy of the outcropping Cretaceous rocks of 

Georgia: U.S. Geol. Survey Bull. 1014, pl. 1, scale 

1:500,000. Maps of seven counties in eastern Coastal 

Plain by John Sandy, under direction of V. J. Hurst for 

Central Savannah River Planning and Devel. Comm., 

1968. 

Idaho.—Geologic Map of the State of Idaho, 1947, 

compiled by C. P. Ross and J. D. Forrester: U.S. Geol. 

Survey; scale 1:500,000. Extensively revised by Philip 
THE GEOLOGIC MAP OF THE UNITED STATES 13 

B. King, as follows: Belt Supergroup and associated 

rocks, northern Idaho: A. B. Griggs, 1973, Geologic map 

of the Spokane quadrangle, Washington, Idaho, and 

Montana: U.S. Geol. Survey Misc. Geol. Inv. Map 1-768; 

scale 1:250,000. Published geologic quadrangle maps by 

J. E. Harrison and others, U.S. Geol. Survey, and synthesis by Harrison. Anna Hietanen, 1962-68, 

Metamorphic and igneous rocks along the northwestern 

border zone of the Idaho batholith: U.S. Geol. Survey 

Prof. Paper 344-A-E; geologic maps on scale 1:48,000. 

Idaho batholith and vicinity, west-central Idaho: Published and unpublished maps by F. W. Cater, Jr., Warren Hamilton, B. F. Leonard, and D. L. Schmidt, U.S. 

Geol. Survey; and R. R. Reid, Idaho Bur. Mines and 

Geol. R. C. Newcomb, 1970, Tectonic structure of the 

main part of the basalt of the Columbia River Group, 

Washington, Oregon, and Idaho: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-587; scale 1:500,000. 

Precambrian, Paleozoic, and Tertiary rocks, east central 

Idaho: Published and unpublished maps by W. J. Mapel, 

E. T. Ruppel, Betty A. L. Skipp, and others of U.S. Geol. 

Survey. Robert Scholten and L. D. Ramspott, 1968, Tectonic mechanism indicated by structural framework of 

central Beaverhead Range, Idaho-Montana: Geol. Soc. 

America Spec. Paper 104, pl. 1; scal3 1:62,500. Upper 

Cenozoic volcanic rocks, Snake River Plain: H. E. Malde, 

1965, Snake River Plain, in H. E. Wright, Jr., and D. G. 

Frey, editors, the Quaternary of the United States: 

Princeton Univ. Press, p. 255-264, fig. 1, scale 

1:1,583,000. H. E. Malde, H. A. Powers and C. H. Marshall, 1963, Reconnaissance geologic map of westcentral Snake River Plain, Idaho: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-373; scale 1:125,000. Revisions 

and corrections by H. E. Malde, U.S. Geol. Survey, 

November, 1970. Paleozoic, Mesozoic, and Tertiary 

rocks, southeastern Idaho: Published and unpublished 

geologic quadrangle maps by F. C. Armstrong, S. S. 

Oriel, E. H. Pampeyan, D. E. Trimble, and others of U. 

S. Geol. Survey, revising and extending earlier mapping by G. R. Mansfield and associates. 

Illinois.-Geologic Map of Illinois, 1967, compiled by 

H. B. Willman and others: Illinois Geol. Survey; scale 

1:500,000. Time-stratigraphic units of Pennsylvanian 

System according to: R. M. Kosanke and others, 1960, 

Classification of the Pennsylvanian strata in Illinois: 

Illinois Geol. Survey Rept. Inv. 214, pl. 1. 

Iowa.-Geologic Map of Iowa, 1969: Iowa Geol. Survey; scale 1:500,000. 

Indiana .-Map of Indiana Showing Bedrock Geology, 

1970: Indiana Geol. Survey Misc. Map 16; scale 

1:2,000,000. Minor modifications from 2-degree sheets 

of Regional Geol. Map Series, partly in manuscript 

1970, scale 1:250,000, supplied through courtesy of 

Robert H. Shaver, Indiana Geol. Survey, November 

1970. 

Kansas.-Geologic Map of Kansas, 1964, compiled by 

J. M. Jewett and others: Kansas Geol. Survey; scale 

1:500,000. Supplemented by Geologic Map of Kansas, 

1937, compiled by R. C. Moore, K. K. Landes, and 

others; Kansas Geol. Survey; scale 1:500,000. 

Kentucky.-Geologic Map of Kentucky, 1954: Kentucky Geol. Survey Ser. 9; scale 1:1,000,000. Supplemented by Geologic Map of Kentucky, 1929, by W. R. 

Jillson: Kentucky Geol. Survey Ser. 6; scale 1:500,000. 

Revised map compiled by Helen M. Beikman, using 

where available 71/2-minute quadrangle maps, scale 

1:24,000, published by U.S. Geol. Survey in 1962 and 

later; and where not available the two State Maps. 

Tertiary units of Mississippi Embayment from Geologic 

Map of Jackson Purchase Region, Kentucky, 1972, 

compiled by W. W. Olive, in Kentucky Geol. Society 

Field Conf. Guidebook; scale 1:250,000. 

Louisiana.-Generalized Geological Map of 

Louisiana, 1959, L. W. Hough, State Geologist: 

Louisiana Geol. Survey; scale approx. 1:1,500,000. Supplemented by two earlier State Maps: Geologic Map of 

State of Louisiana, 1946, compiled by W. E. Wallace, 

Jr.: Shreveport Geol. Society; scale 1:500,000. Geological Map of Louisiana, in manuscript 1948, compiled by 

Rufus LeBlanc, Shell Oil Co.; scale 1:500,000. 

Mississippi Embayment: Map showing Quaternary deposits, in manuscript 1971, by R. T. Saucier, Waterways 

Exp. Sta., Vicksburg, Miss.; scale 1:1,000,000. Geologic 

map [of] alluvial valley floor; sedimentary rocks underlying Recent alluvium, in H. N. Fisk, 1944, Geological 

investigation of the alluvial valley of the Mississippi 

River: Mississippi River Comm., Vicksburg, Miss., pl. 

10, sheet 2: scale 1:500,000. Outcrops of Citronelle Formation (Pliocene): From J. A. Doering, 1956, Review of 

Quaternary surface formations of Gulf Coast region: 

Am. Assoc. Petroleum Geologists Bull., v. 40, p. 

1816-1852, figs. 8-9. Outcrop areas of Midway Group: 

Advice from H. B. Stenzel, written communication, 

July, 1971. 

Maine .-See New England. 

Maryland (and Delaware).-Geologic Map of Maryland, 1968, compiled by K. N. Weaver and others: Maryland Geol. Survey; scale 1:250,000. Supplemented by 

Map of Maryland [and Delaware] Showing Geological 

Formations, 1933, E. B. Mathews, State Geologist: 

Maryland Geol. Survey; scale 1:380,160. Valley and 

Ridge provinces: Not revised. Piedmont province: Revisions by M. W. Higgins, 1972, Age, origin, regional 

relations, and nomenclature of Glenarm Series, central 

Appalachian Piedmont; a reinterpretation: Geol. Soc. 

America Bull., v. 83, p. 989-1026, especially pl. 1. 

Coastal Plain of Maryland and Delaware: State Map 

supplemented by Engineering Geology of the Northeast 

Corridor, Washington, D.C., to Boston, Mass.; Coastal 

Plain and surficial geology [compiled by J. P. Owens]: 
14 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

U.S. Geol. Survey Misc. Inv. Map I-514-B, sheets 2 and 

3; scale 1:250,000. 

Massachusetts.—See New England. 

Michigan.—Bedrock of Michigan, 1968, compiled by 

R. W. Kelley: Michigan Geol. Survey Small-Scale Map 

2; scale 1:2,500,000. Precambrian in Northern Peninsula: Supplemented from: Centennial Geologic Map of 

Michigan (Northern Peninsula), 1936, compiled by H. 

M. Martin: Michigan Geol. Survey Pub. 39, Geol. Ser. 

33; scale 1:500,000. Geologic Map of the Lake Superior 

Region and Structure Sections, 1935, scale 1:1,000,000, 

in C. K. Leith, R. J. Lund, and Andrew Leith, Precambrian rocks of the Lake Superior Region; a review of 

newly discovered geologic features and a revised 

geologic map: U.S. Geol. Survey Prof. Paper 184. R. W. 

Bayley and W. R. Muehlberger, 1968, Basement rock 

map of the United States (exclusive of Alaska and 

Hawaii): U.S. Geol. Survey; scale 1:2,500,000. 

Minnesota.—Geologic Map of Minnesota; bedrock 

geology, 1970, by P. K. Sims: Minnesota Geol. Survey 

Map M-14; scale 1:1,000,000. Representation of 

Paleozoic formations in southeastern Minnesota supplemented from: Geologic Map of Minnesota; St. Paul 

Sheet, 1966, compiled by R. E. Sloan and G. S. Austin: 

Minnesota Geol. Survey; scale 1:250,000. 

Mississippi.—Geologic Map of Mississippi, 1969, 

compiled by A. R. Bicker, Jr.: Mississippi Geol. Survey; 

scale 1:500,000. With additional data from Geologic 

Map of Mississippi, 1945, compiled by W. E. Belt and 

others: U.S. Geol. Survey and Mississippi Geol. Society; 

scale 1:500,000. 

Missouri.—Geologic Map of Missouri, 1961, compiled 

by M. H. McCracken and others: Missouri Div. Geol. 

Survey and Water Res.; scale 1:500,000. 

Montana.—Geologic Map of Montana, 1955, compiled 

by C. P. Ross, D. A. Andrews, and I. J. Witkind: U. S. 

Geol. Survey; scale 1:500,000. Great Plains, eastern 

Montana: No revisions of State Map. Rocky Mountains, 

western Montana: New compilation by Philip B. King 

and Helen M. Beikman, from following sources: Belt 

Supergroup and associated rocks, northwestern Montana: Geologic quadrangle maps by A. B. Campbell, J. 

E. Harrison, M. R. Mudge, W. H. Nelson, and others of 

U.S. Geol. Survey, and W. M. Johns, Montana Bur. 

Mines and Geol. Correlations by A. G. Smith and W. C. 

Barnes, 1966, Correlation and facies changes in the 

carbonaceous, calcareous, and dolomitic formations of 

the Belt-Purcell Supergroup: Geol. Soc. America Bull., 

v. 77, p. 1399-1426. Radiometric dates from J. D. 

Obradovich and Z. E. Peterman, 1968, Geochronology of 

the Belt Series, Montana: Canadian Jour. Earth Sci., v. 

5, no. 3, pt. 2, p. 737-747. Synthesis by J. E. Harrison, 

U.S. Geol. Survey. Boulder batholith and vicinity, 

central-western Montana: Geologic quadrangle maps by 

M. R. Klepper, G. D. Robinson, E. T. Ruppel, Betty A. L. 

Skipp, H. W. Smedes, and others of U.S. Geol. Survey 

and J. C. Maxwell and others of Princeton University. 

Summarized in part by G. D. Robinson, M. R. Klepper, 

and J. D. Obradovich, 1970, Overlapping plutonism, 

volcanism, and tectonism in the Boulder batholith region, western Montana, in R. R. Coats, R. L. Hay, and C. 

A. Anderson, editors, Studies in volcanology: Geol. Soc. 

America Mem. 116, p. 557-576. Southwestern Montana: Published and unpublished quadrangle maps by 

H. L. James, J. B. Hadley, W. B. Meyers, I. J. Witkind, 

and others of U.S. Geol. Survey. Robert Scholten and 

others,1955, Geology of the Lima region, Montana and 

Idaho: Geol. Soc. America Bull., v. 66, p. 345-404, pl. 1; 

scale approx. 1:125,000. Precambrian radiometric dates 

compiled by Maureen G. Johnson, U.S. Geol. Survey; 

Precambrian geology reviewed by H. L. James, U.S. 

Geol. Survey. 

Nebraska.—Geologic Bedrock Map of Nebraska, 

1969, compiled by R. R. Burchett: Nebraska Geol. Survey; scale 1:1,000,000. 

Nevada.—No adequate published State Map available. Compiled by Philip B. King from: Manuscript 

sheets for Geologic Map of Nevada, by J. H. Stewart and 

J. E. Carlson, U.S. Geol. Survey, in preparation 1974; 

scales 1:250,000 and 1:500,000. County geologic maps 

by geologists of U.S. Geol. Survey and Nevada Bur. 

Mines, published since 1960 as Nevada Bur. Mines Bulletins, as U.S. Geol. Survey Misc. Inv. Maps, or in manuscript; scale 1:250,000. 

New England.—Compiled by Philip B. King from: 

State Geologic maps: Preliminary Geologic Map of 

Maine, 1967, compiled by A. W. Hussey II and others: 

Maine Geol. Survey; scale 1:500,000. Geologic Map of 

New Hampshire, 1955, compiled by M. P. Billings: U.S. 

Geol. Survey; scale 1:250,000. Centennial Geologic Map 

of Vermont, 1961, compiled by C. G. Doll and others: 

Vermont Geol. Survey; scale 1:250,000. Geologic Map of 

Massachusetts and Rhode Island, 1917, in B. K. Emerson, Geology of Massachusetts and Rhode Island: U.S. 

Geol. Survey Bull. 597; scale 1:250,000. Bedrock 

Geologic Map of Rhode Island, 1971 in A. W. Quinn, 

Bedrock geology of Rhode Island: U.S. Geol. Survey 

Bull. 1295; scale 1:125,000. Preliminary Geologic Map 

of Connecticut, 1956, compiled by John Rodgers and 

others: Connecticut Geol. and Nat. Hist. Survey; scale 

1:253,440. With modifications from: (1) 71/2-minute 

geologic quadrangle maps in Massachusetts, Rhode Island, and Connecticut, mostly published by U.S. Geol. 

Survey; scale 1:24,000. (2) New England Intercollegiate 

Geol. Conf. Guidebooks, especially for Connecticut valley of Massachusetts, 1967; New Haven, Connecticut, 

and vicinity, 1969; and Rangely Lakes-Dead River 

Basin region, Maine, 1970. (3) Articles and maps in: 
THE GEOLOGIC MAP OF THE UNITED STATES 15 

E-an Zen, W. S. White, J. B. Hadley, and J. B. Thompson, Jr., editors, 1968, Studies of Appalachian geology; 

Northern and Maritime: Interscience Pub., New York; 

especially on nappes and gneiss domes in New Hampshire, Massachusetts, and eastern Connecticut (J. B. 

Thompson, Jr., and others; H. R. Dixon and L. W. 

Lundgren, Jr.) and on Maine (P. H. Osberg and others; 

J. C. Green and V. C. Guidotta; A. M. Hussey II). (4) 

Manuscript maps on northern and southeastern Maine, 

supplied by Louis Pavlides, E. L. Boudette, D. B. 

Stewart, and D. R. Wones; distribution of Paleozoic volcanic rocks in New England, compiled by D. W. Rankin; 

all of U.S. Geol. Survey, 1971. (4) Radiometric dates in 

eastern Massachusetts and vicinity from R. E. Zartman 

and R. F. Martin, 1971, Radiometric age (Late Ordovician) of the Quincy, Cape Ann, and Peabody Granites 

from eastern Massachusetts: Geol. Soc. America Bull., 

v. 82, p. 937-958; also oral communications from R. E. 

Zartman, 1971. Original compilations reviewed and 

corrected by: John Rodgers of Yale University and 

James Skehan of Boston College, 1969 and 1971; and by 

K. G. Bell, H. R. Dixon, Richard Goldsmith, D. S. Harwood, N. L. Hatch, L. R. Page, D. W. Rankin, E-an Zen, 

and others of U.S. Geol. Survey, 1971. 

New Hampshire.—See New England. 

New Jersey.—Geologic Map of New Jersey, 1910-12 

[reprinted 1950]. by J. V. Lewis and H. B. Kümmel: New 

Jersey Dept. Cons. and Econ. Devel. Atlas Sheet 40; 

scale 1:250,000. With revisions as follows: Precambrian 

and Paleozoic of Reading Prong: A. A. Drake, Jr., 1970, 

Structural geology of the Reading Prong, in G. A. 

Fisher, F. J. Pettijohn, J. C. Reed Jr., and K. N. Weaver, 

editors, Studies in Appalachian geology; Central and 

Southern: Interscience Pub., New York, p. 271-291. 

Also manuscript map by A. A. Drake, Jr., U.S. Geol. 

Survey, June, 1971; scale 1:1,000,000. Coastal Plain: 

Engineering geology of the Northeast Corridor, 

Washington, D.C., to Boston, Mass.; Coastal Plain and 

surficial geology [compiled by J. P. Owens]: U.S. Geol. 

Survey Misc. Inv. Map 1-514-B, sheet 1, 1967; scale 

1:250,000. 

New Mexico.—Geologic Map of New Mexico, 1965, by 

C. H. Dane and G. 0. Bachman: U.S. Geol. Survey; scale 

1:500,000. With additions and modifications from various sources, including: Subdivisions of Precambrian, 

Preliminary geologic and relief map of the Precambrian 

rocks of New Mexico, 1961, by R. W. Foster and T. F. 

Stipp: New Mexico Bur. Mines and Min. Res. Circ. 57; 

scale 1:500,000. Faults and other tectonic features from 

maps by V. C. Kelley and others, such as: Upper Rio 

Grande area, 1954, U.S. Geol. Survey Oil and Gas Inv. 

Map OM-157; Ruidoso-Carrizozo area, 1964, New Mexico Geol. Society 15th Field Conf.; Zuni-Defiance region, 

1967, New Mexico Geol. Society, 18th Field Conf. Cornpilation reviewed by G. 0. Bachman, U.S. Geol. Survey, 

March, 1972. 

New York.—Geologic Map of New York, 1962, compiled by J. G. Broughton, D. W. Fisher, Y. W. Isachsen, 

and L. V. Rickard: New York State Mus. and Sci. Serv., 

Geol. Survey Map and Chart Ser. 5; scale 1:250,000. 

Taconic region of eastern part of State revised from 

E-an Zen, 1967, Time and space relationships of the 

Taconic allochthon and autochthon: Geol. Soc. America 

Spec. Paper 97, pl. 1; scale approx. 1:500,000. 

North Carolina:—Geologic Map of North Carolina, 

1958, J. L. Stuckey, State Geologist: North Carolina 

Div. Min. Res.; scale 1:500,000. Coastal Plain: No revisions. Blue Ridge and Piedmont provinces: Extensively 

revised from many sources, including: J. B. Hadley and 

A. E. Nelson, 1971, Geologic Map of the Knoxville 

Quadrangle, North Carolina, Tennessee, and South 

Carolina: U.S. Geol. Survey Misc. Inv. Map 1-654; scale 

1:250,000. D. M. Rankin and G. H. Espenshade, 1972, 

Geologic Map of the Abingdon Quadrangle, Virginia, 

North Carolina, and Tennessee, western half: U.S. 

Geol. Survey Misc. Inv. Map I-709-A; scale 1:250,000 

(eastern half in manuscript). J. R. Conley and G. L. 

Bain, 1965, Composite geologic map of the Carolina 

Slate Belt in North Carolina, west of the Deep River 

-Wadesboro Triassic basin: Southeastern Geol., v. 6, 

no. 3; scale approx. 1:500,000. J. M. Parker III, 1968, 

Structure of easternmost North Carolina Piedmont: 

Southeastern Geol., v. 9, no. 3; scale approx. 1:500,000; 

and written communications from Parker, May 1971. 

Maps and articles by J. B. Hadley, D. M. Rankin, J. C. 

Reed, Jr., and others, in G. W. Fisher, F. J. Pettijohn, J. 

C. Reed, Jr., and K. N. Weaver, Editors, 1970, Studies of 

Appalachian geology; Central and Southern: 

Interscience Pub., New York. Radiometric and other age data 

on plutons in North Carolina, South Carolina, and eastern Georgia, from: P. D. Fullagar, 1971, Age and origin 

of plutonic intrusions in the Piedmont of the southeastern Appalachians: Geol. Soc. America Bull., v. 82, p. 

2845-2862. J. R. Butler, 1972, Age of Paleozoic regional metamorphism in the Carolinas, Georgia, and 

Tennessee: Am. Jour. Sci., v. 272, p. 319-333. Also written communications from Fullagar and Butler, 1971. 

North Dakota.—Bedrock Geologic Map of North 

Dakota, 1969, compiled by C. R. Gordon: North Dakota 

Geol. Survey Misc. Map 10; scale 1:1,000,000. 

Ohio.—Geologic Map of Ohio, 1920 [reprinted 1947], 

by J. A. Bownocker: Ohio Geol. Survey; scale 1:500,000. 

Subcrop extent of Silurian subdivisions and other features revised by Arie Janssens of Ohio Geol. Survey, 

written communication, December 1970. Minor revisions of other areas from published county and quadrangle maps of Ohio Geol. Survey and U.S. Geol. Survey. 
16 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

Oklahoma.—Geologic Map of Oklahoma, 1954, by H. 

D. Miser: U.S. Geol. Survey; scale 1:500,000. 

Oregon .—Western half: Geologic Map of Oregon west 

of 121st Meridian, 1961, by F. G. Wells and D. L. Peck: 

U.S. Geol. Survey; scale 1:500,000. Klamath Mountains 

area revised from: P. E. Hotz, 1971, Geology of lode gold 

deposits in the Klamath Mountains, California and 

Oregon: U.S. Geol. Survey Bull. 1290, pl. 1; scale 

1:500,000. R. G. Coleman, 1972, The Colebrooke Schist 

of southwestern Oregon and its relation to the tectonic 

evolution of the region: U.S. Geol. Survey Bull. 1339, pl. 

1; scale 1:125,000. Eastern half: Geologic Map of Oregon 

East of 121st Meridian, in preparation 1974, compiled 

by G. W. Walker: U.S. Geol. Survey; scale 1:500,000. 

Also covered in part by earlier published 2-degree 

geologic quadrangle maps by G. W. Walker and others: 

U.S. Geol. Survey Misc. Inv. Maps; scale 1:250,000. 

Generalizations by Philip B. King, assisted by G. W. 

Walker. 

Pennsylvania .—Geologic Map of Pennsylvania, 1960, 

compiled by Carlisle Gray and others: Pennsylvania 

Topog. and Geol. Survey; scale 1:250,000. Reading 

Prong of eastern Pennsylvania revised from manuscript 

map by A. A. Drake, Jr., U.S. Geol. Survey, June 1971; 

scale 1:1,000,000. 

Rhode Island.—See New England. 

South Carolina.—No adequate published State Map 

available; partial maps as follows: W. C. Overstreet and 

Henry Bell III, 1965, Geologic map of the crystalline 

rocks of South Carolina: U.S. Geol. Survey Misc. Inv. 

Map 1-413; kale 1:250,000. W. C. Overstreet and Henry 

Bell III, 1965, Geologic map and inferred age relations 

of the crystalline rocks of South Carolina, in The 

crystalline rocks of South Carolina; U.S. Geol. Survey Bull. 

1183, pl. 1; scale 1:500,000. C. W. Cooke, 1936, Cretaceous and Tertiary formations of South Carolina, in Geology of the Coastal Plain of South Carolina: U.S. Geol. 

Survey Bull. 867, pl. 2; scale 1:500,000. Piedmont province: Extensively revised from publications of South 

Carolina State Devel. Board Div. of Geol., including: 

Detailed maps by R. D. Hatcher, Jr., and V. S. Griffin, 

Jr., in north-western South Carolina, and reconnaissance maps elsewhere. County and quadrangle maps by 

D. T. Secor, H. D. Wagener, J. R. Butler, J. F. McCauley, 

and others. Coastal Plain: Revised from data compiled 

by S. D. Heron for Geological Highway Map of the 

Mid-Atlantic Region, 1970: Am. Assoc. Petroleum 

Geologists Geologic Highway Map Ser. 4; scale approx. 

1:2,000,000. 

South Dakota.—Geologic map [of] South Dakota, 

1953, compiled by B. C. Petsch: South Dakota Geol. 

Survey; scale 1:500,000. Supplemented by Geologic 

Map of South Dakota, 1951, compiled by N. H. Darton: 

U.S. Geol. Survey; scale 1:500,000. Subdrift geology 

east of Missouri River from R. F. Flint, 1955, Pleistocene geology of eastern South Dakota: U.S. Geol. Survey Prof. Paper 262, fig. 4. Precambrian of Black Hills 

area revised from data of R. W. Bayley, U.S. Geol. Survey open-file map, 1972. 

Tennessee.—Geologic Map of Tennessee, 1966, by W. 

D. Hardeman, R. A. Miller, and G. D. Swingle; Tennessee Div. Geol.; scale 1:250,000. Tertiary units of Mississippi Embayment area, western Tennessee, revised by 

W. S. Parks, Water Resources Div., U.S. Geol. Survey, 

written communication, November 1971. 

Texas.—Geologic Map of Texas, 1937, by N. H. Darton, L. W. Stephenson, and Julia Gardner: U.S. Geol. 

Survey; scale 1:500,000. Extensively revised as follows 

(letter symbols refer to fig. 6): (A) Eastern, northern, 

and westernmost Texas, where available, from sheets of 

Geologic Atlas of Texas, 1965-72, by V. E. Barnes and 

others: Texas Univ. Bur. Econ. Geology; scale 

1:250,000. (B) Llano region, central Texas, from manuscript mpps by V. E. Barnes, F. B. Plummer, and others, 

Texas Univ. Bur. Econ. Geology; scales 1:125,000 and 

1:250,000. (C) Edwards Plateau region from manuscript 

maps by F. E. Lozo, Jr., Shell Oil Co.; scale 1:250,000. 

(D) South Texas Coastal Plain compiled by Helen M. 

Beikman from manuscript data for Geologic Atlas of 

Texas; manuscript maps by D. H. Eargle, U.S. Geol. 

Survey; Geologic Map of Texas, 1937; and other sources. 

(E) Trans-Pecos region compiled by Philip B. King from 

published quadrangle maps of Texas Univ. Bur. Econ. 

Geology and U.S. Geol. Survey, and from personal 

knowledge. (F) Northwestern Texas, where not otherwise covered, from Geologic Map of Texas, 1937, with 

revisions of Paleozoic area by D. H. Eargle, U.S. Geol. 

Survey. 

Utah.-Geologic Map of Utah, 1961-63, compiled by 

W. L. Stokes, J. H. Madsen, Jr., and L. F. Hintze: Utah 

State Land Board and Univ. of Utah; scale 1:250,000. 

With additions and corrections by M. D. Crittenden and 

H. T. Morris, U.S. Geol. Survey, 1970-71. 

Vermont.—See New England. 

Virginia.—Geologic Map of Virginia, 1963, compiled 

by R. C. Milici, C. T. Spiker, Jr., and J. M. Wilson: 

Virginia Div. Min. Res.; scale 1:500,000. Valley and 

Ridge and Blue Ridge provinces .—Minor revisions only. 

Piedmont province.—Extensive revisions as follows: 

North of James River revised by M.W. Higgins from 

maps by D. L. Southwick, J. C. Reed, Jr., S. K. Neuschel, 

and others, and by extrapolations based on reconnaissance. South of James River revised in part by Philip B. 

King from published and unpublished maps by D. M. 

Rankin, G. H. Espenshade, J. F. Conley, 0. T. Tobisch, 

and Lynn Glover III. Coastal Plain.—No revision. 

Washington.—Geologic Map of Washington, 1961, 

compiled by M. T. Huntting, W. A. Bennett, V. E. 
THE GEOLOGIC MAP OF THE UNITED STATES 17 

West Virginia.—Geologic Map of West Virginia, 

1968, compiled by D. H. Cardwell, R. B. Erwin, and H. P. 

Woodward: West Virginia Geol. and Econ. Survey; scale 

1:250,000. 

Wisconsin.—Geologic Map of Wisconsin, 1949, Wisconsin Geol. and Nat. Hist. Survey; scale 1:1,000,000. 

Precambrian rocks, edge of Cambrian overlap, and 

faults revised from: C. E. Dutton and R. F. Bradley, 

1970, Lithologic, geophysical, and mineral commodity 

maps of Precambrian rocks of Wisconsin: U.S. Geol. 

Survey Misc. Geol. Inv. Map 1-631; scale 1:500,000; 

especially sheets 3 and 5. In the main Precambrian area 

of northern Wisconsin, contacts of Precambrian units 

extrapolated by Philip B. King beyond their extent as 

mapped by Dutton and Bradley. 

104° 102' 100° 98 96 94° 

FIGURE 6.—Index map of Texas, showing areas covered by different 

sources used on the Geologic Map. Letter symbols are explained on 

page 16. 

Livingston, Jr., and W. S. Moen: Washington Div. 

Mines and Geol.; scale 1:500,000. Extensively revised 

by Philip B. King, as follows: Olympic Peninsula, 

northwestern Washington: From published and unpublished maps by W. M. Cady, R. W. Tabor, H. D. Gower, P. 

D. Snavely, Jr., and others of U.S. Geol. Survey. Coast 

Ranges, southwestern Washington: From published 

quadrangle maps by Holly Wagner, E. H. Wolfe, H. D. 

Gower, P. D. Snavely, Jr., and others, U.S. Geol. Survey. 

Volcanic rocks, southern Cascade Range: Revised by C. 

A. Hopson, Univ. of California, Santa Barbara, written 

communication, February, 1972. Prevolcanic rocks, 

northern Cascade Range: Peter Misch, .1966, Tectonic 

evolution of northern Cascades of Washington State, in 

Symposium on the tectonic history and mineral deposits 

of the western Cordillera: Canadian Inst. Min. and 

Geol. Spec. Volume 8, p. 101-148. Maps and other data, 

in part unpublished, by D. F. Crowder, F. W. Cater, R. 

W. Tabor, and C. A. Hopson. Northern and northeastern 

Washington: A. B. Griggs, 1966, Geologic map of western half of Spokane quadrangle, Washington and Idaho: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-464; scale 

1:250,000. General and detailed maps, in part unpublished, by C. D. Rinehart, J. F. Fox, Jr., R. G. Yates, F. 

K. Miller, G. E. Becraft, and others, U.S. Geol. Survey. 

Columbia Plateau: R. C. Newcomb, 1970, Tectonic 

structure of the main part of the basalt of the Columbia 

River Group, Washington, Oregon, and Idaho: U.S. 

Geol. Survey Misc. Geol. Inv. Map 1-587; scale 

1:500,000. 

Wyoming.—Geologic Map of Wyoming, 1955, compiled by J. D. Love, J. L. Weitz, and R. K. Hose: U.S. 

Geol. Survey; scale 1:500,000. Revised in part, as follows: Precambrian rocks from published and unpublished data by R. W. Bayley, Harry Granger, R. C. 

Pearson, and others, U.S. Geol. Survey, and R. S. Houston, Univ. of Wyoming. Heart Mountain fault, northwestern Wyoming: From W. G. Pierce, U.S. Geol. Survey, 1972. Volcanic rocks, Yellowstone National Park: 

From Geologic Map of Yellowstone National Park, 

1972, by geologists of U.S. Geol. Survey: U.S. Geol. 

Survey Misc. Geol. Inv. Map I-711; scale 1:125,000. W. 

R. Keefer, 1972, The geological story of Yellowstone 

National Park: U.S. Geol. Survey Bull. 1374, p1. 1; scale 

approx. 1:500,000; and written communication from R. 

L. Christiansen, U.S. Geol. Survey, November 1970. 

Tertiary sedimentary rocks revised by J. D. Love, written communication, January, 1971. Compilation of 

Wyoming reviewed by J. D. Love, U.S. Geol. Survey, 

and staff of Dept. Geol., Univ. of Wyoming, written 

communication, January 1971. 

Phanerozoic metamorphism .—Areas of Phanerozoic 

metamorphism in western United States, from many 

sources; in Appalachian region from B. A. Morgan, 

1972, Metamorphic map of the Appalachians: U.S. Geol. 

Survey Misc. Geol. Inv. Map 1-724; scale 1:2,500,000. 

Limits of Pleistocene glacial deposits.—Glacial Map of 

the United States East of the Rocky Mountains, 1959, R. 

F. Flint, chairman, Geol. Soc. America; scale 

1:1,750,000. Major revisions, based on later data, made 

by Roger B. Morrison, U.S. Geol. Survey, 1974, as follows: Montana and North Dakota from R. W. Lemke 

and R. B. Colton. South Dakota, Nebraska, Kansas, 

Missouri, and Iowa from the respective State Geological 

Surveys. Indiana from R. V. Ruhe, Indiana University. 

Ohio and Kentucky from Jane L. Forsyth, Bowling 

Green State University. Pennsylvania, New Jersey, 

and New York from C. S. Denny, U.S. Geol. Survey. 
18 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

Subsea bathymetry.—Subsea contours compiled by 

Philip B. King and Gertrude J. Edmonston from the 

following sources, the locations of which are indicated 

in figure 7: (1) and (2) International map of the World, 

United States, scale 1:1,000,000, by the U.S. Geol. Survey. Sheet NL -10, Cascade Range, 1951. Sheet NK-10, 

Mount Shasta, 1951. Sheet NI-11, Los Angeles, 1952. 

Subsea contours in metres. (3) State of California, base 

map with shaded relief and offshore contours, by the 

U.S. Geol. Survey, 1968, scale 1:1,000,000. Contours in 

fathoms, converted to metres. (4) D. C. Krause, 1965, 

Tectonics, bathymetry, and geomagnetism of the southern continental borderland west of Baja California, 

Mexico: Geol. Soc. America Bull., v. 76, fig. 1, p. 260. 

Mercator projection; contours in metres. (5) and (6) 

Bathymetry of the northeast Pacific, by Scripps Institution of Oceanography and Underseas Surveillance 

Oceanographic Center, 1970. Sheets 1 and 2. Mercator 

projection; contours in fathoms converted to metres. (7) 

Submarine topography of the Gulf of California by R. L. 

Fisher, G. A. Rusnak, and F. P. Shepard, in T. H. van 

Andel and G. G. Shor, Jr., editors, Marine geology of the 

Gulf of California: Am. Assoc. Petroleum Geologists 

Mem. 3, 1964. Mercator projection; contours in fathoms, 

converted to metres. (8) Elazar Uchupi, 1968, Map 

showing relation of land and submarine topography. 

Mississippi Delta to Bahia de Campeche; U.S. Geol. 

Survey Misc. Inv. Map 1-521. Elazar Uchupi, 1966, Map 

showing relation of land and submarine topography, De 

Soto Canyon to Great Bahama Bank: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-475. Both maps, scale 

1:1,000,000, contours in metres. (9) Elazar Uchupi, 

1965, Map showing relation of land and submarine topography, Nova Scotia to Florida: U.S. Geol. Survey 

Misc. Geol. Inv. Map 1-451. Scale 1:1,000,000, contours 

in metres. (10) R. M. Pratt, 1968, Physiography and 

sediments of the deep-sea basin, in Atlantic continental 

shelf and slope of the United States: U.S. Geol. Survey 

Prof. Paper 529-B, pl. 1. Mercator projection, contours 

in metres. (11) U.S. Naval Oceanographic Service, Contoured position plotting sheet BG-895. Mercator projection; contours in fathoms, converted to metres. 

USES OF THE GEOLOGIC MAP 

Sometimes, when we explain to nongeologists our 

project for a Geologic Map of the United States, we are 

dismayed when asked, "What good is it?" We compilers, 

enmeshed in our many problems of assembling, collating, and generalizing the source data for the map, find it 

difficult to produce a ready answer to this question. 

Nevertheless, the values and uses of an accurate 

Geologic Map of the United States are manifold, not 

only to geologists, but to the public at large. 

First of all, of course, the map displays the rocky 

foundations on which our country is built and is a summation of the nearly two centuries of investigation of 

this foundation by a succession of geologists. It is thus a 

reference work that present and future geologists of the 

country can consult and is of prime importance in the 

education of earth scientists in schools and colleges. 

Further, it can be consulted by geologists in other countries and continents who wish to learn about the geology of the United States; they will compare the map 

with similar national or continental maps of their own 

countries. 

In terms of resources useful to man, the Geologic Map 

lays out accurately the major regions of bedrock in the 

United States upon which many facets of our economy 

depend. It illustrates the areas of stratified rocks that 

are the sources of most of our fuels, and the areas of 

crystalline, plutonic, and volcanic rocks that contain 

important parts of our mineral wealth. The map shows 

areas of complex folding a... faulting, parts of which are 

still tectonically unstable and subject to earthquake 

hazards. To some extent the bedrock represented on the 

map also influences the surface soils, which are of interest in agriculture and engineering works. 

Beyond this, the practical value of the map is less 

tangible, although it can be an important tool for the 

discerning user. Clearly, the map will not pinpoint the 

location of the next producing oil well or the next 

bonanza mine, nor will it give specific advice for the 

location of a dam or a reactor site; these needs can only 

be satisfied on maps on much larger scales, designed for 

specific purposes. Nevertheless, the sapient exploration 

geologist can find upon it significant regional features 

not apparent to the untrained user. Many great petroleum pools occur in stratigraphic traps, or "wedge belts 

of porosity," caused by overlap or truncation, the regional occurrence of which can be seen on the map. Important mineral deposits cluster along regional tectonic 

trends or chains of plutons of specific ages. Finally, the 

Geologic Map will be used in national planning activities in conjunction with other national maps showing environmental features such as climate, vegetation, 

and land use—for the location of power transmission 

corridors, highways, National Parks, wilderness areas, 

reclamation projects, and the like. 

METHODS OF COMPILATION 

Many people, including a surprising number of 

trained geologists, ask the question: How does one go 

about compiling a geologic map of the United States (or 

any small-scale regional geologic map)? No doubt various methods of compilation are possible, yet some general principles apply to all, if an acceptable product is to 

be obtained. We can explain our own methods, which we 

have evolved through trial and error. 

First of all, compilation involves geological comprehension and human skill; no mechanical shortcuts 
FIGURE 7.—Index map of the United States, showing areas covered by the different sources used for the subsea contours on the Geologic Map. Numbers are explained on page 18. 
20 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

are possible. High-altitude or satellite imagery is undoubtedly valuable for interpreting the geology of other 

planets, or even of poorly known regions of the earth, 

but it is merely of peripheral interest in regions where 

large amounts of ground data are available, as in the 

United States. Such images illustrate the broad 

geomorphic features and tectonic lineaments, but they 

reveal little of the nature, relations, or sequences of the 

rocks from which these features are made; also, in the 

United States, wide areas covered by the imagery show 

more of the soil, vegetation, and the works of man than 

of the fundamental geology. Further, there appears to 

be little value in reducing large-scale data into smallscale data by computer. We are not familiar with the 

details of research that has been done on this matter, 

but it is our impression that the computer simply reduces selected lines from the source maps in a manner 

that could be done as well by photography. Precision of 

linework is attained, but there is no generalization that 

would make the product comprehensible. 

We begin instead, where possible, with geologic 

source maps on medium scales, approximately between 

1:500,000 and 1:250,000, or five to ten times our final 

scale of 1:2,500,000 (fig. 8). A certain amount of 

generalization has already been made on these 

medium-scaled geologic maps, yet they still retain 

much of the original geology in manageable form. 

Where only the raw geologic data are available, on 

scales of 1:24,000 to 1:62,500, it has been necessary for 

us to make our own generalization to the medium scale 

before proceeding further. On the other hand, source 

materials on scales of 1:1,000,000 or smaller are ordinarily ill adapted for our purpose, unless they cover 

areas of very simple geology. On these, the hand of 

another compiler has been interposed between us and 

the sources; we must accept on faith his judgment as to 

what should be represented rather than making judgments of our own. 

Beginning with the ideal medium-scaled source 

maps, we make an effort to comprehend the geological 

meaning of the area represented—its geologic history, 

stratigraphy, and tectonics—in order to determine 

what features can most appropriately be selected for use 

on the final map. We then trace these features on clear 

plastic. Some items on the original maps can easily be 

sacrificed, such as subdivisions within gross stratigraphic units, convolutions of contacts produced by erosion or topography, little faults unrelated to the gross 

tectonic pattern, patches of some ubiquitous lava or 

gravel scattered over bedrock, and strips of river alluvium. Other items should be emphasized or even exaggerated, such as inliers of Precambrian rocks amidst 

younger rocks, and the lay of formations and contacts 

produced by folding and faulting. 

Something should be said about the rock units 

selected for tracing. The compiler of each State Map or 

other source map classifies the rocks in a manner most 

appropriate for his area, but which may be inappropriate for an adjoining State or area. In compiling a 

Geologic Map of the United States it would be a simple 

matter merely to accept and copy without coordination 

the classifications in the different areas, but this would 

not result in a meaningful representation for the whole 

country. The compiler of a national map must therefore 

have in mind what he wishes to achieve in a unified 

classification for the country and make his tracings 

accordingly—although this tentative classification may 

have to be more or less modified as the work proceeds. 

These tracings are then reduced photographically 

and replotted. Ordinarily the reduction is to some intermediate scale-1:1,000,000 in regions of complex 

geology, and 1:2,000,000 in regions of simpler geology. 

The results are expressive for their scales, but when a 

further reduction is made to the final 1:2,500,000 scale, 

it is obvious that still greater sacrifices will be necessary. 

The final generalization is always painful to the compiler, because he is thoroughly aware of the significant 

geological features he wishes to portray, yet has very 

little space in which to do so. He is constrained by the 

limits of legible printing of lines and colors, and by the 

eventual user's limits of comprehension. Reduction and 

generalization of the geology to the 1:2,500,000 scale 

brings it down to about the limit at which actual ground 

features can be represented; on smaller scales the compiler must indulge in fantasy. On the 1:2,500,000 scale 

he must endeavor to retain some grasp of reality and to 

present a digest of the significant aspects of the geology. 

For some complex areas this is not possible, even on 

the 1:2,500,000 scale. For these areas King recalls the 

sage advice of Nelson Horatio Darton, a master compiler of an earlier generation: Do not attempt to show 

details of geologic pattern or structure; show merely 

"what is there"—patches of the significant formations, 

not necessarily arranged in any meaningful picture. In 

parts of the United States Map, especially in the Basin 

and Range province of the Western States, we reluctantly have cast our ideals aside and resorted to this 

drastic procedure, producing within the mountain 

ranges a crazy quilt of colored patches of selected units, 

leaving the user to consult maps on larger scales for the 

actual details. 

CONTENTS OF THE GEOLOGIC MAP 

The present Geologic Map of the United States follows the same format as the preceding Geologic Map of 

the United States of 1932. Ideally, both have been de- 
THE GEOLOGIC MAP OF THE UNITED STATES 21 

signed to represent the geological features that the user 

could find if he should visit any locality within its limits, 

that is, the bedrock formations that lie at the surface at 

that locality. In many parts of the country, especially in 

the arid regions of the Southwest, this is literally true. 

In other parts of the country there are lesser or greater 

departures from this ideal, owing mainly to concealment of the bedrock by surficial material. 

Thus, the geologic map is primarily a bedrock map 

and not a surficial geology map. Surficial geology maps 

represent in much detail the surface geology and materials, mainly of Quaternary age, that overlie the bedrock 

and classify them as to kind and origin. Bedrock is 

shown, at most, only in actual outcrops; hence, these 

maps can give little hint as to the fundamental bedrock 

pattern and structure. Making a surficial geology map 

is a worthy enterprise in itself, but one with which we 

are not involved; such maps of all or large parts of the 

country have already been prepared by others (Thorp 

and Smith, 1952; Flint, 1959). 

Consequently, the Geologic Map of the United States 

does not represent the glacial and other deposits of 

Pleistocene age that blanket large parts of the Northern 

Interior States, and loess or drifted sand which are 

extensive in other places. In such areas our representation of the bedrock must perforce be a subcrop or subdrift map sometimes based more on the results of drilling and geophysical data than on outcrops. In the 

Northern Interior States we have marked the limits of 

the later and earlier glaciations to suggest areas in 

which the bedrock is likely to be extensively concealed. 

The Geologic Map does, however, represent the Quaternary deposits along the Atlantic and Gulf Coasts and in 

intermontane areas in the West, where they are essential features of the bedrock pattern. Details of procedure 

are discussed at several places further on (see p. 31). 

The Geologic Map of the United States is not a tectonic 

map. Tectonic maps classify the surface bedrock according to its tectonic rather than its stratigraphic evolution, and they sometimes represent rocks and structures at considerable depths beneath the surface. They 

also symbolize the folding and faulting to which the 

rocks have been subjected and classify the faults as to 

kind and origin. Again, the making of a tectonic map is 

a worthy enterprise in itself, with which we are not here 

involved (although King has been so involved in the 

past); tectonic maps of the United States and of North 

America have already been published (Longwell, 1944; 

Cohee, 1962; King, 1969). 

Nevertheless, the bedrock patterns on a geologic map 

have tectonic implications, and these should not be 

slighted. Where the rocks have been folded, the folding 

should be emphasised by the patterns of the formations, 

and where the formations have been displaced by faults, 

the faults should be represented. Some small-scale 

geologic maps have omitted faults entirely; others have 

shown them only where they offset a map unit. On the 

present map, faults are shown not only to explain offsets 

of the map units, but for their own sake, to illustrate the 

structural grain of the region (see p. 28). 

The Geologic Map of the United States is not constructed according to any particular tectonic principle 

or theory—the permanence of continents, the oceanization of continental material, continental accretion, continental displacement, plate tectonics, or the like. If 

such theories have a place on maps, it is on tectonic 

rather than geologic maps. A geologic map should present a reasonably factual statement of the bedrock that 

actually exists on the continent. It contains the data on 

which a theoretician can build, if he chooses, and hopefully it provides constraints for the more exuberant 

manifestations of theoretical geology. 

The Geologic Map of the United States represents 

only the geology of the continental territory of the United States; the geology of the continental territory of 

Canada and Mexico within the limits of the geographic 

base is not represented. National geologic maps of 

Canada and Mexico have been published (Geological 

Survey of Canada, 1969; Sanchez Mejorada and Lopez 

Ramos, 1968). For our own edification, we have plotted 

on our copy of the United States Map the geology of 

Canada and Mexico within the limits of the base, as 

shown on the national maps of those countries. The 

results are interesting, and the general fit across the 

international boundaries is satisfactory, but there are 

problems in detail of classification and unification that 

it would be presumptuous for us to attempt to resolve. 

Finally, the map does not represent the offshore geology on the continental shelves and continental slopes. 

Geologic maps of variable quality have been made of 

parts of the offshore areas by marine geologists,2but the 

geology of other parts is still imperfectly known; accurate representation of all the offshore areas of the 

United States is still a project for the future. On the 

Geologic Map we have, however, represented the positions of the continental shelves and slopes by means of 

the first 200-metre contour, and of 500-metre contours 

thereafter, and with this guidance the user can, if he 

wishes, mark whatever additional data meet his fancy. 

The sources from which the contours were compiled 

have been listed earlier (see p. 18 and fig. 7). 

CLASSIFICATION OF THE ROCK UNITS 

The general plan of classification of the rock units on 

the Geologic Map of the United States is illustrated by 

'See, for example, the geologic map of the sea bottom in the North Atlantic and Gulf of 

Mexico adjacent to North Ameries by Emery and Uchupi (1972, fig. 87). 
35° 35° 

22 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

A 106° B 106° 

FIGURE 8.—Geologic maps of the Sandia Mountains, N. M., to illustra to the process of generalizing data for the Geologic Map of the United 

States. A,Representation of the area on the primary source, the Geologic Map of New Mexico of 1965; original scale 1:500,000. B, The 

area when generalized, somewhat revised, and replotted on a scale of 1:1,000,000. C, The area as shown on the Geologic Map of the 

United States, a further generalization from B; original scale 1:2,500,000. The representation in C appears crude by comparison with 

A and B, but contains the maximum detail possible for publication scale. 

the map legend. The legend of the present map differs 

from that of the previous map of 1932 in that all items 

are combined into a single tabulation, rather than being 

broken up into separate tabulations for each of the 

geologic provinces. This change is now possible because 

of the progress that has been made during the intervening 40 years in correlation and coordination of the geology of the country. 

On the legend, the Phanerozoic rock units are 

classified according to both age and kind. (The Precambrian rocks are treated in a similar manner so far as 

possible, but they have special problems and will be 

treated in a later report.) Rocks of approximately the 

same age are shown at the same horizontal level in the 

legend—for example, Lower Cretaceous strata and 

Lower Cretaceous granitic rocks. Successive vertical 

columns show different kinds of rocks. Classification 

begins in the first column with what might be considered as the "normal stratified sequence," largely 

marine and obeying the classic laws of superposition, 

and in succeeding columns proceeds to various groupings of the units, then to other facies of similar age such 

as continental and eugeosynclinal, to contemporaneous 

volcanic and plutonic rocks, and finally to the metamorphic equivalents of the others. 

The classification of the rock units is, if possible, 

time-stratigraphic—that is, units which are of approximately the same geologic ages at all places, such as 

systems, series, and stages. Rock-stratigraphic units, 

which may be of different ages from place to place, are 

used only where they illustrate some special geologic 

feature, or where the age classification is uncertain. 

Unlike the legend for the Geologic Map of 1932, very 

few formations and other specific stratigraphic units 

are mentioned; discussion of these will be taken up in 

later reports. 

The first column of the normal stratified sequence 

lists the smaller subdivisions that are used on the map, 

commonly series or groups within the systems. Ordinarily, these can be shown on a map of this scale only in 

regions of simple geology, where the systems occupy 

wide outcrop bands. Places where such subdivisions can 

be represented differ from one system to another, hence 

the first column does not represent a sequence that 

occurs in a single region. In general, the Paleozoic systems can be divided in most detail in the Eastern In- 
THE GEOLOGIC MAP OF THE UNITED STATES 23 

EXPLANATION 

Q 

Quaternary 

Quaternary volcanics 

Pliocene 

Eocene 

Tertiary int rusives 

Upper Cretaceous 

Jurassic 

0 10 20 30 40 50 MILES 

I 

I I 

I 

I I I 1 

IL 

0 10 20 30 40 50 KILOMETRES 

35 

Triassic 

Upper Leonardian 

Lower Leonardian 

Wolfcampian 

l'pper Paleozoic 

Precambrian 

C 106° 

FIGURE 8—Continued. 

terior Region, the Permian in the Western Interior, the 

Cretaceous in the Western Interior and the Atlantic and 

Gulf Coastal Plains, and the Tertiary in the Coastal 

Plains. In a few places, the rocks of the timestratigraphic units dip so gently, or are so thick, that 

they occupy areas too broad to express the geologic 

features adequately, and smaller subdivisions are desirable. The Lower Ordovician of the Ozark Plateau, 

and the Montana Group in the northern Montana plains 

are thus further divided into units 01 a and b, and uK3a 

and b, respectively. 

Most of the geologic systems that form wide outcrop 

bands can be divided on the map into three or four 

comprehensive time-stratigraphic units, but the situation is less satisfactory in the Permian. The Permian 

dips gently and forms wide outcrops in the Midcontinent Region, New Mexico, and northern Arizona. The 

Permian forms smaller, less continuous areas in western Texas, but the rocks here are of fossiliferous marine 

facies and are the basis for the standard subdivision of 

the system. In each of these areas the Permian can be 

subdivided in some detail. Especially impressive is the 

long belt of outcrop in the Midcontinent Region, from 

north-central Texas to Nebraska, where six subdivisions can be traced, to a large extent on continuity of 

outcrops. Nevertheless, the obvious subdivisions in 

each area are not necessarily correlative, and their correlation is in part controversial. In the Permian, unlike 

other systems, recourse therefore had to be made to 

"operational units," which are illustrated in a diagram 

in the lower part of the legend. Permian stratigraphic 

problems will be treated at greater length in a later 

report. 

In the remainder of the United States, the geologic 

systems must be shown as single map units, or several 

systems must be combined, as shown in the second and 

third columns of the legend. Map units that combine the 

systems into more comprehensive groupings are both a 

necessity and a plague to the compiler. In strongly deformed regions, where the strata are turned up steeply, 

outcrop bands of even the major units become very narrow, and the niceties of stratigraphic differentiation, 

appropriate for areas of simpler geology, are out of the 

question. 

In the Eastern United States, we therefore resort to 

hybrids—DS for Devonian and Silurian, 0€ for Ordovician and Cambrian, and the like. This means either that 

the two systems form outcrop bands too narrow to be 

separated successfully on a map of this scale, or else that 

the two systems form a homogeneous body of rocks. In 

making our compilations we have discovered that some 

geologists have used the hybrids in another sense—DS 

for Devonian or Silurian when they are not certain 

which. Where possible, we have avoided this second 
24 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

meaning and have made arbitrary decisions; if the 

weight of evidence is more toward a Devonian than a 

Silurian age, the unit is mapped as Devonian; if we are 

in error, the error can be corrected later. 

In the Cordilleran region of the Western United 

States, even this hybridization is insufficient, and we 

have resorted to the more general groupings of lPz, uPz, 

and 1Mz, for lower Paleozoic (Cambrian to Devonian), 

upper Paleozoic (Mississippian to Permian), and lower 

Mesozoic (Triassic and Jurassic), respectively. This 

usage will make stratigraphers and other precisionists 

unhappy; it will fail to reveal to them, for example, the 

nearly complete absence of the Silurian in most of the 

Rocky Mountains, or the Triassic in the Sierra Nevada. 

The alternative would have been to resort to complex 

letter combinations, varying from one part of the map to 

another, such as CD (Carboniferous and Devonian), 

DSO (Devonian, Silurian, and Ordovician) and De (Devonian, Silurian, Ordovician, and Cambrian) used on 

the Geologic Map of North America of 1965 (Goddard, 

1965)—each requiring a separate color on the map and 

box in the legend. 

Within the Paleozoic areas of the West, the Cambrian 

and Permian at the base and top of the sequence occupy 

significantly large areas in a few places, and are of 

interest both stratigraphically and structurally. These 

large areas are separately shown; elsewhere the two 

systems are merged with the lower and upper Paleozoic. 

Following the normal stratigraphic sequence are columns for various facies. In the Tertiary deposits of the 

West it is important to distinguish between marine and 

continental deposits—the marine Tertiary along the 

Pacific and Gulf Coasts, and the continental Tertiary of 

the interior, which forms wide areas in the Great Plains 

and the intermontane basins of the Rocky Mountains. 

Problems multiply in the pre-Tertiary rocks, and consistent separation of continental deposits becomes impossible. How should one classify coal measures, red 

beds and evaporites, or sheets of fossil sand dunes, all of 

which form broad units in normal stratified sequences, 

which are continental in a sense, yet have at least some 

tenuous marine connections? In general, these are not 

shown as continental deposits on the map. In the preTertiary rocks, only the more obvious continental deposits are so indicated—Cretaceous adjoining orogenic 

areas in the Rocky Mountains, Jurassic in the Northern 

Interior, Permian near the Wichita Mountain axis in 

Oklahoma, and Devonian in the Northern Appalachians. 

Another facies that is separated comprises the 

eugeosynclinal deposits. Modern tectonic studies indicate that "eugeosynclinal" is a broad generic term that 

embraces many specific kinds of rocks formed in different environments—marginal seas, island arcs, deep-sea 

trenches, and ocean floors. Be that as it may, the 

eugeosynclinal suite embraces rocks markedly different from the usual marine and continental deposits of 

the interior of the continent—immature clastic sediments, cherts, and large volumes of volcanics and volcaniclastic sediments. While the generic characters are 

plain, separation into specific varieties is likely to be 

subjective and would, further, unduly clutter representation on the scale of the Geologic Map of the United 

States. 

Eugeosynclinal deposits are represented in the 

coastward parts of the Appalachians (where they are of 

lower Paleozoic age), and the Cordillera (where they are 

of Paleozoic and Mesozoic ages). In addition, eugeosynclinal deposits of Tertiary age, very much like those of 

the earlier ages, occur in the Olympic Peninsula of 

northwestern Washington and are separately mapped. 

Differentiation of rocks of eugeosynclinal facies emphasizes important structural features in the United 

States, as where they have been thrust for many miles 

over normal marine carbonate rocks of similar age in 

the Northern Appalachians and the Great Basin. 

Volcanic rocks likewise form stratified or quasistratified sequences, which are equivalent to, or merge 

laterally into the stratified sedimentary sequences. 

Those of Cenozoic age occur primarily in the Cordilleran region of the Western States, where they are areally 

extensive and offer the greatest opportunities for 

classification and subdivision. On the present map, we 

have intentionally avoided use of the units Tv and QPv 

of the 1932 map, for undifferentiated volcanic rocks, 

believing that the data are now sufficient, or nearly so, 

to permit a meaningful regional subdivision. Basis for 

classification is primarily by age (based on fossils and 

radiometric data), but felsic or siliceous varieties are 

differentiated where data are available; in addition, 

several other compositional varieties are shown in the 

Pacific Northwest. Details of classification of the 

Cenozoic volcanic rocks will be considered in a later 

reports. 

In the pre-Tertiary systems, volcanic rocks are distinguished in few places. They unquestionably form 

large volumes of the eugeosynclinal deposits, but as 

these are in part volcanic by definition, their volcanic 

components can generally be surmised. In the lower 

Paleozoic eugeosynclinal deposits of the Appalachians, 

however, volcanic rocks form well-marked entities, the 

areally more extensive of which are separately mapped. 

Among the plutonic rocks, granitic varieties are the 

most extensive and the most amenable to classification 

by age, mainly on the basis of radiometric data but 

partly on their geologic relations to the country rocks. 

Mafic varieties are less extensive and are not subdivided in detail. The ultramafic rocks are a class by 
THE GEOLOGIC MAP OF THE UNITED STATES 25 

themselves and are not designated by, age; large parts of 

them, at least, are fragments of mantle material of 

enigmatic age which have arrived at their present positions by tectonic rather than magmatic processes. 

Metamorphic rocks are indicated primarily by overprints on the parent rock units, except in parts of the 

Piedmont province of the Appalachians and in the Cascade Range of the Pacific Northwest, where the ages of 

the parent rocks are as yet undetermined; such rocks 

are designated as "metamorphic complexes." The 

metamorphic overprint is not used in the Precambrian 

rocks; the designation of certain units as "orthogneiss" 

or "paragneiss" seems sufficient to indicate their 

metamorphic nature. 

Rocks shown as "metamorphic" are primarily those of 

amphibolite grade or higher, that is, with garnet, kyanite, sillimanite, and other diagnostic minerals. Rocks 

that have been altered to greenschist grade, with chlorite, biotite, and similar diagnostic minerals, are not 

represented as metamorphic. Near the West Coast, in 

California and Oregon, upper Mesozoic eugeosynclinal 

rocks (uMze, Ke) have been subjected to high-pressure 

low-temperature metamorphism, producing various 

blueschist minerals. In this domain regionally 

metamorphosed rocks containing glaucophane, lawsonite, and pumpellyite are shown as metamorphic; lower 

grade rocks with laumontite and similar minerals are 

not. In a few places on the map the metamorphic overprint is used to express geologically significant 

metamorphic rocks or metamorphic contrasts, without 

regard to mineral content; thus some of the rocks of the 

Olympic Mountains, Wash., are shown as metamorphic, 

even though they are low grade mineralogically. 

SYMBOLIZATION OF ROCK UNITS 

On the Geologic Map itself, the rock units are differentiated by colors, patterns, and letter-number symbols. Of these, the colors present the greatest problems 

and hence will be dealt with in most detail. 

Colors on a geologic map have two facets—geological 

philosophy and the technology of lithography and printing. The latter need not concern us greatly here, as it is a 

matter of the techniques of producing colored maps; 

these change from generation to generation, although 

the general results are much the same. The geological 

philosophy is more fundamental, and one upon which 

there are still significant differences of opinion and 

usage. 

One can, if one wishes, produce an empirical representation, in which the choice of colors on the map has 

no general meaning—usually for the purpose of creating contrasts between map units, thereby enhancing 

legibility. An excellent example is the Geologic Map of 

Pennsylvania (Gray and others, 1960), in which the 

colors are used unsystematically, yet eloquently portray the structure and stratigraphy of the State. This 

method is best adapted to large-scale maps, or regional 

maps of restricted areas, and would be inappropriate for 

the Geologic Map of the United States. 

The best alternative is to match the orderly sequence 

of rock units from oldest to youngest with an orderly 

sequence of prismatic colors (consult the Munsell color 

notation system, which has been adopted by the American Standards Association). As stated by Willis (1912, 

p. 27): 

Let it be agreed that the sequence red, purple, violet, blue, green, and 

yellow shall be adopted to represent the succession of formations, 

groups, or series of sedimentary rocks from older to younger and let 

the order of colors be invariable according to the principle stated 

above, no matter what part or how much of the geologic column is 

represented. Then red will always represent something older than 

that which is shown in purple, or violet, or blue, etc. Blue will always 

be older than that shown in green or yellow. In looking at any geologic 

map thus colored the student would at once know which were the older 

and which were the younger sedimentary rocks. The essential features of the sequence and structure would be immediately obvious. 

Most systems of coloring geologic maps use this general 

principle, although with greater or lesser departures 

from it, as we shall see. 

Efforts to achieve a systematic scheme for coloring 

geologic maps are nearly a century old, and their history is pertinent. By the 1870's, the proliferation of 

geological investigations in both Europe and North 

America made obvious the need to systematize 

results—in stratigraphy, mineralogy, paleontology, 

and the making of geologic maps. This led to the convening of the First International Geological Congress in 

Paris in 1878, the results of which were inconclusive. 

Decisions were therefore deferred until the Second 

Congress (Bologna) in 1881 and the Third Congress 

(Berlin) in 1885.3Only the results that pertain to the 

making of geologic maps need concern us here; many of 

the recommendations made on the other subjects have 

only historical interest. 

The prime need at the time was a comprehensive 

scheme of symbolization for use on a Geologic Map of 

Europe, then being compiled by an international committee. Although some geologists protested that the 

results were provisional and experimental and applied 

only to the European project (Frazer, 1888, p. 95), there 

were misgivings by others at the time that they would 

crystallize into a permanent general usage (Gilbert, 

1887, p. 432)—a foreboding that has been amply 

justified by subsequent events. Immediately thereafter, 

the color scheme adopted by the 1881 and 1885 Congresses was used by C. H. Hitchcock (1887, p. 466-467) 

'The results have been published in the respective reports of the first three Congresses, in 

which the official language was French. For the American reader, they were usefully summarized by the secretary of the American Committee, Persifor Frazer (1888). In addition. G. 

K. Gilbert (1887) presented a lengthy critique of the results of the Third Congress. 
26 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

for coloring his Geologic Map of the United States (see p. 

6), and today it is commonly referred to as the "International system" by European geologists, who have 

urged its adoption on a worldwide basis. 

Meanwhile, however, J. W. Powell was appointed 

second Director of the U.S. Geological Survey in March, 

1881, and in his first official report, written a few 

months later, announced a scheme of stratigraphic 

nomenclature, map coloring, and patterns to be used 

thenceforth in Survey publications (Powell, 1882, p. 

xliii-liii) even though: "On the 26th of September next 

[1881] a congress of geologists of the world will assemble 

at Bologna, Italy, to confer on this subject. It is unfortunate that advantage cannot be taken of the deliberations of so great a body of savants in the publication of 

these monographs, but the exigencies of the work will 

not permit of longer delay even for so important a purpose" (p. xlii). Viewed from the perspective of nearly a 

century the justification for this precipitate action 

seems specious; it was probably dictated by immediate 

political problems in Washington.4Somewhat later he 

presented the methods used by the U.S. Geological Survey to an international audience in a paper at the Berlin 

Congress (Powell, 1888, especially p. 236-239), delivered in his behalf by W J McGee. 

The scheme proposed by Powell has laid the groundwork for usage in publications of the U.S. Geological 

Survey to the present time. Detailed specifications for 

usage in these publications were promulgated in 1890, 

after areas of diverse geology in many parts of the country had been sampled by mapping, and after conferences 

with 18 of the leading Survey geologists of the time 

(Powell, 1890, p. 56-79); they differ in detail from the 

original proposal of 1881, but the broader features remain the same. Thus, Powell's original map colors, with 

subsequent elaborations, have become the United 

States, or "American color system." 

The principal differences between the "American" 

and the "International" color systems are in the 

stratified sedimentary rocks; the intrusive and volcanic 

rocks in both systems are shown in more brilliant tints, 

with a preference for the reds and oranges. The two 

systems are compared in table 1; the original proposal 

for each is followed by samples of subsequent usage, 

including that on the present Geologic Map of the United States. 

The reasons for the differences between the two systems are ably explained by Willis (1912, p. 24-26): 

The European international color scheme embodies the results of 

prolonged consideration by the international committee who were 

charged by the Geological Congress with the duty of preparing the 

map of Europe. In it can be recognized some elements of the French 

usage, particularly in the colors employed for the Mesozoic and Tertiary terranes. German influence appears in the selection of tones for 

'Stegner (1954, p. 271-272) presents some interesting speculations on the circumstances. 

the Paleozoic terranes, and the familiar association of gray with 

Carboniferous and of pink with the ancient crystalline schists is an 

obvious result of general practice. So also is the use of strong brilliant 

colors for the igneous rocks. The writer is not definitely informed 

regarding the discussion of principles through which the result was 

reached, but a study of the color schemes in the light of what is 

published concerning the controlling principles, it would seem that 

the committee recognized (1) established usage, (2) the order of prismatic colors from purple through blue and green to yellow for that 

portion of the scheme relating to the Triassic and post-Triassic terranes, and (3) the arbitrary principle that Mesozoic terranes should be 

distinguished from Paleozoic by a very decided contrast of light and 

shade, the Paleozoic terranes being indicated by dark colors. 

The European color scheme is exceedingly well adapted to delineate 

the geology of Europe and would apply very well to that portion of 

western North America in which the Mesozoic and Tertiary formations occupy large areas in contrast to the Paleozoic terranes, as they 

do in Europe also. The color scheme thus commends itself through the 

beautiful appearance of the map. It must not be forgotten, however, 

that Europe represents a special form of geologic structure. The continent is made up of extensive areas of Mesozoic and Tertiary strata 

surrounding relatively small exposures of Paleozoic terranes. This 

arrangement of younger strata about older nuclei is, from the standpoint of the cartographer, the most important feature which the continent presents. The committee with good reason sought to emphasize 

the fact and through that emphasis the map of Europe gains in 

expression and educational value. The greater part of the map is 

easily legible, being covered only by the light colors which are used for 

the Mesozoic and Tertiary, and the difficulties which arise in attempting to read the geology of the minor Paleozoic areas are not forced 

upon the attention. 

But the international scheme is unfitted to lands in which the 

Paleozoic terranes predominate and are minutely subdivided, for the 

density of the colors selected for the Paleozoic would produce a map 

that would offend good taste and be illegible. Moreover, inasmuch as 

the range of prismatic colors from purple, blue, and green to yellow is 

preempted in the European color scheme for Mesozoic and Tertiary 

terranes and the reds assigned to the ancient crystalline and eruptive 

rocks, the choice of colors remaining available for the Paleozoic is 

much too limited for satisfactory discriminations. This is at once 

evident on an examination of the Paleozoic areas as represented on 

the international map—such, for instance, as the coal fields of Belgium and France, or the peninsula of Brittany, or Wales and Scotland. 

Although the distinctions are limited to a few great systems they are 

recognizable only on close inspection and the areas are indistinguishable from one another at a little distance. A geologic map of eastern 

North America printed in these dark colors with so little difference of 

hue or shade would fail to present adequately the great Appalachian 

zone as distinguished from the broad plateaus of the coal measures 

and the domelike uplifts of the Cincinnati axis. In the Precambrian 

also the number of formations recognized in North America is greatly 

in excess of those distinguished in Europe, and the simplicity of the 

European scheme renders it insufficient to delineate the geology of the 

Lake Superior region and the Canadian Shield. 

The validity of Willis' evaluation is substantiated by 

the results of attempts to apply the so-called "International system" to continents where the gross geologic 

structure and surface distribution of the geologic systems differ significantly from those of Europe. The inadequacy of the "International system" for Australia is 

lamentably evident on the otherwise beautifully 

printed sheets for this part of the Geological Map of the 

World (Bureau of Mineral Resources, Geology, and 
THE GEOLOGIC MAP OF THE UNITED STATES 27 

TABLE l.—Comparison between "American" and "International" systems of coloring stratified rocks on maps 

System 

American color system International color system 

U.S. Geol. Survey 

2d Ann. Rept. 

1881' 

U.S. Geol. Survey 

Folio 227 

19452 

Geologic Map of 

United States 

19743 

2d and 3d 

Internat. Geol. Cong. 

Bologna and Berlin 

1881, 18854 

Eastern 

Siberia 

19643 

Geologic Map of 

France 

19688 

Quaternary Gray Brownish yellow Galey P 

ra 

 yellow Undecided Gray Gray 

Tertiary Yellow Yellow ocher 

Light yellow 

Pale brown 

Pale flesh 

Dark yellow 

Greenish yellow 

Yellow 

Light yellow 

Yellow 

Greenish yellow 

Light yellow 

Dark yellow 

Orange yellow 

Cretaceous 

Green 

Olive green 

Olive green 

Yellow green 

Cool green 

Green Green Green 

Jurassic Blue green Blue green Blue Blue Blue 

Triassic Peacock blue Peacock blue Violet Violet Violet and purple 

Permian 

Blue 

Light blue Cool blue 

Gray 

Warm brown Gray 

Pennsylvanian 

Blue 

Gray 

Dark gray Dark gray 

Mississippian Warm blue 

Devonian 

Purple 

Blue gray Blue Brown Cool brown Brown 

Silurian Blue purple Purple 

Greenish gray 

Olive green Olive gray 

Ordovician Red purple Rose and pink Pale green Olive green 

Cambrian Brick red Red and coral Rose Warm brown 

Precambrian Brown Brownish red 

Gray brown 

Yellow brown 

Brown 

Bluish gray 

Brick red 

Rose Brown Pale brown 

'Powell, 1882, p. xi-Iv. 

'Butts, 1945, specifications of folio series on inside covers ("Geologic Atlas of the 

United States"). 

'King and Beikman, 1974, this report and map. 

"For summary, see Frazer, 1888. 

"Kransky, 1964. 

'Service de la Carte Geologique du France, 1968. 

Geophysics, 1965). The Tectonic Map of the country 

(Tectonic Map Committee, Geological Society of Australia, 1971) and recent maps of individual states use 

an approximation of the "American system" and produce a much clearer picture of the regional geology. It is 

of interest to compare the systems of Europe and the 

United States with that adopted on the Geological Map 

of Canada (Geological Survey of Canada, 1969); as in 

the "American system" it follows a prismatic scale, but 

the blue colors are extended downward to the base of the 

Paleozoic, reserving the red, orange, and brown colors 

for the Precambrian, in which rocks of many kinds and 

ages must be differentiated. 

The colors used on the present Geologic Map of the 

United States conform as far as possible to the traditional "American system," in which the prismatic scale 

of colors embraces the whole geological sequence, from 

earliest Precambrian into the Quaternary. Some departures are necessary, it is true, due to modern methods of 

lithography, and to obtain greater emphasis of some 

units. (Similar freedom has been exercised within the 

so-called "International system," as is evident in the 

last two columns of table 1.) In order to clarify the 

growing complexity of the Precambrian sequence, the 

rocks of division X are separated from the prevailing 

reds and browns of the other divisions by the use of tints 

of bluish gray; and the Oligocene and Miocene Series of 

the Tertiary are distinguished from the prevailing yellows of the others by the use of flesh and pale-brown 

tints. 

Traditionally, on geologic maps published by the U.S. 

Geological Survey, the meaning of colors has been enhanced by the use of patterns, as explained in the text 

that accompanied all the folios of the Geologic Atlas: 

"Patterns composed of parallel straight lines are used to 

represent sedimentary formations deposited in the sea, 

in lakes, or in other bodies of standing water. Patterns 

of dots and circles represent alluvial, glacial, and eolian 

formations. Patterns of triangles and rhombs are used 

for igneous formations. Metamorphic rocks of unknown 

origin are represented by short dashes irregularly 

placed; if the rock is schist the dashes may be arranged 
28 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

in wavy lines parallel to the structure planes." Use of 

patterns was more feasible with the older methods of 

lithography than the methods used at present, in which 

it is more practical to use flat tints; but they can still be 

achieved by overprints on the flat colors—as has been 

done on recent maps of the U.S. Geological Survey and 

on maps published in the Soviet Union and elsewhere. 

One need only to study a map without patterns to become painfully aware of their mnemonic value; not even 

the use of vivid, contrasting colors for plutonic rocks and 

lavas (as on the Geologic Map of France, 1968) conveys 

the distinctions as clearly and immediately as do patterns. 

On the Geologic Map of the United States, overprinted patterns are used to indicate plutonic rocks, 

metamorphic rocks, and some of the volcanic rocks. For 

the granitic class of plutonic rocks we have used the 

"short dashes irregularly placed" (there is no better 

descriptive term for this excellent and expressive pattern); it implies massive crystalline rocks, so that its 

former use in the folios for metamorphic rocks has become inappropriate. The pattern is superposed on a 

color expressing the age of the granitic pluton (which 

can now be determined from radiometric data). For 

metamorphic rocks a dense halftone overprint is substituted; the "random dashes" of earlier maps were too 

weak to differentiate these rocks clearly. For the volcanic rocks we use various v-patterns, a simplified form 

of the "rhombs and triangles" of the folios. 

It is most desirable that colors on a map be identified 

by letter/number symbols to assist the user in comparing the map with its legend. The handicap of a colored 

map without symbols is at once apparent to the user of 

the otherwise excellent sheets of the 1:200,000 Geologic 

Map of Switzerland, in whose complex parts there are 

many small patches and bands of color that he must 

endeavor to match with one of an assortment of similar 

colors in the legend. 

The simplest form of symbolization is by numbers, 

which are appropriate where there are only a few units, 

but confusing when they number 50 or more, as on some 

Canadian maps. Being entirely noncommittal, numbers have no mnemonic value—an advantage or a disadvantage, depending on the circumstances. 

Much more common are single or multiple letters, or 

letter combined with numbers, several systems of which 

have been used—no one better than the other. In the 

specifications for the Geologic Map of Europe adopted by 

the International Geological Congress, geologic ages of 

strata were expressed by roman lowercase letters, 

modified by suffixed numbers, and different kinds of 

eruptive rocks were shown by Greek letters. On many 

other geologic maps, including those of the U.S. Geological Survey, general age is expressed by capital roman 

letters representing the geological systems, modified by 

suffixed lowercase letters. The symbols used on the 

Geologic Map of the United States resemble those of the 

latter system; variants are introduced by prefixing the 

initials 1, m, and u (for "lower," "middle," and "upper"), 

to avoid complicating the suffix, and by use of suffixed 

numerals rather than letters for the smaller age divisions, reserving lowercase letters for descriptive 

modifiers, such as c for "continental" and v for "volcanic." Throughout, we have avoided long strings of 

modifying suffixed letters, which often become annoying acronyms. The only exceptions are the symbols for 

Tertiary eugeosynclinal deposits of the Olympic Peninsula, Wash.—Tmoe and Toee, for "Tertiary MioceneOligocene eugeosynclinal" and "Tertiary OligoceneEocene eugeosynclinal." Not all the units shown on the 

Geologic Map require qualification by a lowercase 

suffixed letter. Many of them represent a whole geologic 

system (or several systems); for these, the capital letters 

alone are sufficient. 

REPRESENTATION OF FAULTS 

As indicated earlier (p. 21), faults are shown on the 

Geologic Map of the United States, not only to explain 

offsets of map units, but for their own sake, to express 

the structural grain of the area. The density of faults 

represented on the geologic map thus equals that which 

would appear on a tectonic map of the country, but they 

are marked simply as faults, not as low-angle or highangle thrust faults, normal faults, or strike-slip faults; 

for this information the user should consult the appropriate tectonic map. 

By the method adopted, faults are shown not only at 

contacts between map units, but within map units. 

Some of these are major faults with large displacements. In Arkansas, the great frontal thrusts of the 

Ouachita Mountains all lie within the combined 

Atokan and Morrowan Series (lPi ), which is here more 

than 4 miles thick; the lower part of the unit is displaced 

against the upper, as would be evident on a more detailed map. Other faults within map units are themselves minor, but are components of major structures; 

those lying in the volcanic units of eastern Oregon are 

merely a sampling of the dense swarms that appear on 

maps of larger scale, which are arranged in regional 

sets of several directions. 

In the Basin and Range province of the Western 

United States we have made a special effort to represent 

range-front faults where geomorphic evidence (steep 

mountain faces, even base lines, and the like) requires 

their existence; more timid compilers often fail to show 

them, thereby creating the illusion of an unfaulted terrane. Commonly, the range-front fault lies a short distance out from the foot of the range beneath the al- 
EXPLANATION 

Rocks of the mountain areas 

(Tertiary and older) 

Rocks of intermontane basins 

(mainly Quaternary) 

Xs. 

Sevier thrusts 

(middle Cretaceous) 

:97,00:1•74

0000.. 

Metamorphic infrastructure 

(probably related to Sevier 

thrusting) 

1111111111I 

Roberts Roberts thrust (late Devonian to early Mississippian) 

Golcanda thrust (late 

Permian to early Triassic) 

ill 

91 

1 • 

ii,. 

12.4-- 

L

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e 

• 

'1 0 

4 

#4 

sl 

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D 

NEVADA 

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. 

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 • 

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41' 

40° 

38° 

100 150 MILES 

100 150 KILOMETRES 

7 

50 

0 50 

THE GEOLOGIC MAP OF THE UNITED STATES 29 

luvium; on a large-scale map it would be shown as a 

dotted line parallel to and closely adjacent to the bedrock contact of the range. On the small scale of the 

present Geologic Map, only the fault itself is shown, and 

the bedrock contact is not. 

Although the faults on the geologic map are 

unclassified, their patterns suggest something of their 

geometry. For example, in the Taconic region of eastern 

New York State, an array of sinuous fault traces (many 

closing on themselves) expresses flat or gently dipping 

thrusts and contrasts strongly with the straight or angularly bent traces of the high-angle faults of the 

Adirondack uplift and those on the borders of the belts 

of Triassic rocks. 

Low-angle thrust faults geometrically like those in 

the Taconic area of eastern New York State are components of the internal structure of the ranges in the Great 

Basin section of the Basin and Range province (fig. 9). 

They are older than the range-front faults just mentioned, which greatly disrupt them. The major lowangle thrusts of the Great Basin section are recognizable from range to range by distinctive rocks on their 

upper and lower plates, but their original continuity is 

difficult to represent on the geologic map because of the 

confusing array of other rocks and structures; dotted 

lines are used in a few places to suggest the obvious 

connections. 

The regional extent of these faults is indicated on the 

accompanying figure, which shows the inferred traces of 

the frontal thrusts of the Sevier orogenic belt in Utah (of 

mid-Cretaceous age), of the Roberts thrust in northcentral Nevada (of late Devonian—early Mississippian 

age), and of the Golconda thrust a little farther west (of 

late Permian—early Triassic age). On large-scale maps 

the experienced eye could detect each of these by its 

characteristic "trademark," but these "trademarks" are 

necessarily blurred on the much generalized, smallscale Geologic Map of the United States. Nevertheless, 

even on this map the different segments of the Roberts 

thrust are apparent from the juxtaposition of 

116° 114 112 

FIGURE 9.—Map of the Great Basin in Nevada and Utah, showing regional extent of major low-angle thrust faults that are represented 

on the Geologic Map of the United States as exposed fragments in the mountain areas. The thrusts involve only the Paleozoic and 

Mesozoic strata, whereas the mountain areas also include plutonic and stratified rocks younger than the thrusting. 
110° 107' 

95° 94° 

0 50 

50 

T0 

100 MILES 

100 KILOMETRES 

I 1 1 

45° 

11 

PzQ 

7----U't 

.... 

e % 

1 

I OpE I 

I ‘ / 

• ... 

....,_.. 

Vir 

---4 L., 

43* 

42' 

3 

30 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

eugeosynclinal lower Paleozoic rocks (1Pze) and normal 

lower Paleozoic rocks (IPz) on its upper and lower plates. 

Explanation is needed of the nearly circular fault 

traces of small to medium diameters which appear in 

places on the Geologic Map (fig. 10). They are of multiple origins, some being the rims of calderas (produced by 

terrestrial volcanism), others the edges of astroblemes 

(produced by extraterrestrial impact). Parts of these are 

shown by dashes, not to imply that they are hypothetical but to suggest that the marginal faulting around the 

central structure is discontinuous. As with the other 

faults, they are not further symbolized on the geologic 

map. Moreover, they are shown only where they conspicuously affect the surface bedrock pattern. Many 

more calderas and astroblemes could be represented on 

a tectonic map, but they would not conspicuously affect 

surface geology; such calderas are old, worn down, and 

largely buried, and the astroblemes are little structures 

within single map units. We have made one exception of 

the great caldera rim in Yellowstone National Park, 

nearly 40 mi (65 km) in diameter, even though it is 

extensively concealed by ash-flow tuffs and rhyolite 

EXPLANATION 

Q 

Quaternary sediments 

Quaternary volcanics 

Tertiary sediments 

Tertiary volcanics 

Ti • 

Tertiary intrusives 

Mesozoic 

Paleozoic 

Ordovician 

Precambrian 

FIGURE 10.—Circular faults shown on the Geologic Map of the United States. A and B are associated with calderas, C and D with astroblemes. A, Yellowstone and Island Park calderas, northwestern Wyoming and adjacent Montana. B, Calderas in San Juan Mountains, Colorado. C, Monson structure, central Iowa. D, Wells Creek Basin, west Tennessee. Contacts are the same as on the Geologic 

Map, but units are grouped in the legend. 
THE GEOLOGIC MAP OF THE UNITED STATES 31 

flows resulting from the eruption; it is one of the major 

structural features of the United States and should not 

be ignored. 

REPRESENTATION OF CONTACTS 

Throughout the Geologic Map of the United States, 

contacts between map units (where not faulted) are 

represented by fine solid lines except where one set of 

map units merges with another along the strike; here 

the colors of the two are juxtaposed without a contact 

line. A conspicuous example is in northwestern Iowa 

and south-western Minnesota, where subdivisions of 

the Upper Cretaceous that are separately shown to the 

west give place eastward to undifferentiated Upper 

Cretaceous. Along the outcrop belts in the folded Appalachians, subdivisions of the Paleozoic systems similarly give place along the strike to undivided systems, 

but these features are of smaller areal extent, and are 

only apparent on close inspection of the map. 

A "state-line unconformity" occurs between North 

and South Dakota, in an area of heavy drift cover where 

the contact between the Colorado and Montana Groups 

of the Upper Cretaceous (uK2 and uK.3) fails to match by 

several counties on the bedrock maps of the respective 

States, the contact has been reconciled by sketching 

across the state line. Other "state-line unconformities" 

(discrepancies between map units as represented in adjoining States) abounded on our initial compilations but 

were resolved upon inquiry. 

Subdivisions of the Eocene Series in the Mississippi 

Embayment of western Tennessee and the Atlantic 

Coastal Plain of southwest Georgia are inaccurately 

located; in Tennessee the contact between Tee and Tea is 

concealed by a blanket of Pleistocene loess, and in Georgia by residuum. The location of the contacts between 

the Eocene and the Oligocene (To) and the Oligocene 

and the Miocene (Tm) in Georgia are also in doubt. 

Drilling beneath these blankets is insufficient to clarify 

the actual bedrock pattern, and for want of better information we have projected the contacts hypothetically 

across them. 

The southwestern part of the Blue Ridge province of 

northern Georgia was inadequately mapped at the 

time of compilation, but a hypothetical contact between 

supracrustal rocks (Z) and basement rocks (Ym) was 

mapped. For a more accurate representation, see the 

new Geologic Map of Georgia (in press, 1974). 

Dotted lines, expressing contacts buried by younger 

deposits, are used sparingly on the Geologic Map, for the 

most part to indicate connections between closely adjacent areas of outcrop but also in southwestern Minnesota and in the Mississippi Embayment. 

Those in Minnesota are boundaries between Precambrian units beneath a blanket of Upper Cretaceous 

strata and glacial drift, as shown on the Bedrock 

Geologic Map of Minnesota (Sims, 1970), and are supported by a variety of drilling and geophysical data. 

Those in the Mississippi Embayment are contacts 

between various series of the Tertiary and subdivisions 

within the Eocene Series buried beneath the Quaternary deposits of the alluvial valley of the Mississippi 

River. The Quaternary deposits (Pleistocene and 

Holocene) are several hundred feet thick and are an 

essential feature of the bedrock pattern. The Tertiary 

units are exposed on each side of the alluvial valley and 

are connected beneath it in subcrop, where they are 

represented by dotted lines. These lines explain buried 

features of interest, especially the large outliers of 

Jackson Group (Tea) north of the normal belt of outcrop, 

where they are preserved in the downwarp of the Desha 

basin. The extent of the Tertiary units in subcrop is well 

known from many drill data, which were first assembled by Fisk (1944, pl. 10); representation on the 

Geologic Map includes some later refinements. 

In the areas on the Geologic Map where extensive 

subcrop is represented by dotted contacts, it is clarified 

by letter symbols of the buried units in parentheses. 

SUBCROP GEOLOGY 

The present Geologic Map of the United States, like 

the map of 1932, is intended to represent bedrock rather 

than surficial deposits. The map shows principally the 

distribution of the Tertiary and older rocks, and the 

surficial deposits of the country are largely of Quaternary age. Quaternary deposits are shown on the map 

where they are thick enough, or tectonically significant 

enough, to be an essential part of the bedrock pattern. In 

some parts of the country, bedrock is represented even 

where the cover of surficial deposits is extensive and 

outcrops are sparse; here, outcrops must be supplemented by drill and geophysical data to produce a 

subcrop map. The most extensive area of such surficial 

cover is in the part of the Northern Interior States 

subjected to continental glaciations during Pleistocene 

time, but smaller areas occur elsewhere outside the 

glacial limits. 

NORTHERN INTERIOR STATES 

In the Northern Interior States the extent of the 

surficial cover is suggested on the geologic map by lines 

showing the limits of the latest (Wisconsin) glaciations 

and of the older glaciations. Concealment of the bedrock 

is greatest in the area of the Wisconsin glaciations, but 

it is nearly equalled in a few parts of the area of the older 

glaciations. 

The extent of the concealment is illustrated by the 

accompanying maps of eastern South Dakota (fig. 11). 

West of the Missouri River there is little surficial cover, 

and the bedrock is mapped from outcrops. East of the 
EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

EXPLANATION 

Terminal moraine 

Ground moraine and 

glacial outwash 

Lacustrine deposits 

rib 

Areas not glaciated 

100° 

Q 

Glacial deposits 

Pliocene continental deposits 

Undifferentiated older Tertiary 

Lance Formation 

Montana Group 

Colorado Group 

Precambrian Y 

Sioux Quartzite 

Precambrian W 

0 

200 KILOMETRES 

32 

FIGURE 11.—Maps of eastern South Dakota, to illustrate problems of representing bedrock 

cover of surficial deposits. A, Surficial, or Quaternary glacial deposits, generalized from 

as shown on Geologic Map of the United States. Outcrops of bedrock east of Missouri 

South Dakota of 1953; contacts elsewhere, shown by dotted lines, are those shown on 

States and are subcrop representations based on subsurface data. 

geology in areas with extensive 

Flint (1959). B, Bedrock geology 

River are from Geologic Map of 

the Geologic Map of the United 
EXPLANATION 

Pennsylvanian 

...... m...j 

Mississippian 

F

p 

Devonian 

Silurian 

Ord 

Ordovician 

r A 

Southern limit 

of Wisconsin 

glaciation 

Preglacial river 

channel 

THE GEOLOGIC MAP OF THE UNITED STATES 33 

river the cover of Wisconsin glacial deposits is nearly 

complete, including massive terminal moraines, and 

intervening areas of ground moraine, outwash, and 

lacustrine deposits. Bedrock emerges only in a few 

places along the streams, with two or three outcrops to a 

county at most, and in some counties none at all. 

Most of the glaciated region in the Northern Interior 

States is a terrane of gently dipping Paleozoic strata 

that had been dissected into a dendritic pattern by 

stream erosion prior to the glaciations. The contacts 

between the map units show the crenulations characteristic of such dissection, including long narrow projections of older units into areas of younger, which express 

preglacial stream valleys, now filled and obliterated. 

Especially striking examples of these features occur 

along the Ordovician-Silurian contact in western Ohio 

and eastern Indiana. Segments of preglacial stream 

valleys have been known in this region from water-well 

drilling since the turn of the century, but only in the last 

few decades has it been recognized that they are all 

parts of a single major river system, quite different from 

the present major Ohio River system (Horberg, 1945, p. 

356-359; Janssen, 1952). The master stream was the 

Teays River, named for a now-empty valley near 

Charleston, W. Va. (Tight, 1903, p. 50). Its headwaters 

were the present New and Kanawha Rivers, which 

drain from the Appalachian Highlands. Northwest of 

the present Ohio River, the valley of the Teays passes 

under glacial deposits and has no surface expression, 

but it can be traced in subcrop across Ohio and Indiana 

(where it and its tributaries produced the crenulations 

in the above-noted Ordovician-Silurian contact), and 

into central Illinois, where it joined the ancestral Mississippi River near the present course of the Illinois 

River (fig. 12). 

Other preglacial valleys in northwestern Missouri 

are illustrated on an inset map accompanying the 

Geologic Map of Missouri (McCracken and others, 1961) 

and produce crenulations in the contacts between Pennsylvanian map units unrelated to modern drainage. 

Another crenulation, on the Precambrian-Cambrian 

contact in central Wisconsin, has been shown on many 

earlier geologic maps and was thought to have been 

produced by the ancestral Wisconsin River; however, 

modern reviews of the subcrop data indicate that this 

valley, if it exists, does not penetrate the top of the 

Precambrian in this manner (Dutton and Bradley, 

1970, sheet 5). 

In the northern two-thirds of Minnesota the prevailing terrane is Precambrian rather than Paleozoic, and 

in its western part the amount of surficial cover again 

requires recourse to subcrop mapping. On the Geologic 

Map of 1932, this part was mostly represented as 

Quaternary, for want of better data. Much more information on the bedrock is available now, from drilling 

and geophysical surveys, so the patterns of Precambrian units can be extended westward across the State 

to join the Precambrian in the valley of the Red River 

0 50 100 

1 

50

1 

0 100 

150 200 MILES 

1 1 

1 1 

150 200 KILOMETRES 

FiGuitz 12.—Generalized geologic map of eastern Middle Western States, to show relation of subcrop geology of preglacial river systems. 

Preglacial drainage compiled from Tight (1903), Horberg (1945 ), and other sources. 
34 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

shown on the bedrock geologic map of North Dakota 

(Sims, 1970; Gordon, 1969). 

The situation is complicated by the fact that in part of 

northern Minnesota a thin sheet of unconsolidated Cretaceous deposits intervenes between the Pleistocene 

and the Precambrian (fig. 13). These deposits are the 

Coleraine Formation (Sloan, 1964, p. 8-15), which has 

been exposed in mine workings on the south flank of the 

Mesabi Range and is known elsewhere from drilling and sparse natural outcrops. Part of the formation 

is marine, and its fossils indicate that it is equivalent to 

the Upper Cretaceous Colorado Group (uK2) that occurs 

in North and South Dakota to the west. 

We believe that, for purposes of the Geologic Map of 

the United States, the feature of primary interest in 

northern Minnesota is the Precambrian bedrock, and 

we have accordingly extended it in subcrop across most 

of this part of the State. The Pleistocene deposits can be 

sacrificed without regret, even though they attain 

thicknesses of many hundreds of feet in places. Omission of the Cretaceous Coleraine Formation is less defensible, and under other circumstances it should 

perhaps be represented, yet to do so here would greatly 

obscure the essential Precambrian pattern. We have 

96° 94° 92° 

EXPLANATION 

L(aut

s

(b i

Upper Cretaceous 

uKa- Upper Cretaceous as shown 

on Geologic Map of the United 

States 

uKb- Thin eastward extension of 

upper Cretaceous 

Jurassic continental 

deposits 

Paleozoic 

Cambrian and Ordovician 

Precambrian Z 

Precambrian Y 

V- Sediments and volcanics 

Ym -Mafie intrusive 

Precambrian X 

X- Metasediments 

Kg - Gra nit iex 

I -Wgs; 

Precambrian W 

W - Metasediments, metavolcanics, 

and gneiss 

Wg-Granitica 

150 MILES 

150 KILOMETRES 

FIGURE 13—Geologic map of northern Minnesota, showing the extent of thin Upper Cretaceous deposits (Coleraine Formation) that are 

not represented on the Geologic Map of the United States. Compiled from Sloan (1964) and Sims (1970). 

0 50 100 

0 50I 100 
THE GEOLOGIC MAP OF THE UNITED STATES 35 

therefore classified the Cretaceous with the Pleistocene 

as part of the overburden on the Precambrian subcrop 

but have shown its known extent in figure 13. 

EOLIAN DEPOSITS 

In several places outside the glaciated area of the 

United States, eolian deposits of Pleistocene and 

younger age cover areas so extensive that the bedrock 

beneath them is represented in subcrop (see Thorp and 

Smith, 1952). 

In northwestern Nebraska an area of about 20,000 

square miles was shown as Quaternary on the Geologic 

Map of 1932, on the authority of N. H. Darton and G. E. 

Condra (fig. 14). This is the Sand Hills region, whose 

dunes and drifted sand, or Sand Hills Formation, lie on 

the Pliocene continental deposits of the Ogallala Formation (Tpc), from which they were ultimately derived 

(Reed and others, 1965, p. 199). Although the Nebraska 

Sand Hills are a prominent geomorphic feature of the 

Great Plains, they are merely surficial cover and hence 

are omitted from the present Geologic Map. 

East of the alluvial valley of the Mississippi River, in 

Mississippi, Tennessee, and Kentucky, the Tertiary 

bedrock of the uplands is mantled by loess, a windblown 

dust derived from the alluvial valley, when it was in its 

braided-channel phase during the late Pleistocene, and 

104° 102° 100° 

before it entered its present meander-belt phase 

(Krinitzsky and Turnbull, 1967, p. 7-9; Snowden and 

Priddy, 1968, p. 129-140). The loess is as much as 100 

feet (30 m) thick in the bluffs next to the alluvial valley 

but thins irregularly eastward to a featheredge. On the 

State geological maps the loess belt is shown as about 25 

miles (40 km) wide in Mississippi and more than 50 

miles (80 km) wide in Tennessee; it actually extends 

east of the alluvial valley for 100 to 150 miles (160-250 

km), but the remainder is thinner and less continuous 

(Thorp and Smith, 1952). Although the Mississippi Valley loess is appropriately shown on the State geologic 

maps, it would be inappropriate on the Geologic Map of 

the United States. In Tennessee it conceals the 

Claiborne-Jackson contact (Te2-Te3). 

In southeastern Washington and adjacent States 

another loess deposit, the Palouse Formation, extensively covers the basalts of the Columbia River Group 

(Tmv) and was probably derived during Pleistocene 

time from the front of the Cordilleran ice sheet to the 

north (Richmond and others, 1965, p. 238). On the 

Geologic Map of Washington (Huntting and others, 

1961), much of this part of the State is mapped as 

Quaternary, including not only the Palouse (Qce), but 

also various units of glacial outwash and stratified drift, 

so that the true bedrock pattern is not apparent. Actu98° 

EXPLANATION 

Ss • 

"Quartz sand. in part 

windblown" of Thorp 

and Smith (1953) 

I Qa 

42° "Alluvium and wind-blown 

sand" of Geologic Map 

of the United States 

(1932) 

Tpc.: 

Pliocene continental deposits (Ogallala Formation) 

Miocene and Oligocene 

continental deposits 

40' 

Cretaceous 

0 50 100 150 200 MILES 

0 50 100 150 200 KILOMETRES 

FIGURE 14.—Map of western Nebraska, showing bedrock geology as represented on the Geologic Map of the United States, superposed 

on which are the areas of Quaternary sand dunes and drifted sand (Sand Hills Formation) as represented on the Geologic Map of the 

United States of 1932 and by Thorp and Smith (1952). 
36 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

ally, the Palouse Formation is a surficial cover on the 

Columbia River Group in the uplands, whereas the 

other Quaternary deposits occur in structural depressions where they lie on older Pleistocene and on Tertiary deposits. On the Geologic Map of the United States 

we have therefore omitted the Quaternary deposits in 

the uplands but have retained those in the depressions, 

in the same manner as shown by Newcomb (1970). 

age. There is something to be said for showing the deposits in Maryland and New Jersey in the same manner 

as the Citronelle, but to do so would obscure the already 

small-scaled pattern of the bedrock outcrops, and it 

would be difficult to know how far to extend them because their correlation with surficial deposits in other 

parts of the Coastal Plain is uncertain; they are therefore omitted. 

RADIATING STRIKES 

ATLANTIC COASTAL PLAIN 

In the Atlantic Coastal Plain, from South Carolina 

northward to New Jersey, we have followed the usage 

on the Geologic Map of 1932 and have shown the 

Quaternary only in the coastal areas and represented 

the inland areas as bedrock of Miocene age and older. 

Actually, Pleistocene and possible Pliocene deposits 

cover parts of the surface of the inland areas, in places to 

such an extent that representation of the older strata 

must be by subcrop mapping. 

The surficial deposits are shown separately on the 

Geologic Map of Maryland (Weaver and others, 1968) 

and as overprints on the geologic maps of New Jersey 

and Virginia (Lewis and Kiimmel, 1910-12; Milici and 

others, 1963); map data for the other States are less 

definite (fig. 15). The deposits have been variously interpreted as between marine and continental, as to 

whether they are classifiable according to altitude (that 

is, whether they formed on surfaces representing different stands of the sea during the Pleistocene), their relation to glaciation, and their relation to crustal warping; 

the place for resolution of these problems should be on a 

surficial geology map, rather than on the Geologic Map 

of the United States. 

On the source maps, the older surficial deposits are 

better defined than the younger, as they form erosional 

remnants and outliers on the higher divides of the country. One of them, the Brandywine Formation is preserved on the uplands between Chesapeake Bay and the 

Potomac River in southern Maryland. Another, the 

Bridgeton Formation, is extensive in southern New 

Jersey, and a little farther north are smaller remnants 

of the apparently older Beacon Hill Formation. All of 

these are alluvial or fluviatile deposits whose ages are 

speculative at best. The Brandywine may be Pliocene 

(Hack, 1955, p. 25-40), as well as the Beacon Hill; the 

Bridgeton may be early Pleistocene, yet it is not clearly 

separable from the presumably younger Pensauken 

Formation (Richards, 1965, p. 130-131). These formations resemble in origin and geographic habit the Citronelle Formation of the Gulf Coast (differentiated on 

the Geologic Map as a continental deposit of Pliocene 

age, Tpc), although they are not necessarily of the same 

In closing this general discussion of the Geologic Map 

of the United States, a few remarks should be made 

about a curious feature (or pseudofeature) apparent to 

anyone who views the map from a little distance—the 

"radiating strikes" or belts of outcrop which fan out in 

all directions from the Arbuckle Mountains uplift in the 

southern Midcontinent Region of southern Oklahoma. 

The feature was observed years ago by Arthur Keith5on 

the basis of the general mapping available at the time; 

it is much more apparent on the Geologic Maps of the 

United States of 1932 and 1974. 

The "radiating strikes" involve a number of disparate 

geological elements that can be sorted out as follows 

(fig. 16): 

(1) Strikes of belts of Pennsylvanian and Permian 

strata in the Prairie Plains homocline, across Oklahoma into Kansas on the north, and into northcentral Texas on the south. 

(2) Tectonic features of Paleozoic age that cross the 

homocline transversely in Arkansas and Oklahoma. 

East of the Arbuckle area they include the south flank of 

the Ozark uplift and folds and faults in the Arkoma 

basin and Ouachita Mountains. West of the Arbuckle 

area they include the axes of the Anadarko basin and 

the Wichita Mountains uplift. 

(3) Strikes of homoclinal belts of Cretaceous rocks on 

the north and west flanks of the East Texas embayment 

in the Gulf Coastal Plain. 

The southern Midcontinent Region is geologically 

and tectonically complex, with many features of different ages crossing each other or superposed, only parts of 

which are revealed in the surface bedrock pattern; 

abundant subsurface data indicate many other features 

and in places quite a different history than would be 

inferred from the surface geology alone. Hence, many of 

the "radiating strikes" are illusory, or coincidental at 

most. The only truly valid features are the radiating 

strikes of the belts of Pennsylvanian and Permian 

strata in the Prairie Plains homocline. Their convergence toward the Arbuckle Mountains uplift indicates that tilting of the strata near the uplift was more 

'Arthur Keith, lecture at University of Texas, Austin, while visiting professor, 1926. We 

have been unable to find a reference to the subject in his publications. 
THE GEOLOGIC MAP OF THE UNITED STATES 37 

6° 75° 74° 77° 

EXPLANATION 

SURFICIAL UNITS BEDROCK UNITS 

D 

Alluvium and coastal 

deposits (Holocene?) 

Eocene 

Low-level deposits (late 

Pleistocene?) Talbot 

Formation of Maryland; 

Cape May Formation of 

New Jersey 

Intermediate-level deposits (mid-Pleistocene?) 

Wicomico Formation of 

Maryland; Pensauken 

Formation of New Jersey 

Pre-Cretaceous 

High-level deposits (earl) 

Pleistocene? and Pliocene?) Brandywine Formation of Maryland; 

Bridgeton and Beacon 

Hill Formations of New 

Jersey 

pK 

39° 

•.9 

Miocene 

Paleocene 

uK I 

Upper Cretaceous 

Lower Cretaceous 

0 10 20 30 40 50 MILES 

1 I 1 1 1 11 

0 10 20 30 40 50 KILOMETRES 

FIGURE 15.—Geologic map of the Atlantic Coastal Plain in Maryland, Delaware, and New Jersey, showing the relation of the bedrock 

units that appear on the Geologic Map of the United States to surficial deposits of Quaternary and late Tertiary age. Compiled from 

state geologic maps and from Owens (1967). 

steeply westward than farther north or south— although even where steepest it amounts to no more than 

than a few feet per mile. 

Despite the questionable nature of this feature it has 

recently been exploited by Burke and Dewey (1973, p. 

420-421), with the aid of some subsurface data, as a 

triple or quadruple rift junction in the continental plate 

produced by global tectonic movements during late 

Paleozoic time (styled the "Dallas junction"). The 

merits of this proposal remain to be evaluated. 
Tertiary 

Cretaceous 

Upper Paleozoic (Permian 

and Pennsylvanian) 

Lower Paleozoic (Mississippian and older; some Precambrian) 

Strike of Permian and Pennsylvanian strata (in Prairie 

Plains homocline) 

Tectonic features of Paleozoic 

age (faults and fold axes, 

indicated by appropriate 

symbols) 

Strike of homoclinal Cretaceous strata (on flanks of 

East Texas embayment) 

32° 

38 EXPLANATORY TEXT TO ACCOMPANY THE GEOLOGIC MAP OF THE UNITED STATES 

125 250 375 500 MILES 

0 125 250 375 500 KILOMETRES 

FIGufts 16.—Map of the southern Midcontinent Region in Oklahoma, Arkansas, and Texas, showing "radiating strikes" in Paleozoic and 

Cretaceous rocks. Lines are generalized from contacts shown on Geologic Map of the United States. 

0 

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