in the National Atlas 

United States of America

About the Geologic Map

of the

U.S. Department of the Interior

U.S. Geological Survey

Circular 1300
By John C. Reed and Charles A. Bush

About the Geologic Map 

in the National Atlas

of the United States of America

Circular 1300

U.S. Department of the Interior

U.S. Geological Survey
PACIFIC OCEAN

ATL

U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Reed, J.C., and Bush, C.A., 2007, About the geologic map in the National Atlas of the United States of America: U.S. 

Geological Survey Circular 1300, 52 p. 
Contents

Introduction.....................................................................................................................................................1

Geologic Provinces........................................................................................................................................2

Conterminous United States ...............................................................................................................2

Central Interior Region................................................................................................................2

Canadian Shield ..................................................................................................................2

Interior Lowlands................................................................................................................3

Appalachian and Ouachita Mountain Systems ......................................................................4

Sedimentary Appalachians...............................................................................................4

Crystalline Appalachians...................................................................................................6

Ouachita Mountain System...............................................................................................8

Coastal Plain Province..............................................................................................................10

Atlantic Coastal Plain.......................................................................................................10

Florida Peninsula ..............................................................................................................10

Gulf Coastal Plain..............................................................................................................12

Cordilleran Mountain System ..................................................................................................14

Laramide Rocky Mountains ............................................................................................14

Colorado Plateau ..............................................................................................................16

Rocky Mountain Fold and Thrust Belt ...........................................................................18

Basin and Range Province..............................................................................................18

Rio Grande Rift ..................................................................................................................19

Paleogene Volcanic Fields of the Eastern Cordillera..................................................20

Accreted and Subduction-Related Terranes of the Western Cordillera .................22

Lava Plains and Plateaus of the Columbia Intermontane Region.............................26

Alaska. ..................................................................................................................................................30

Brooks Range and Arctic Slope ..............................................................................................30

Central Intermontane Plateaus and Basins ..........................................................................32

Aleutian-Alaska Range Mountain System.............................................................................33

Volcanoes of the Aleutian Islands, Alaska Peninsula, and Wrangell Mountains, 

and the Bering Sea Volcanic Field ............................................................................34

Hawaii. ..................................................................................................................................................38

Puerto Rico and the U.S. Virgin Islands ..........................................................................................38

Faults. ...........................................................................................................................................................39

Continental Ice Sheets................................................................................................................................40

Impact Structures........................................................................................................................................42

Selected References...................................................................................................................................44

Glossary.........................................................................................................................................................45

iii
 Figures

1. Geologic time scale......................................................................................................................1

2. Index map showing geologic provinces and subprovinces 

in the conterminous United States. ...........................................................................................3

3−12. Satellite photographs showing:

 3. Appalachian Valley and Ridge Province near Harrisburg, 

Pennsylvania. .........................................................................................................................7

 4. Ouachita Mountains, Arkansas and Oklahoma................................................................9

 5. South Florida and the Everglades.....................................................................................11

 6. The Mississippi Delta and New Orleans, Louisiana. .....................................................13

 7. The Laramide Rocky Mountains. ......................................................................................15

 8. The Grand Canyon, Arizona. ..............................................................................................17

 9. The Basin and Range Province, Nevada.........................................................................21

 10. Mount Saint Helens, Washington. ....................................................................................25

 11. The San Andreas fault at Point Reyes, California. .........................................................27

 12. The Los Angeles Basin, California....................................................................................29

13. Index map showing geologic provinces 

and subprovinces in Alaska......................................................................................................31

14. Satellite photograph showing Glacier Bay 

and the St. Elias Range, Alaska................................................................................................35

15. Photograph showing Mount McKinley, Alaska. ....................................................................37

16. Index map showing the chain of islands 

that make up the State of Hawaii.............................................................................................38

17. Satellite photograph showing edge 

of the continental ice sheet in South Dakota.........................................................................41

18. Satellite photograph showing Meteor Crater, Arizona.........................................................43

 Table

1. Impact structures in the United States. ..................................................................................42

iv

PACIFIC OCEAN

ATLANTIC OCEAN

PACIFIC OCEAN

ATLANTIC OCEAN
Geologic Provinces 1

PACIFIC OCEAN

ATLANTIC OCEAN

By John C. Reed and Charles A. Bush

Introduction

The geologic map in the National Atlas of the United States of America shows the age, 

distribution, and general character of the rocks that underlie the Nation, including Alaska, 

Hawaii, Puerto Rico, and the Virgin Islands (but excluding other small island possessions). 

(The National Atlas of the United States can be accessed at URL http://nationalatlas.gov/

natlas/Natlasstart.asp.) The map depicts the bedrock1 that lies immediately beneath soils 

or surficial deposits except where these deposits are so thick and extensive that the type of 

bedrock beneath them can only be inferred by deep drilling or geophysical methods, or both. 

Thus, it does not show the extensive glacial deposits of the North Central and Northeastern 

States, the deep residuum of the Southeastern and South Central States, the relatively thin 

alluvium along many major rivers and basins, and extensive eolian deposits on the high 

plains. However, it does show, in a general way, the thick alluvial deposits along the lower 

Mississippi River and on the Atlantic and Gulf Coastal Plains, and in the deep basins of the 

western cordillera.

The rocks are classified as either sedimentary, volcanic, plutonic, or metamorphic, 

and their geologic ages are given in terms using a simplified version of the 1999 Geological 

Society of America geologic time scale (fig. 1). In some places rocks depicted as sedimentary are interlayered with volcanic rocks, including tuff, volcanic breccia, and volcanic 

flows. Conversely, many of the rocks shown as volcanic include interlayered sedimentary 

rocks. Plutonic rocks are classified by age and as granitic, intermediate, mafic, or ultramafic, but no similar classification has been attempted for the volcanic rocks in this version of the map. Where sedimentary or volcanic rocks have been metamorphosed but still 

retain clear evidence of their depositional age and origin, the extent of the metamorphism 

is shown by a pattern. Where the metamorphism has been so intense that the rocks bear little 

resemblance to the rocks from which they were derived, they are mapped as gneiss, but the 

age given is generally the age of the original rocks.

The map in the National Atlas is a generalization of a new geologic map of North 

America that has recently been published by the Geological Society of America. The 

original compilation was prepared at a scale of 1:2,500,000 for publication at a scale of 

1:5,000,000. This generalized version is intended for viewing at scales between about 

1:10,000,000 and 1:7,500,000. Figures 2 and 13 are index maps showing the various geologic provinces described in the following section. Figure 2 shows provinces in the conterminous United States; figure 13 shows those in Alaska.

ERA MILLIONS OF

YEARS AGO EON

Phanerozoic

Cenozoic

Mesozoic

Paleozoic

Proterozoic

Archean

pre-Archean

 Late Proterozoic

Middle Proterozoic

 Early Proterozoic

 Late Archean

Middle Archean

 Early Archean 3,800?

2,500

490

543

443

417

354

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

323

290

248

206

144

65

1.8 Neogene

Paleogene 23.8

PERIOD

middle* upper* lower*

1

Terms in bold type are defined in the Glossary.

Figure 1. Geologic time scale. Asterisk (*) denotes informal unit.

About the Geologic Map

in the National Atlasof the

United States of America
2 About the Geologic Map in the National Atlas of the United States of America

Figure 2. Index map showing geologic provinces and subprovinces 

in the conterminous United States. Heavy red lines show boundaries 

between major provinces; medium-weight blue lines show boundaries of 

subprovinces. Pink lines show smaller subdivisions, and in some places 

dashed pink lines show minor subdivisions. Lines are dashed where 

boundaries are concealed by younger deposits. Pink stipple shows early 

and mid-Tertiary volcanic provinces; blue stipple shows late Tertiary and 

Quaternary volcanic provinces and subprovinces. Small subdivisions 

and geologic features mentioned in the text are identified by circled 

letters and numbers. Note that the numbers and letters apply only within 

a particular province or subprovince and may be repeated in other 

subprovinces to identify different features.

Geologic Provinces

 Conterminous United States

The conterminous United States occupies a significant part of southern North America, 

and its complex geology reflects the complex geology of the continent (fig. 2). The nucleus of the 

continent is a stable block of metamorphic and igneous rocks assembled from about half a dozen 

plates of continental crust at various times during the Precambrian. Most of this block of continental crust, called the North American craton, has remained stable during much of subsequent 

geologic time. However, parts of the eastern margin of the craton were deformed and metamorphosed during formation of the Appalachian mountain system, and extensive parts of the craton 

in the west have been affected by folding, faulting, and uplift during formation of the Cordilleran 

mountain system. Large parts of the craton were blanketed by sediments deposited in shallow seas 

that washed across it during the Paleozoic and Mesozoic and by materials eroded from highlands 

that developed along its margins from time to time. For descriptive purposes the area of the 

conterminous United States can be divided into four principal provinces: (1) Central Interior 

Region, which comprises the craton and its veneer of little-disturbed sedimentary rocks; 

(2) the Appalachian and Ouachita mountain systems, which border the craton on the east and 

south; (3) the coastal plains of the Southern and Southeastern States, which are composed of 

Mesozoic and Tertiary rocks that lap onto the southeastern margin of the Appalachians, largely 

cover the Ouachita mountain system, and lap directly onto the interior lowlands of the Central 

Interior Region in the Mississippi Valley; and (4) the Cordilleran mountain system, which 

flanks the craton on the west and extends to the Pacific Coast. Each of these can be further 

divided into subprovinces and belts that share similar geologic histories and display similar 

geologic features.

Central Interior Region

The Central Interior Region extends from the western foothills of the Appalachians 

to the eastern front of the Rocky Mountains, and from the Canadian border south to the 

northern limit of the Gulf Coastal Plain. Thus, it encompasses nearly half of the area of the 

conterminous United States.

Canadian Shield

Precambrian rocks shown on the map in Minnesota and Wisconsin are part of the southern 

margin of the Canadian Shield, the vast area in which the Precambrian rocks that form the 

North American craton are exposed at the surface. These rocks are widely exposed in central 

Canada, but in the United States they are poorly exposed beneath the extensive cover of 

Quaternary glacial deposits (not shown on this map). The Archean gneiss (unit An) and 

granite (unit Ag) mapped in northwestern Minnesota are among the oldest rocks of the 

shield; some are as old as 3.6 billion years. The gneiss represents highly metamorphosed 

volcanic and sedimentary rocks that accumulated in or near volcanic arcs during the early stages 
PACIFIC OCEAN

ATLANTIC OCEAN

CENTRAL INTERIOR

REGION

FLORIDA

PENINSULA

ADIRONDACK

UPLIFT

MICHIGAN 

BASIN

APPALACHIAN

BASIN

INTERIOR

LOWLANDS

CINCINNATI ARCH

ILLINOIS

 BASIN

OZARK

 UPLIFT

CANADIAN SHIELD

LLANO

UPLIFT

MARATHON UPLIFT

DAVIS

MOUNTAINS

VOLCANIC FIELD

MISSISSIPPI EMBAYMENT NASHVILLE

 DOME

OGALLALA APRON

WILLISTON

 BASIN

BLACK HILLS

 UPLIFT

POWDER RIVER BASIN DENVER

BASIN

MOGOLLON-DATIL

 VOLCANIC FIELD

SAN

ANDREAS FAULT

SIERRA - WASATCH

VOLCANIC FIELD

COLORADO

 PLATEAU

 SAN JUAN

VOLCANIC FIELD

RIO GRANDE RIFT

SNAKE RIVER PLAIN

BOULDER

BATHOLITH

ROCKY MTS FOLD AND THRUST BELT

OREGON

PLATEAUS

 COLUMBIA

RIVER PLATEAU

C 1

8

A

B 6

D

4

3 B

A 1

9

C

2

7 5

SEDIMENTARY APPALACHIANS

CRYSTALLINE

APPALACHIANS

ATLANTIC

COASTAL PLA

NI

APPALACHIAN MOUNTAIN

SYSTEM

COASTAL PLAIN PROVINCE

GULF COASTAL PLAIN

OUACHITA

MOUNTAIN

SYSTEM

LARAMIDE

ROCKY MTS

ABSAROKA

VOLCANIC FIELD

IDAHO BATHOLITH

D

B A

C

F

G

3

H

3

2

1

SONORAN

SUBPROVINCE

GREAT

BASIN

SUBPROVINCE

BAS

NI AND RAN

GE PROVINCE

I

ACCRET OI N - AND SUBDUCTION - RELATED TERRANES

CO

RDILLERA

N

MOUNTAIN

SYSTEM

OF THE WESTERN CORDILLERA

E

 Geologic Provinces 3
4 About the Geologic Map in the National Atlas of the United States of America

of formation of the craton. The granite intruded the sedimentary and volcanic rocks 

during their deposition or eruption or shortly thereafter.

The Early Proterozoic volcanic and granitic rocks (units Xv and Xg) shown in 

Wisconsin are part of a belt of island arcs added to the southern edge of the craton 

about 2.5 billion years ago. The Early Proterozoic sedimentary rocks (unit X) in 

northern Wisconsin and south-central Minnesota represent materials eroded from 

this newly added arc and deposited on top of the Archean rocks.

Finally, the Middle Proterozoic volcanic and sedimentary rocks (units Yv and 

Y) in southern Minnesota and northern and western Wisconsin fill a fault-bounded 

trough, the Midcontinent Rift, that formed about 1,000 million (1 billion) years 

ago. The rift lies beneath Paleozoic sedimentary rocks in southern Minnesota but 

can be traced southward beneath the covering rocks in drillholes and by gravity

and magnetic anomalies all the way to central Kansas.

Interior Lowlands

South of the Canadian Shield the Precambrian basement rocks of the craton 

are largely mantled by a blanket of flat-lying sedimentary rocks that is generally a 

few hundred to thousands of feet thick. In the eastern and central parts of the lowlands the exposed sedimentary rocks are largely of Paleozoic age; in the western 

part of the lowlands the exposed rocks are chiefly Mesozoic and Tertiary. Most of 

the strata in the eastern part of the lowlands are limestone, shale, and sandstone

formed from sediments deposited in shallow seas that periodically washed over the 

craton during the Paleozoic. However, some are shale, sandstone, and conglomerate 

composed of materials shed from the exposed parts of the craton or from the bordering 

Appalachian highlands. The broad areas of Cretaceous rocks in the western parts 

of the lowlands are mostly shale, siltstone, limestone, and sandstone formed from 

sediments deposited in a shallow seaway that extended across the craton from the 

present position of the Gulf of Mexico to the Arctic Ocean at that time. The Tertiary 

rocks along the western margin of the central lowlands are chiefly conglomerate, 

sandstone, and siltstone intermixed with volcanic ash deposited on land in basins 

and as thin veneers on the Cretaceous rocks during the rise of the Rocky Mountains.

The continuity of the sedimentary blanket is broken by a number of subcircular domes in which older rocks are exposed at the surface and subcircular to 

elongate basins in which younger strata are preserved. Commonly, sedimentary 

layers surrounding these features thin toward the uplifts and thicken toward the 

basins, this pattern suggesting that these features formed over long periods. Some 

of the principal features of the central interior lowlands are described below:

The Adirondack Uplift is a dome that exposes Middle Proterozoic metamorphosed sedimentary rocks (unit Y), gneiss (unit Yn), and plutonic rocks, including 

granite (unit Yg) and anorthosite (unit Ya) in its core. The dome is surrounded by 

lower Paleozoic rocks (unit lg) that dip gently away from it in all directions.

The Cincinnati Arch and Nashville Dome are broad, elongate uplifts in 

which lower Paleozoic rocks (unit lg) are exposed in the cores and middle 

Paleozoic rocks form the flanks (unit mg). 

The Ozark Uplift is a broad, gentle dome in which lower Paleozoic rocks 

(unit lg) are extensively exposed in the center and middle Paleozoic rocks 

(unit mg) form the flanks. Basement rocks, chiefly Middle Proterozoic granite 

(unit Yg) and rhyolite (unit Yv) are exposed locally beneath the lower Paleozoic 

strata in the core.

The Llano Uplift, a dome in central Texas, exposes a core of Middle Proterozoic rocks (units Yg, Yn) similar to those in the Adirondack Uplift surrounded 

by lower Paleozoic strata (unit lg) and around its western, southern, and eastern 

margins by Cretaceous strata (unit K).

The Black Hills Uplift is a gentle, slightly elongate dome with a core composed 

largely of Early Proterozoic gneiss (unit Xn) and surrounded by middle Paleozoic (unit 

mg), lower Mesozoic (unit lc), and Cretaceous strata (unit K).

The Marathon Uplift in western Texas exposes folded and faulted lower 

Paleogene rocks (unit lg) like those in the Ouachita Mountains (see below) and less 

deformed upper Paleogene rocks (unit ug) that are tilted northward beneath Cretaceous strata (unit K) surrounding the uplift and that are themselves warped into a 

broad dome.

The Appalachian Basin is an elongate basin extending along the southeastern margin of the interior lowlands, adjacent to the main part of the Appalachian 

mountain system. Basement rocks along the trough of the basin are as much as 

16,000 feet below sea level, and the basin is filled with middle and upper Paleozoic sedimentary rocks (units mg, ug), most of them representing materials 

eroded from the rising Appalachian highlands to the southeast. The upper Paleozoic rocks in the trough of the basin are chiefly sandstone, shale, and conglomerate, but they include many thick and continuous coal beds that constitute more 

than half of the bituminous coal reserves of the United States.

The Michigan Basin is a saucer-shaped depression in which the basement 

rocks are as much as 16,000 feet below sea level. Drilling shows that the lower 

and middle Paleozoic strata (units lg, mg), which are only about 300 feet thick in 

adjoining areas, thicken to thousands of feet in the center of the basin, which indicates that the basin was subsiding as these rocks were being deposited. The center 

of the basin is occupied by lower Mesozoic (Jurassic ) rocks (unit lc), the only 

occurrence of rocks of this age in the eastern part of the interior lowlands.

The Illinois Basin is a shallow, elliptical basin filled with upper Paleozoic

rocks (unit ug) beneath which basement rocks are downwarped to depths of 

more than 10,000 feet. 

The Williston Basin and the Powder River Basin are irregular downwarps

beneath which basement rocks are depressed to about 13,000 feet below sea 

level. At depth these basins contain somewhat thickened sections of Paleozoic 

and lower Mesozoic strata, but near the surface the basins are defined by downwarped Upper Cretaceous (unit K) and Paleogene (unit pgT) rocks. The Paleo-
Geologic Provinces 5

gene rocks are chiefly shale, sandstone, and conglomerate derived from the rising 

Rocky Mountains to the west. In the Powder River Basin they contain thick beds 

of low-sulfur subbituminous and bituminous coal.

The Denver Basin lies at the western edge of the interior lowlands, immediately adjacent to the eastern front of the Rocky Mountains. The deepest part of 

the basin is near its western edge, where basement rocks are as much as 7,000 

feet below sea level (the elevation of the land surface there is about 5,000 feet 

above sea level). Sedimentary beds along the western flank of the basin slope 

steeply toward the basin or stand vertically, in contrast to their gentle slopes 

into the basin on the eastern, northern, and southern flanks. The basin is largely 

filled with Upper Cretaceous rocks (unit K), which reach a thickness of more than 

5,000 feet. Upper Paleozoic and lower Mesozoic rocks beneath them are only 

slightly thicker than they are on the outer flanks of the basin. The youngest strata 

in the basin are Paleogene sedimentary and volcanic rocks that were deposited 

during initial uplift of the neighboring Laramide Rocky Mountains.

The Ogallala Apron is a broad, flat expanse of gravel, sand, and silt deposited by streams flowing eastward from the southern Rocky Mountains during 

rapid regional uplift during the Neogene. These stream-laid deposits (unit nT) 

make up the Ogallala Formation, one of the most important aquifers in the Western United States, which supplies ground water used for extensive irrigation in 

the High Plains of western Texas, Oklahoma, Colorado, Kansas, and Nebraska. 

The Ogallala originally formed a continuous, east-sloping apron that extended 

from the mountains at least 400 miles to the east. Downcutting by streams has 

dissected the apron so that the underlying Cretaceous and older rocks are exposed 

along major river valleys. Along much of the mountain front the Ogallala Apron 

is separated from its mountain sources by erosion along the Pecos, Canadian, 

Arkansas, and South Platte Rivers and their tributaries. 

 Appalachian and Ouachita Mountain Systems

The Appalachian mountain system includes the sinuous upland belt that 

extends for 1,800 miles from northern Alabama northeastward through Maine 

and the Maritime Provinces of Canada to Newfoundland. It is flanked on the 

northwest by the Central Interior Region, and on the southeast by gently dipping 

Mesozoic and Cenozoic strata of the Atlantic Coastal Plain. The Appalachian 

system is the product of complex plate tectonic interactions between North 

America and oceanic and continental plates that lay to the southeast and south 

during the Late Proterozoic and the Paleozoic. The system consists of two major 

belts sometimes referred to as the “sedimentary Appalachians,” so called because 

they consist chiefly of folded and faulted, but little-metamorphosed, sedimentary 

rocks, and the “crystalline Appalachians,” which consist chiefly of metamorphic 

and plutonic rocks.

Amidst the Paleozoic rocks of the Appalachians are several faultbounded basins filled with unmetamorphosed lower Mesozoic red sandstone 

and shale (unit lc) interleaved with sheets of mafic intrusive rock (unit 

lcm). These basins form a discontinuous series that extends from North 

Carolina to central Massachusetts, and similar basins are found in Nova 

Scotia. The basins formed during the initial stages of opening of the present 

Atlantic Ocean as North America separated from Europe and Africa after 

formation of the Appalachian mountain system. Less well understood are 

the Mesozoic granite plutons (unit cg) in the White Mountains of central 

New England, which may have formed as a result of movement of the North 

American plate over a stationary plume of hot material rising from deep in 

the Earth’s mantle.

The Ouachita mountain system is exposed only in a few scattered areas in 

Arkansas, Oklahoma, and Texas. The rest of the system, which forms the southern 

border of the North American craton, lies buried beneath younger deposits.

Sedimentary Appalachians

Nonmetamorphosed sedimentary rocks form the northwestern part of 

the Appalachian mountain system adjacent to the craton. Their boundary 

with the cratonic cover rocks is at the transition between broad open folds 

in the cratonic cover to the northwest and the sharply folded and faulted 

rocks to the south and east. The rocks of the sedimentary Appalachians (units lg, 

mg) are chiefly limestone, shale, and sandstone formed from sediments that were 

deposited during the same interval of time as the sedimentary cover of the eastern 

parts of the interior lowlands. Many sediments were deposited in seawardthickening wedges along the edge of the craton, but others accumulated in deep 

basins that formed during uplift of the mountain system. All of the strata have 

been folded, and in parts of the belt thick sheets of sedimentary rocks have been 

displaced many miles northwestward onto the craton along thrust faults. The 

sedimentary Appalachians can be divided into two principal segments, the Valley 

and Ridge Province and the Taconic Province.

The Valley and Ridge Province is the segment between northeastern Alabama and northeastern Pennsylvania. It is characterized by long, 

northeast-trending ridges (fig. 3) formed by the upturned edges of resistant 

sedimentary layers (commonly sandstone) and intervening valleys cut in 

softer strata (commonly limestone and shale). In the southwestern part of this 

segment thrust faults are very conspicuous, and some can be traced along the 

trend of the belt for hundreds of miles. The slices of rocks above and between 

these faults have been transported tens of miles or more toward the craton. In 

the northeastern part the rocks are folded into linear anticlines and synclines, 

but thrust faults have not been recognized at the surface, although major 

thrusts may be hidden at depth. Most, and perhaps all, of the deformation in 

the Valley and Ridge took place during the late Paleozoic.

The Taconic Province is the narrow segment of the sedimentary Appalachians that extends from northeastern Pennsylvania through eastern New 
6 About the Geologic Map in the National Atlas of the United States of America

York and western Vermont to the Canadian border. The rocks are chiefly lower Paleozoic 

strata (unit lg), mostly sandstone, limestone, dolomite, and shale that have been folded and 

locally carried westward in thin thrust sheets, some of which include Late Proterozoic rocks 

(units Z, Zg). In contrast to the Valley and Ridge Province, where most of the folding and 

faulting took place near the end of the Paleozoic, most of the thrusting and folding in the 

Taconic Province took place late in the early Paleozoic during the Ordovician. 

Crystalline Appalachians

Like the sedimentary Appalachians, the crystalline Appalachians can be divided into 

two principal segments, a southern segment that extends from northeastern Alabama to eastern 

Pennsylvania, and a northern segment that includes most of New England.

The southern crystalline Appalachians comprise the broad belt of metamorphic and 

plutonic rocks that underlie the Blue Ridge and Piedmont Plateau and pass beneath the 

younger sedimentary rocks of the Atlantic Coastal Plain on the southeast and east. The 

northwestern part of the belt chiefly includes metamorphosed Late Proterozoic sedimentary 

and volcanic rocks (units Z and Zv) that overlie Middle Proterozoic gneiss (unit Yn) similar 

to those exposed in the Adirondacks. In the southwestern part of the belt both the gneiss and 

the overlying Late Proterozoic rocks have been carried northwestward on thrust faults across 

the Paleozoic sedimentary rocks of the Valley and Ridge Province. Farther southeast, broad 

expanses of metamorphosed sedimentary and volcanic rocks of Late Proterozoic or early 

Paleozoic age (units Zg, Zgv, lg, lgv) represent sedimentary and volcanic materials that 

accumulated along the margin of the continent or in and between chains of volcanic islands 

and were added to North America during plate collisions. Some of the gneiss (unit Yn) in the 

southeastern part of the belt may at one time have been parts of Africa. 

All these rocks occur in slices along major thrust faults that formed during the docking

of the various tectonic plates and plate fragments with the continent. At least three principal 

collisional episodes have been distinguished, each accompanied by intrusion of plutonic 

rocks, metamorphism, and formation of faults and folds. These include the Taconic episode near the end of the early Paleozoic (accompanied by plutons mapped as unit lgg), the 

Acadian episode in the early part of the middle Paleozoic (accompanied by a few plutons 

mapped as units mgg and mgm), and the Alleghenian episode in the late Paleozoic (accompanied by plutons mapped as unit ugg). The Alleghenian event was the result of the collision of northwestern Africa with North America, and it was during this event that the thrust 

faults in the Valley and Ridge Province formed. 

The New England crystalline Appalachians extend from the valley of the Hudson 

River and Lake Champlain eastward to the coast. Thus, they encompass most of the New 

England States. Like the southern crystalline Appalachians, they comprise metamorphosed 

and strongly folded sedimentary and volcanic rocks intruded by a variety of plutonic rocks 

of several ages. Along the western edge of the province Middle Proterozoic gneiss (unit Yn) 

like that in the Adirondack Dome are exposed in a discontinuous series of elongate domes 

that extends from the Hudson River northward to central Vermont. These basement rocks 

are overlain by sedimentary and volcanic rocks of Late Proterozoic and early Paleozoic age 

(Z, lg), some of which have been carried westward on thrust faults so that they now overlie 

Figure 3. Appalachian Valley and Ridge Province near Harrisburg, 

Pennsylvania. View is toward the northwest from above Harrisburg. The 

West Fork of the Susquehanna River drains south from the Allegheny 

Plateau (AP) through Lockport (LP) and joins the main stem of the 

Susquehanna at Duncannon (D).

The linear ridges and valleys between Harrisburg (H) and Lockport 

(LP) are the eroded edges of folded layers of sedimentary rocks. The 

ridges are composed of sandstone and conglomerate that are very 

resistant to erosion, and the valleys are underlain by more easily 

eroded shale and limestone. The ridges are largely forested and 

appear dark brown in the photograph, which was taken in the winter. 

The valleys are largely farmlands that appear lighter in color and 

show a checkerboard pattern of cultivated fields. The ridges to the left 

(southwest) of the Susquehanna River are chiefly Tuscarora Sandstone 

of Silurian age in anticlines; those to the right (northeast) of the river 

are formed of Pocono Sandstone of Mississippian age in canoeshaped synclines. The folds were formed during the Alleghenian 

episode of mountain building about 300 million years ago, when the 

sedimentary layers were skidded northwestward and crumpled like a 

rug being pushed across a smooth floor.

The Allegheny Plateau is underlain by less folded Pennsylvanian 

sedimentary rocks of the Appalachian Basin. South Mountain (SM) is 

the northern end of the Blue Ridge, part of the crystalline Appalachians.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center (photograph STS059-205-67; available from http://

eol.jsc.nasa.gov). Direction of view is northwest; altitude 114 nautical 

miles. Distance from Harrisburg to Lockport is about 70 miles.
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 Geologic Provinces 7

NORTH

H

LP

D

AP

SM
8 About the Geologic Map in the National Atlas of the United States of America

unmetamorphosed lower Paleozoic strata. This belt of Middle Proterozoic gneiss and its 

cover rocks were apparently part of North America at the beginning of the Paleozoic; most 

of the rocks to the east originated in or between tectonic plates or plate fragments that were 

added to the continent during the Paleozoic.

The same episodes of plate collision accompanied by metamorphism and intrusion of 

plutonic rocks that affected the southern crystalline Appalachians are recognized in the New 

England crystalline Appalachians. During the Taconic episode (near the end of the early 

Paleozoic) fragments of tectonic plates that included the lower Paleozoic rocks (unit lg) in 

New Hampshire and southwestern Maine were added to the margin of the continent. Granite 

plutons (unit lgg) were emplaced, and the rocks were folded, faulted, and metamorphosed. 

Following the Taconic episode, the earlier folds and faults were truncated by erosion and a thick 

and widespread blanket of middle Paleozoic sandstone, conglomerate, shale, and limestone 

(unit mg) accumulated across the older rocks. These strata have no counterparts in the southern 

crystalline Appalachians. The middle Paleozoic Acadian episode was apparently related to 

docking of plate fragments that now make up much of southeastern Maine, eastern New 

Hampshire, and central Massachusetts. It also resulted in widespread folding and metamorphism 

of the middle Paleozoic cover rocks (unit mg) to the northwest and in the emplacement of 

widespread plutonic rocks (units mgg and mgm) in New Hampshire and central Maine. The 

Alleghenian episode (near the end of the Paleozoic) included the accretion of plate fragments 

that now occupy extreme eastern Maine (units mgv and mgm) and southeastern Massachusetts (units Zg, ug) to the edge of the continent and the intrusion of granite plutons (unit ugg).

Ouachita Mountain System

The Ouachita mountain system is a belt of sedimentary rocks deformed during the late 

Paleozoic. This belt (fig. 4) is similar to the Valley and Ridge Province of the Appalachians. 

The Ouachita system extends westward from the southern end of the Appalachian system in 

central Alabama through Mississippi and Arkansas, into western Texas. Although the system 

is almost 1,300 miles long, it is exposed only in the Ouachita Mountains of western Arkansas 

and eastern Oklahoma and in the Marathon Uplift in southwestern Texas. The remaining 90 

percent of the system is buried beneath younger deposits and is known only from deep drilling and from geophysical studies. Like the Valley and Ridge Province, the Ouachita system 

consists of Paleozoic rocks that have been folded and carried toward the continental interior 

in thrust sheets, overriding the Paleozoic sedimentary rocks, chiefly sandstone, shale, and 

limestone that blanket the southern part of the craton. In contrast to the sedimentary rocks of 

the cratonic cover, which were mostly deposited in shallow seas, the rocks carried northward 

in the thrust sheets are chiefly shale, siltstone, sandstone, chert, and novaculite deposited in 

much deeper water along the edge of the continent. Most of the deep-water sedimentary rocks 

are of essentially the same age as the shallow-water cratonic cover rocks to the north and thus 

are also shown on the map as units lg and mg. The upper Paleozoic rocks (unit ug) consist 

of sandstone, shale, and some conglomerate deposited during various phases of movement of 

the thrust sheets during the Mississippian, Pennsylvanian, and Early Permian. Some of these 

rocks were carried northward in the thrust sheets, and some accumulated in basins along the 

edge of the craton north of the main belt of thrusting and folding.

Figure 4. Ouachita Mountains, Arkansas and Oklahoma. View is 

southeast from near Fort Smith, Ark. (FS) toward Broken Bow Reservoir 

(BB) in Oklahoma. River in the upper right corner is the Red River (RR) 

that marks the boundary between Texas and Oklahoma.

The linear ridges and valleys of the Ouachita Mountains (foreground), 

like those of the Appalachian Valley and Ridge Province, represent the 

edges of folded and faulted sedimentary rocks that have been etched 

by erosion so the more resistant layers of sandstone and conglomerate 

stand as ridges, while less resistant layers of shale and limestone form 

valleys. The layers were folded and faulted during an episode of mountain 

building about the same time as the folds in the Appalachian Valley and 

Ridge Province were formed. At that time thick sheets of sedimentary 

rocks were thrust northward (toward the observer) onto flat-lying and 

little-deformed rocks of the continental interior. The different pattern of 

cultivation south of Broken Bow Reservoir marks the northern edge of 

Cretaceous and younger strata of the Gulf Coastal Plain.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center (photograph STS058-091-059; available from 

http://eol.jsc.nasa.gov). Direction of view is south-southeast; altitude 

154 nautical miles. Distance from Fort Smith to Broken Bow Reservoir is 

about 130 miles.
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 Geologic Provinces 9

SOUTH

FS

BB

RR
10 About the Geologic Map in the National Atlas of the United States of America

Coastal Plain Province

The southern and southeastern margins of the conterminous United States are covered by wide coastal plains composed of sediments that were deposited along the passive 

edges of the continent as North America separated from Africa to form the present Atlantic Ocean and from northern South America to form the Gulf of Mexico. For descriptive 

purposes it is convenient to divide the province into three segments: the Atlantic Coastal 

Plain, the Florida Peninsula, and the Gulf Coastal Plain.

Atlantic Coastal Plain

The Atlantic Coastal Plain extends from Long Island, N.Y., southward to northern 

Florida, widening from a few miles in northern New Jersey to more than 100 miles 

in South Carolina and Georgia. It is underlain by a seaward-thickening wedge of Cretaceous, 

Tertiary, and Quaternary sediments that overlie metamorphic and igneous rocks of the crystalline Appalachians. The wedge thickens southeastward from its inner edge to more than 

6,500 feet along the coast and extends southeastward beneath the continental shelf, where it 

locally reaches a maximum thickness of 33,000 feet. The continental separation that led to 

the accumulation of the coastal plain sediments began during the early Mesozoic, as indicated by the fault-bounded basins filled with Triassic and Jurassic sedimentary and igneous 

rocks in the crystalline Appalachians. The oldest rocks exposed on the Atlantic Coastal Plain 

are Cretaceous, although drilling and geophysical studies show that lower Mesozoic rocks 

are present at depth along the coast and beneath the continental shelf. Most of the exposed 

Cretaceous (unit K) and Tertiary (units pgT and nT) sedimentary rocks were formed from 

sand, silt, and clay deposited in shallow waters on the continental shelf or near deltas where 

rivers emptied into the sea. Some of these rocks were formed from sand, gravel, and clay 

deposited on land during low stands of sea level. The Quaternary deposits include riverdeposited sand and gravel, and sand deposited on beaches and in sand dunes, as well as 

organic materials laid down in coastal swamps. On Long Island, the Quaternary deposits are 

chiefly till in glacial moraines, and in New Jersey they include sand and gravel carried by 

rivers flowing off the melting glacial ice.

Florida Peninsula

The Florida Peninsula (fig. 5) is underlain by nearly flat-lying sedimentary rocks 

of Cretaceous and Tertiary age that locally attain a thickness of more than 20,000 feet. 

Unlike the rocks of the Atlantic Coastal Plain, which overlie metamorphic and igneous 

rocks of the crystalline Appalachians, those in Florida overlie little-metamorphosed 

Paleozoic sedimentary rocks and Precambrian igneous and metamorphic rocks that may 

represent a part of Africa that remained attached to North America during the opening 

of the modern Atlantic Ocean. The oldest rocks exposed in the peninsula are chiefly 

limestone, evaporite, shale, and sandstone of early Tertiary age (unit pgT). These rocks 

are largely mantled by upper Tertiary rocks (unit nT), chiefly sandstone, shale, and 

Figure 5. South Florida and the Everglades. View southeast from over 

west coast north of Fort Myers (FM) toward Palm Beach (PB) and Miami 

(M). The Florida Peninsula is underlain by thousands of feet of flat-lying 

sedimentary rocks, chiefly limestone, deposited during Cretaceous, 

Tertiary, and Quaternary time. The present landscape of southern Florida 

is the result of variations in sea level during the Pleistocene Ice Ages. As 

glaciers grew, water was withdrawn from the oceans and sea level fell, 

at times by several hundred feet. During interglacial intervals, glaciers 

melted and water returned to the oceans, raising sea level and flooding 

southern Florida. During the low stands of sea level, limestone was 

dissolved and eroded to form caves and sinkholes; during high stands, 

limestone was deposited and beaches and dunes were created. As the 

most recent ice age ended, about 12,000 years ago, sea level began to 

rise and continues to rise today. The rising sea level impeded drainage of 

fresh water and led to the accumulation of thick layers of peat beginning 

about 5,000 years ago. These peat deposits form the Everglades (EG) and 

the Big Cyprus (BC) Swamp. Lake Okeechobee (LO) occupies a solution 

depression in the limestone. Natural drainage is southward from central 

Florida through Lake Okeechobee into the Everglades. Construction of 

levees, dikes, and drainage canals has disrupted the natural drainage 

in the region with drastic effects on the ecosystems. Diagonal streak in 

lower part of the photograph is an airplane contrail; plumes of smoke 

from burning vegetation are visible in the right part of the picture.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS51C-143-0032; available from 

http://eol.jsc.nasa.gov). Direction of view is southeast; altitude not given. 

Distance from Fort Myers to Palm Beach is about 115 miles.
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 Geologic Provinces 11

SOUTH

PB

M

FM

LO

EG

BC
12 About the Geologic Map in the National Atlas of the United States of America

Figure 6. The Mississippi Delta and New Orleans, Louisiana. View 

northwest from over the mouth of the Mississippi River toward New 

Orleans (NO). The Mississippi River drains more than one-third of 

the conterminous United States. Each year it carries about 125 cubic 

miles of water to the Gulf of Mexico. More than two-thirds of this flow 

enters the Gulf down the main channel below New Orleans, shown in 

this photograph. The remainder of the flow enters the Gulf through the 

Atchafalaya, a branch of the Mississippi that enters the Gulf about 115 

miles to the west. The river carries more than 180 million tons of silt, 

clay, and sand into the Gulf each year, exclusive of the material rolled 

along the bottom or carried in solution. The sediments are as much as 

8,000 feet thick near the river mouth and as much as 16,000 feet farther 

south beneath the Gulf of Mexico. Slumping and slow creep of the soft 

sediments causes the widespread faults shown on the geologic map and 

continuing subsidence of the delta.

The distinct color change in the lower delta reflects the difference 

between vegetation growing in freshwater swamps near the river mouth 

and that growing in salt marshes farther inland, where the supply of 

freshwater has been cut off as the delta advances. The Chandeleur 

Islands (CI) are barrier islands built by reworking of the delta sediments 

by waves and currents.

This photograph was taken in January 1985, long before the 

devastation caused by hurricane Katrina.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS51C-143-0027; available from 

http://eol.jsc.nasa.gov). Direction of view is northwest; altitude not given. 

Distance from tip of the delta to New Orleans about 80 miles.

limestone formed from sediments deposited in shallow seas on the continental shelf, 

and by Quaternary deposits (unit Q), chiefly sand deposited along beaches and in sand 

dunes, and by mud deposited in coastal swamps.

Gulf Coastal Plain

The Gulf Coastal Plain extends from the panhandle of Florida westward into 

southern Texas and then southward into central Mexico. It ranges in width from about 

200 miles in Alabama and Mississippi to as much as 300 miles in Texas, but a major 

northward projection of the coastal plain, the Mississippi Embayment, extends nearly 

500 miles northward up the Mississippi River to its junction with the Ohio. Like the 

Atlantic Coastal Plain, the Gulf Coastal Plain consists of a seaward-thickening wedge 

of Mesozoic, Tertiary, and Quaternary rocks formed from sediments that were deposited 

along the subsiding margin of the continent as North America separated from adjacent 

tectonic plates during the early Mesozoic.

The oldest rocks exposed at the surface in the Gulf Coastal Plain are Cretaceous. 

In Texas these rocks are chiefly limestone, shale, and sandstone of Early Cretaceous 

age, older than any known on the Atlantic Coastal Plain. East of the Mississippi, the 

Cretaceous rocks are all Upper Cretaceous, like those of the Atlantic Coastal Plain, but 

they include chalk and limestone. Most of the Cretaceous sediments were deposited on 

shallow continental shelves, along beaches, or in deltas where rivers entered the sea.

The exposed Tertiary rocks (units pgT, nT) are chiefly shale, sandstone, and limestone that originated as sediments deposited in shallow seas, in river deltas, and on land 

in coastal plains. The Quaternary rocks include beach, coastal swamp, and dune deposits, 

in addition to sand, silt, and clay deposited in river deltas.

In addition to its greater geographic extent and in the much greater proportion 

of limestone and chalk in the Cretaceous and Tertiary strata, the Gulf Coastal Plain 

differs from the Atlantic Coastal Plain in two other important aspects. First, the older 

Mesozoic strata, which are not exposed but which underlie much of the southern part 

of the coastal plain, contain a thick layer of salt, which flows readily under pressure 

when deeply buried by younger sediments. Second, the Mississippi River system, which 

drains much of the central part of the continent, has delivered vast amounts of soft mud, 

clay, and sand to the Mississippi delta and to the Gulf of Mexico during the Tertiary and 

Quaternary. These deposits reach a thickness of as much as 8,000 feet on the delta at the 

river mouth (fig. 6) and as much as 16,000 feet beneath the floor of the Gulf. As the salt 

flows under the weight of these deposits, some of it moves upward in pipelike bodies 

called salt domes, some of which penetrate all the way to the land surface or to the sea 

floor. The upwarped strata along the margins of these domes form traps for petroleum 

and natural gas, and the domes that approach the surface are commonly mined for salt. 

Flowage of the salt plus downslope movement of the soft Tertiary and Quaternary sediments toward the deep Gulf of Mexico Basin has produced a complex series of faults in 

the coastal plain deposits, some of which are still moving. Some of these growth faults

are shown on the map.
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 Geologic Provinces 13

NORTH

NO

CI
14 About the Geologic Map in the National Atlas of the United States of America

Cordilleran Mountain System

The mountain ranges, basins, and plateaus that lie between the Central Interior 

Region and the Pacific Coast are part of the Cordilleran mountain system, a belt of 

highlands that extends for almost 4,000 miles along the western margin of North America 

from Alaska through parts of Canada, the conterminous United States, Mexico, and Central America. The widest part of the Cordilleran mountain system is in the western conterminous United States, where it reaches a width of about 1,000 miles between Denver 

and San Francisco. Just as the Appalachian and Ouachita mountain systems are products 

of plate interactions along the eastern and southern margins of the North American craton 

largely during the Paleozoic, the Cordilleran mountain system resulted from plate interactions along the western margin of the craton largely (but not entirely) during the Mesozoic 

and Cenozoic. Sizable parts of the mountain belt consist of material from oceanic plates 

that was shoved against, beneath, or onto the western edge of North America as oceanic 

plates to the west were subducted beneath or slid past the western continental margin. 

As a result of these processes an area of almost 400,000 square miles (almost one and 

one-half times the size of Texas) was added to the western edge of the continent in the 

conterminous United States alone.

For purposes of description, the Cordilleran mountain system is divided into a 

number of subregions, each characterized by a different geologic history, but many of the 

boundaries between these subregions are gradational and the lines drawn between them 

are somewhat arbitrary.

Laramide Rocky Mountains

The Laramide Rocky Mountains are a series of asymmetric mountain ranges cored 

(fig. 7) by Precambrian basement rocks and their cover of Paleozoic and Mesozoic sedimentary rocks that were originally part of the craton. These ranges are generally bounded 

by sharp flexures in the sedimentary cover or by moderately to steeply inclined faults that 

slope beneath the uplifted block. The ranges are separated by basins filled with debris 

shed during and after the uplift, which took place during a relatively short interval in latest Cretaceous and earliest Tertiary time. This episode of uplift is called the Laramide

orogeny. It takes its name from the Laramie Formation, a deposit of sandstone and shale 

shed from the uplifts into neighboring basins in Colorado and Wyoming. For descriptive purposes the Laramide Rocky Mountains are divided into two parts, the northern 

Laramide Rocky Mountains in Montana, Wyoming, and northernmost Utah, and the 

southern Laramide Rocky Mountains in Colorado and New Mexico.

The principal features of the northern Laramide Rocky Mountains are the elongate 

fault-bounded basement uplifts and the intermontaine basins. The uplifts are indicated by 

circled numbers on the index map (fig. 2), the basins by circled letters.

The uplifts include the northwest-trending Bighorn Uplift (1) and other uplifts in 

southwestern Montana, the Beartooth (2), and Wind River (3) uplifts in northwestern 

Wyoming, the northwest-trending Laramie Uplift (4) in south-central Wyoming, and the 

east-west-trending Uinta Uplift (5) in Utah. The basement rocks in all of these except the 

Figure 7. The Laramide Rocky Mountains. View northwest from over 

Pueblo, Colorado. Most of the ranges in this view represent the cores of 

uplifts that rose between 70 and 45 million years ago during the Laramide 

period of mountain building. Erosion during and after the uplift stripped 

away the softer sedimentary rocks, exposing the harder sedimentary 

strata and Precambrian basement rocks that form the present mountain 

ranges. At the same time the surrounding basins were filled with the 

materials eroded from the uplifts. The basement rocks beneath the 

basins lie tens of thousands of feet below the same rocks exposed in 

the cores of the uplifts. During later uplift of the entire region erosion 

cut deep canyons, such as the Royal Gorge and the Black Canyon of the 

Gunnison. The linear valley west of the Sangre de Cristo Range and east 

of the Sawatch Range is part of the Rio Grande Rift.

Laramide uplifts (identified by numbers) include Sangre de Cristo 

Range (1), Front Range (2), Sawatch Range (3), Wind River Range (4), 

Uinta Mountains (5), Bighorn Mountains (6), Beartooth Mountains (7), 

and Teton Range (8). Laramide basins (identified by letters) include 

Denver Basin (a), South Park (b), North Park (c), Uinta Basin (d), Green 

River Basin (e), Bighorn Basin (f). Other features include Great Salt 

Lake (GSL), Black Canyon of the Gunnison (BCG), Royal Gorge (RG), and 

various ranges in the Fold and Thrust Belt (ftb).

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS040-604-011; available from 

http://eol.jsc.nasa.gov). Direction of view is northwest; altitude 155 

nautical miles. Distance from north end of the Sange de Cristo Range to 

Great Salt Lake is about 400 miles.
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 Geologic Provinces 15

NORTH

1

2 3

4

5

6

7 GSL

BCG

RG

ftb ftb

a b

c

d

e f

8
16 About the Geologic Map in the National Atlas of the United States of America

Uinta Uplift are granite and gneiss of Archean age (units Ag, An). The core of the Uinta 

Uplift comprises little-metamorphosed Proterozoic sandstone (unit a) that apparently accumulated in a local downwarp in the craton.

The basins include the Bighorn Basin (A) in northwestern Wyoming, the Wind River

Basin (B) in central Wyoming, and the Green River (C) and Red Desert Basins (D) in southcentral Wyoming. All these basins are floored by Paleogene rocks (unit pgT) composed of 

materials eroded from the uplifts and deposited during the late stages of Laramide uplift and 

after the uplift ceased. The Archean basement rocks (units A, An, Ag) beneath the deepest 

basins lie as much as 20,000 feet below sea level; the same rocks in some of the adjacent 

uplifts stand at elevations of more than 13,000 feet above sea level. Thus, the relative change 

in elevation of the surface of the basement rocks during the Laramide orogeny was as much 

a 6 miles. Erosion of the Laramide uplifts also furnished material to the Powder River and 

Williston Basins to the east.

The southern Laramide Rocky Mountains differ from the northern Laramide Rocky 

Mountains in that most of the uplifts and basins trend nearly north-south and that the basement rocks exposed in the cores of the uplifts are all of Proterozoic age. The basement rocks 

are schist, gneiss, and granitic rocks (units Xn, Xg, Yg) that range in age from 1.8 to 1.0 billion years and thus are considerably younger than those in uplifts to the north. The uplifts in 

the southern Laramide Rocky Mountains include the Front Range (6) and Park Range–Sierra

Madre (7) in northern Colorado and southernmost Wyoming, the Sawatch (8) in central 

Colorado, the Sangre de Cristo (9) in southern Colorado and northern New Mexico, and the

Sandia and San Andreas (not distinguished on map) in central and southern New Mexico.

Intermontane basins include North Park and South Park in Colorado, which are too 

small to be shown on the map. The Laramide uplifts also furnished material to the Denver 

Basin to the east. If the basins in the southern Laramide Rocky Mountains were ever as 

extensive as those in the northern Laramide Rocky Mountains, much of the basin fill has 

since been removed by erosion.

Colorado Plateau

The Colorado Plateau is an irregular, but roughly equidimensional, region of 

relatively flat-lying Paleozoic and Mesozoic sedimentary rocks that lies west of the 

southern Laramide Rocky Mountains and the Rio Grande Rift, discussed in a later section. The plateau extends from northeastern Utah southward for nearly 450 miles into 

central Arizona, and from the Rio Grande Rift in southern Colorado and northern New 

Mexico for almost 400 miles westward through northern Arizona. It is capped largely 

by upper Paleozoic, lower Mesozoic, and Cretaceous rocks. Lower Paleozoic strata and 

the basement rocks that underlie them are exposed only in the Grand Canyon in northwestern Arizona (fig. 8) and along the southern margin of the plateau in central Arizona. 

The basement rocks are all of Proterozoic age and are closely related to the Proterozoic 

rocks exposed in the Laramide uplifts in the southern Laramide Rocky Mountains. The 

Colorado Plateau is an isolated piece of the craton separated from the Interior Lowlands 

by uplift of the Laramide Rocky Mountains. The plateau itself has also been uplifted, 

so that the surface of the basement rocks beneath it stands 2,500 feet or more above the 

Figure 8. The Grand Canyon, Arizona. The Grand Canyon, more than a 

mile deep and 277 miles long, was carved by the Colorado River through 

flat-lying sedimentary rocks of the Colorado Plateau largely during the 

last 5 million years. The river enters the Grand Canyon through Marble 

Canyon (MC). The Little Colorado River (LC) flows north through a 

smaller canyon to join the Colorado as shown in the lower left part of 

the photograph. The snow-covered Coconino Plateau (CO) and Kaibab 

Plateau (KI) and the surrounding flat desert are capped by siltstone, 

sandstone, and limestone of Permian age. Older Paleozoic strata are 

exposed in the canyon walls, where resistant layers of sandstone and 

limestone form jagged towering cliffs, and layers of less resistant shale 

form smoother and gentler slopes. Downstream from the mouth of the 

Little Colorado gently tilted Late Proterozoic sedimentary rocks and 

lava about 1.1 billion years old are exposed at river level. Finally, in the 

deepest parts of the canyon, the river has cut into metamorphic rocks 

and granite more than 1.7 billion years old. The straight southwesttrending linear features that cross the canyon as shown in the left center 

of the photograph and extend onto the surface of the Coconino Plateau 

mark faults along which the sedimentary layers have been offset. Some 

of the faults are marked (f).

The distance across the narrowest part of the canyon from the snowcovered part of the Coconino Plateau to the snow-covered part of the 

Kaibab Plateau is about 8 miles.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph; STS060-83-005 available from 

http://eol.jsc.nasa.gov). Direction of view is southwest; altitude 192 

nautical miles. The horizontal distance across the center of the area 

shown is about 60 miles.
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 Geologic Provinces 17

SOUTH

CO

LC

MC

KI

f

f

f

f

f
18 About the Geologic Map in the National Atlas of the United States of America

basement surface in the Interior Lowlands and the top of the plateau stands 

6,500–8,000 feet above sea level.

Deep basins (circled letters on the map) filled with thousands of feet 

of Paleogene rocks (unit pgT) fringe the plateau on the north and east. 

These include the Uinta Basin (A) in northeastern Utah, the Piceance Basin

(B) in northwestern Colorado, and the San Juan Basin (C) in southwestern 

Colorado and northwestern New Mexico. The basin-filling sediments are 

chiefly debris shed from the flanking Laramide uplifts but include some 

deposits that accumulated in large lakes that occupied parts of the Uinta 

and Piceance Basins.

In the central part of the plateau the flat-lying sedimentary rocks 

were intruded by igneous rocks (unit pgTi) that were emplaced very late 

in the Paleogene. These intrusive rocks form tabular and mushroomshaped bodies that bowed up the sedimentary strata above them and were 

eventually exposed by erosion so that they stand as isolated groups of 

mountains.

Along the southern edge of the plateau are several young volcanic 

fields in which lavas, chiefly basalt and related rocks (units nTv, Qv) 

were erupted from groups of central volcanoes. Among these is the San

Francisco volcanic field (1), whose prominent volcanic peaks rise above 

Flagstaff, Ariz.

Rocky Mountain Fold and Thrust Belt

In westernmost Wyoming, eastern Idaho, and western Montana 

the Rocky Mountain fold and thrust belt is quite different geologically 

from the Laramide Rocky Mountains. Instead of the broad fault-bounded 

Laramide uplifts in which basement rocks are exposed, the mountain 

ranges in the fold and thrust belt consist largely of sedimentary rocks that 

originally were tens of miles or more west of their present positions. These 

strata now lie in a stacked series of thin sheets that have been folded and 

carried eastward toward the craton along thrust faults, mostly during the 

Mesozoic. This episode of folding and thrust faulting is often referred to as 

the “Sevier orogeny.” The Sevier started during the early Mesozoic, before 

the Laramide orogeny, and ended near the end of the Cretaceous, while the 

Laramide orogeny was still in progress. Although they overlap in time, the 

geological structures produced by these two orogenies are quite different.

The Rocky Mountain fold and thrust belt extends northward from the 

conterminous United States through western Canada and into the Brooks 

Range in northern Alaska. The same structures also extend southward 

through Utah, Nevada, and Arizona, but there they are sundered by later 

faults of the Basin and Range Province (described below) to such a degree 

that they cannot be shown on a map of this scale. From eastern Arizona the 

belt continues southeastward through eastern Mexico into northern Central 

America. The geological structure of the fold and thrust belt is very similar 

to that of the Valley and Ridge Province of the Appalachians, although 

both the age and the direction of transport on the thrusts are different (but 

in both cases the transport direction is away from the continental margin 

and toward the craton).

The segment of the fold and thrust belt in the conterminous United 

States can conveniently be divided into two segments, one in Idaho and 

Montana and one in western Wyoming.

In the Idaho-Montana segment of the fold and thrust belt the rocks 

in the thrust sheets are chiefly sandstone, argillite, and thin beds of 

dolomite of Middle Proterozoic age (unit Y). These strata were deposited 

in an extensive basin on what was then the western edge of the continent. 

Paleozoic and Mesozoic rocks appear in the thrust sheets in this segment 

only along its northeastern and southeastern margins. In western Montana 

the continuity of the north-northwest-trending thrust faults is broken by 

a series of northwest-trending faults called the Lewis and Clark zone. 

The northeast side of each of these faults has moved northwestward with 

respect to the southwest side, displacing the structures of the fold and 

thrust belt.

South of the Lewis and Clark zone, large bodies of Cretaceous and 

Paleogene granitic rocks (units Kg, pgTg) intruded the rocks of the fold and 

thrust belt, some during, but most after, movement on the thrust faults. The 

largest of these bodies are the Idaho batholith and the Boulder batholith. 

In a broad zone surrounding the Idaho batholith the Middle Proterozoic 

sedimentary rocks (unit Y) were metamorphosed to gneiss (unit Yn).

The Wyoming segment of the fold and thrust belt lies chiefly in 

western Wyoming but extends into southeastern Idaho and north central 

Utah. The sedimentary rocks in the thrust sheets in this segment range 

in age from Late Proterozoic through Late Cretaceous, but because of 

the scale of the map they have been grouped simply as Paleozoic and 

Mesozoic (units g and c ). No large bodies of granitic rocks like those in 

Montana and Idaho have intruded this segment of the fold and thrust belt.

Basin and Range Province

West and south of the Colorado Plateau is a broad region 

characterized by linear mountain ranges separated by sediment-filled 

basins (fig. 9). This region, known as the Basin and Range Province, 

extends north into Idaho and encompasses a large part of northwestern 

Utah, most of Nevada, and southeastern California, and extends eastward 
Geologic Provinces 19

through southern Arizona and New Mexico, and southward into northern 

Mexico. In Utah and most of Nevada the trends of the ranges are nearly 

north-south, but in northwestern Nevada trends are northeasterly and in 

Arizona and New Mexico many ranges trend northwest. Rocks exposed in the 

ranges include Late Proterozoic, Paleozoic, and Mesozoic sedimentary rocks 

(units Z, g, c) and Tertiary volcanic rocks (units pgTv, nTv). The materials 

filling the basins are chiefly fan-shaped deposits of gravel and sand eroded 

from the nearby ranges but include sediments deposited in extensive lakes 

that occupied the basins during the ice ages, when the climate was wetter. 

Much of the province in Utah and Nevada is part of the Great Basin, an area 

from which no streams escape to the sea.

Unlike most mountain ranges, the mountains of the Basin and Range 

Province are produced by pulling apart of the Earth’s crust, rather than 

by compression of the crust as in the fold and thrust belt. The ranges are 

bounded by faults on one or both flanks, along which the mountain blocks 

have been raised relative to the intervening basin blocks. The faulting that 

produced the Basin and Range Province began in the late Paleogene, but most 

of the movement took place during the Neogene. Some of the faults are still 

moving today, as evidenced by new fault scarps formed during earthquakes 

in the past few decades. The total extension of the crust across the Basin and 

Range is estimated to have been 100 miles or more.

The fold and thrust belt originally extended through much of the Basin 

and Range Province, but the thrust faults have been cut apart by the younger 

and more steeply inclined basin and range faults so that the older structures 

are difficult to decipher. The Basin and Range Province in the conterminous 

United States can be divided into two subprovinces: the northern, or Great 

Basin, subprovince, and the southern, or Sonoran, subprovince.

The boundary between the Basin and Range Province and the Colorado 

Plateau is delineated by a number of volcanic centers that were active during 

and after the extensional faulting. These centers erupted large volumes of 

dark lava and widespread blankets of light-colored volcanic ash.

The Great Basin subprovince includes the parts of the Basin and Range 

Province in Utah and Nevada and a small segment in central Idaho north of 

the Snake River Plain. In Utah and eastern Nevada the rocks exposed in the 

ranges are Late Proterozoic and Paleozoic (units Z, g) and resemble those 

in the adjacent Wyoming segment of the fold and thrust belt. However, in 

central Nevada the Paleozoic rocks (unit g) in the ranges include chert, 

shale, and basalt that were deposited in much deeper water and were thrust 

eastward onto the edge of the continental shelf during the late Paleozoic. In 

western Nevada the ranges also include lower Mesozoic sandstone, shale, and 

volcanic rocks that were thrust onto the edge of the continent near the end of 

the early Mesozoic. 

Faults in this part of the Basin and Range Province are currently 

more active than those elsewhere in the province. Continuing fault 

movement and the desert climate prevent drainage from the Great Basin 

from reaching the sea.

The Sonoran subprovince comprises the parts of the Basin and Range 

Province in southeastern California, Arizona, and New Mexico. In this 

region the ranges generally trend north-northwest. The uplifted blocks 

include Proterozoic rocks (units Xn, Xg, Yg) similar to those in Colorado 

and New Mexico; these rocks presumably form the basement beneath 

the Colorado Plateau. They also include Paleozoic rocks like those of the 

Colorado Plateau in thrust sheets that were emplaced during the Sevier 

orogeny. In addition, in southern Arizona they locally include lower 

Mesozoic sedimentary and volcanic rocks that accumulated in and between 

chains of volcanoes along the margin of the continent. Movement on the 

range-bounding faults in the southern Basin and Range terminated by 

about 5 million years ago, so that sand and gravel eroded from the ranges 

has blanketed the faults—thus very few of them are shown on the map.

Rio Grande Rift

The Rio Grande Rift in Colorado (fig. 7), New Mexico, and western 

Texas separates the southern Laramide Rocky Mountains from the 

Colorado Plateau. This linear valley through which the Rio Grande flows 

southward to El Paso, and then southeastward toward Big Bend National 

Park, was formed during the same episode of extension of the Earth’s crust 

as the Basin and Range Province, with which it connects in central New 

Mexico.

The rift valley is generally bounded by faults on one or both sides. 

Unlike the faults in the southern Basin and Range, many of those that 

bound the rift are still active, as shown by fault scarps that cut Quaternary 

gravels and glacial moraines. The rift is filled with hundreds to thousands 

of feet of Neogene gravel, sand, and clay (unit nT) interlayered with 

Neogene lava flows, chiefly basalt (unit nTv). Much of it is floored 

by Quaternary alluvium and wind-blown sand (unit Q). A number of 

volcanoes of Quaternary age punctuate the floor of the rift; none of these 

is currently active, but most of them still retain little-modified volcanic 

shapes. Opening of the rift led to uplift of the flanking ranges, so that 

many of the highest peaks in the conterminous United States lie along the 

rift margins in central and southern Colorado.
20 About the Geologic Map in the National Atlas of the United States of America

Paleogene Volcanic Fields of the Eastern Cordillera

During the Paleogene, following the end of the Sevier and Laramide orogenies, 

volcanic rocks (unit pgTv) were erupted in a number of volcanic fields in the Laramide 

Rocky Mountains, Colorado Plateau, Basin and Range Province, and Rio Grande Rift. 

In most places the eruptions started with andesite and similar lava from central volcanoes, 

together with breccia and mud-flow deposits. These early eruptions were followed 

by voluminous eruptions of volcanic ash with compositions resembling rhyolite that 

accompanied formation of great subcircular volcanic depressions called calderas. The 

calderas are thought to have been formed by the catastrophic collapse of the roofs of 

large near-surface bodies of granitic magma; the volcanic ash erupted was in many 

cases so hot and contained so much entrapped air that it flowed like a liquid (these 

deposits are known as ash flows). In many places the ash was hot enough to weld under 

its own weight when it came to rest, forming a hard, glassy rock called welded tuff. Some 

sheets of ash-flow tuff cover hundreds of square miles and have volumes of tens or 

hundreds of cubic miles.

The Paleogene volcanic fields were formed during an interval of about 30 million 

years, beginning at about 51 million years ago. However, their ages are not all the same. 

The eruptions began first in Idaho and northwestern Wyoming and progressed southward 

into northern Nevada, and then into central Nevada and southern Utah. Farther south, 

eruptions began in trans-Pecos Texas and progressed northwestward into the southern 

Rocky Mountains and southern Colorado Plateau and into the Sonoran subprovince 

of the Basin and Range in southern Arizona. These regular changes in the ages of 

the volcanic field are thought to be related to patterns of subduction of oceanic 

plates from the Pacific beneath western North America, but the exact mechanism is 

not yet clear.

The Challis volcanic belt (not labeled on the map) encompasses the Paleogene 

volcanic rocks (unit pgTv) east of the Idaho batholith in central Idaho. Early andesitic 

rocks were erupted there between 51 and 48 million years ago, and extensive rhyolite 

ash flows erupted from calderas between 48 and 45 million years ago. The ash flows 

were apparently fed from granitic bodies (unit pgTg) intruded into the Idaho batholith 

during the same time interval.

The Absaroka volcanic field lies in northwestern Wyoming, much of it in 

Yellowstone National Park. The volcanic rocks, chiefly andesitic lava, breccia, and 

mudflows (unit pgTv), were erupted from a chain of volcanoes between 50 and 43 

million years ago, blanketing the older Laramide structures beneath thousands of feet of 

volcanic deposits. The Absaroka field is unique in that calderas and extensive ash-flow 

sheets are missing.

The Sierra-Wasatch volcanic field extends across Nevada and western Utah from 

the foothills of the Sierra Nevada to the Wasatch Range. Early eruptions, chiefly andesite 

and related rocks, began in the northern part of the belt between 41 and 38 million years 

ago. Andesite eruptions farther south began between 37 and 34 million years ago, and 

gave way to extensive ash-flow eruptions from calderas that lasted from about 34 to 

Figure 9. The Basin and Range Province, Nevada. View is southwest 

from near Carlin, Nev. This view shows much of the southern part of 

the Basin and Range Province. The dark mountains partly obscured by 

scattered puffy clouds seen near the upper right edge of the photograph 

are part of the Sierra Nevada (SN). Part of the San Joaquin (SJ) Valley in 

California is visible just beyond them. The nearly circular blue lake near 

the foot of the Sierra Nevada is Mono Lake (ML).

The parallel fault-bounded mountain ranges and intervening basins 

shown in the central part of the photograph are typical of the Basin and 

Range Province. The ranges are tilted blocks of Paleozoic and Mesozoic 

sedimentary rocks intruded by Cretaceous and Tertiary granite and capped 

by Tertiary volcanic rocks. The basins are filled with great volumes of 

sediment eroded from the neighboring ranges. 

Most of the area pictured is within the Great Basin, a large closed 

basin from which nothing drains to the sea. The green area shown near 

the central part of the right edge of the photograph is Carson Sink (CS), 

where the Carson River, which drains part of the eastern slope of the Sierra 

Nevada, empties into a shallow basin, where it ponds and evaporates. 

The lighter-colored areas in the valleys between the ranges are playas 

where drainage from the nearby ranges collects and evaporates, leaving 

salty mud flats. At the time the photograph was taken a few of the playas 

apparently contained enough shallow water to glitter in the sun. During the 

Ice Ages much of this part of the Great Basin was filled with a huge freshwater lake known as Lake Lahontan, which covered 8,500 square miles and 

was as much as 900 feet deep.

Many of the range-bounding faults in this part of the Basin and 

Range Province are still active. On December 14, 1954, a magnitude-7.1 

earthquake occurred near Fairview Peak, near the south end of the fish 

hook–shaped range visible near the center of the photograph. Four minutes 

later a magnitude-6.9 earthquake occurred along faults on the west side of 

Dixie Valley, the valley immediately east (left) of Carson Sink, forming scarps 

as much as 10 feet high.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS41G-121-184; available from http://

eol.jsc.nasa.gov). Direction of view is southwest; altitude not given. Horizontal 

distance across center of area shown is about 250 miles. 
PACIFIC OCEAN

ATLANTIC OCEAN

 Geologic Provinces 21

SN

SJ

ML

CS

SOUTH
22 About the Geologic Map in the National Atlas of the United States of America

about 20 million years ago. By the end of the Paleogene eruptions, the ash 

flows formed a nearly continuous volcanic plateau that extended completely 

across the future site of the Basin and Range Province and that has since 

been dismembered by faults to form most of the ranges in the southern part 

of the Great Basin subprovince.

The San Juan volcanic field in southwestern Colorado is probably the 

best studied of all the Paleogene volcanic fields. It consists of andesite lava 

and breccia erupted from central volcanoes, chiefly between 35 and 30 million years ago, and great sheets of rhyolite ash-flow tuff that erupted from 

calderas between 31 and 25 million years ago. At least half a dozen major 

calderas have been recognized, and between 15 and 20 ash flows have been 

mapped, with volumes ranging from a few tens to hundreds of cubic miles.

The Mogollon-Datil volcanic field lies near the southeastern corner of 

the Colorado Plateau. The early andesitic rocks there were erupted starting 

about 49 million years ago, and extensive eruptions of rhyolite ash flows 

from calderas took place between 34 and 28 million years ago.

The Davis Mountains volcanic field lies in trans-Pecos Texas along the 

northeast flank of the Rio Grande Rift. Eruptions of andesitic rocks there 

began about 49 million years ago (about the same time as in the Challis and 

Absaroka fields) and gave way to eruptions of extensive ash flows between 

38 and 32 million years ago. The volcanic field overlies rocks similar to 

those of the interior lowlands to the northeast, but these rocks were folded 

and faulted during the Laramide orogeny. Later the volcanic rocks were broken by faults related to the Rio Grande Rift. The Davis Mountains field is a 

northeastern outlier of the huge volcanic field of the Sierra Madre Occidental, which blankets much of north-central Mexico.

The same trend of younger ages to the north and west continues through 

Arizona into southeastern California, where the early andesitic rocks are as 

young as 27 million years old and ash flows and calderas range from 26 to 22 

million years old. Thus, some of these rocks are shown as unit nTv on the map.

Accreted and Subduction-Related Terranes of the Western 

Cordillera

West of the Basin and Range Province and the Rocky Mountain fold 

and thrust belt many of the exposed Paleozoic and Mesozoic rocks are 

parts of accreted terranes that reached their present positions during the 

Mesozoic and Tertiary. Some of the accreted rocks originated in chains of 

volcanic islands, inter-island basins, volcanic plateaus, and seamounts that 

were lodged against the western edge of the continent as oceanic plates in 

the Pacific were subducted beneath the North American plate. Other parts of 

the collage of accreted terranes were probably parts of North America that 

were displaced laterally along the continental margin to reach their present 

positions. Studies of the fossils and of magnetic orientations of minerals in 

the accreted rocks suggest that many of them originally lay hundreds or even 

thousands of miles south of their present locations. Still other parts of the 

western tectonic collage consist of materials added to the western edge of 

North America, either as igneous rocks generated as tectonic plates from the 

Pacific were carried beneath the continent, or as sediments that accumulated 

in marginal basins related to the subduction zones and thrust eastward onto 

the continent.

The northern Cascade Mountains (A) extend across northern 

Washington from the San Juan Islands in Puget Sound to the Columbia 

River. The mountains consist chiefly of complexly folded and faulted 

sedimentary and volcanic rocks, chiefly of Mesozoic age, invaded by 

myriad granitic plutons of Mesozoic and early Tertiary age. Many of these 

rocks have been metamorphosed, and some have been converted to gneiss 

and migmatite. In the western part of this belt sheets of rocks have been 

carried westward along east-dipping thrust faults and all of the rocks have 

been displaced by steeply dipping faults, along many of which rocks west 

of the fault have moved northward with respect to rocks east of the fault.

Most of these rocks probably originated in island arcs or basins 

adjacent to them. Paleomagnetic measurements indicate that many of 

them originated thousands of miles to the south of their present positions. 

The rocks in the eastern part of the northern Cascade belt (the Okanogan 

Highlands) were probably added to North America late during the 

Paleozoic; those to the west were added during the Cretaceous.

The Olympic Mountains (B) and the Oregon and Washington Coast

Ranges (C) lie between the Klamath Mountains and Vancouver Island 

in British Columbia. These mountains consist of a belt of Paleogene 

sedimentary and volcanic rocks (units pgT, pgTv). Most of the sedimentary 

rocks are mica- and feldspar-rich sandstone and siltstone and a few 

interlayered volcanic rocks that accumulated in an elongate marginal basin 

west of a chain of volcanoes, chiefly during the Eocene and Oligocene. 

The total thickness of these deposits is as much as 6,500 feet. To the east, 

these rocks interfinger with, and are overlain by, sediments deposited in 

shallow water or on land and with volcanic rocks related to the volcanic 

chain. Throughout most of the length of the basin the subduction zone lay 

offshore, but in the core of the Olympic Mountains, melange and associated 

rocks related to the subduction zone have been pushed under the rocks of 

the basin along thrust faults.

The rocks of the Blue Mountains (D) are similar to those in the 

northern Cascades. They are exposed in a discontinuous belt that includes 

the Blue Mountains of northeastern Oregon and extends northeastward to 

exposures in Hells Canyon on the Snake River between Idaho and Oregon. 
Geologic Provinces 23

The rocks include intermediate and mafic volcanic rocks (unit gcv); 

limestone, sandstone, and shale of Triassic age; and a belt of melange

consisting of disrupted oceanic crust and deep-water sedimentary rocks 

of Paleozoic age that probably formed in a subduction zone (all of the 

sedimentary rocks and melange are grouped as unit gc). Paleomagnetic 

measurements indicate that many of these rocks also originated far to 

the south; they were transported northward and added to North America 

during the early Mesozoic. Such measurements also show that the rocks of 

the Blue Mountain belt were rotated about 60° clockwise since the Early 

Cretaceous. In western Idaho, rocks of the Blue Mountain belt are intruded 

by Late Cretaceous granitic rocks of the Idaho batholith.

A discontinuous chain of great volcanoes (E) forms the crest of the 

Cascade Range in Oregon and extends southward into northern California and 

northward across Washington into British Columbia. The most conspicuous 

volcanic vents are steep-sided cones of andesite lava and pyroclastic rocks, 

but these are interspersed with lower shield volcanoes that have erupted 

chiefly basalt. The lava and associated volcanic rocks of the Cascade chain 

are shown as unit Qv in Oregon, but in Washington they are not extensive 

enough to show on a map of this scale. The oldest eruptions in the volcanic 

chain started about 7 million years ago, but many of the volcanoes are still 

active, as confirmed by the explosive eruption of Mount St. Helens (fig. 10) 

in 1980. The Cascade volcanoes pose the most significant volcanic hazard in 

the conterminous United States. Although the eruptions themselves can be 

destructive, the greatest threats are volcanic mudflows generated by melting 

of glaciers and snowfields during an eruption that can course for miles 

down the flanks of the volcanoes and along river valleys into the urbanized 

lowlands nearby.

The Cascade volcanoes are a result of the continued subduction of the 

Juan de Fuca plate (part of the floor of the Pacific Ocean) beneath Oregon, 

Washington, and southern British Columbia. In places the volcanoes are 

built on older volcanic rocks marking the volcanic chain that existed during 

accumulation of the sedimentary rocks in the Coast Ranges to the west.

Most of the rocks exposed in the Klamath Mountains (F) in outhwestern 

Oregon and northeastern California and the northern Sierra Nevada (G) in 

east-central California formed in island arcs and adjacent basins, many of 

which originally lay far to the south. All of these rocks have been telescoped 

by movements along thrust faults, and in many places the thrust sheets are 

interleaved with large slices of ophiolite that represent the oceanic crust on 

which the arcs and basins formed. In most cases these thrusts root toward the 

west and movement of the upper plate has been toward the east, but in several 

cases the faults root to the east and movement has been toward the west. 

The westernmost of these displaced terranes closely resemble those in the 

western part of the Northern Cascades and the Blue Mountains and 

probably reached their present positions along the edge of the 

continent at the same time, during the early Mesozoic. The easternmost 

displaced terranes contain belts of Paleozoic rocks that probably originated 

in a volcanic archipelago along the margin of the continent and in shallow 

seas and deeper offshore basins. These terranes seem to have arrived in their 

present positions during the late Paleozoic or earliest Mesozoic, at the same 

time as some of the thrust faulting in the northern Basin and Range Province. 

In many places the accreted rocks were invaded by plutons of Jurassic and 

Early Cretaceous granitic rocks.

The Sierra Nevada batholith, the Great Valley sequence, and the

Franciscan Complex (H) are quite different in their geology, but they are 

intimately related in terms of their history and tectonic setting and are 

therefore treated together in this discussion.

The Sierra Nevada batholith (1) is a belt of granitic rocks almost 

300 miles long and as much as 70 miles wide that underlies the higher 

parts of the Sierra Nevada in eastern California. The batholith is part of 

a discontinuous band of granitic plutons that extends southward into the 

Coast Range of southwestern California and northward to include scattered 

plutons in northwestern Nevada and eastern Oregon and the Idaho batholith 

in Idaho and Montana. It may also include granitic plutons in the Northern 

Cascades in eastern Washington.

The Sierra Nevada batholith is not a single, huge body of granitic 

rock, but an amalgamation of literally hundreds of individual plutons 

that crop out over areas ranging from several square miles to hundreds of 

square miles. Although the batholith is shown on the map as Cretaceous 

granitic rocks (unit Kg), the age of the individual plutons ranges from Late 

Jurassic to Late Cretaceous. In general, the older component plutons lie 

along the western margin of the batholith and the plutons become younger 

in the eastern parts of the batholith. In the northern part of the batholith 

the plutons intruded accreted terranes of the northern Sierra Nevada and 

Klamath Mountains; farther south, they invaded Precambrian rocks that 

have been part of North America since the beginning of the Paleozoic, as 

well as their sedimentary cover rocks. 

The plutons of the Sierra Nevada batholith and its extensions to 

the north and south represent the magma chambers that fed a chain of 

volcanoes resulting from subduction of tectonic plates of the Pacific 

beneath the western margin of North America, beginning in the early 

Mesozoic and continuing into the Late Cretaceous.

The Great Valley of California (2) is a flat-floored linear depression 

about 400 miles long and as much as 50 miles wide that occupies much of 

central California between the Sierra Nevada and the northern Coast Ranges. 

The valley is underlain by a sequence of Jurassic and Cretaceous sedimentary 

rocks, chiefly mudstone and sandstone that are referred to as the Great Valley 
24 About the Geologic Map in the National Atlas of the United States of America

sequence (shown as unit c on the map). The Great Valley sequence is largely covered 

by Neogene and Quaternary deposits that make up the floor of the present valley but is 

widely exposed along the margins of the valley, especially around its northern end. On 

the western side of the valley, the Great Valley sequence is locally nearly 10 miles thick 

and dips steeply eastward; on the east side the sequence is much thinner and dips gently 

westward. Detailed studies of the materials in the sand and mud show that the sediments 

of the Great Valley sequence were shed from the volcanic chain of the Sierra Nevada 

and from its roots and were deposited in a deep linear basin between the Sierra volcanic 

chain and a low, linear ridge to the west. Such fore-arc basins are common features of 

modern convergent plate margins.

The Coast Ranges of northern California lie west of the Great Valley and the 

Klamath Mountains and east of the San Andreas fault. The exposed rocks are coeval 

with the Great Valley sequence, but are quite different in character. Most of them 

belong to the Franciscan Complex (3), an enigmatic package of rocks that includes 

lower Mesozoic, Cretaceous, and Paleogene sedimentary rocks, chiefly dark shale 

and sandstone, but also basalt, chert, and metamorphic rocks of various kinds. Parts 

of the complex are melange, a chaotic mixture of blocks of sandstone, basalt, chert, 

and conglomerate in a matrix of dark, scaly mudstone. The blocks range from a few 

inches in diameter to hundreds of feet across. Unlike the Great Valley sequence, no 

orderly arrangement of layers is discernible in the Franciscan. The rocks are broken 

by pervasive low- and high-angle faults that destroyed any vestige of original layering 

and reduced the rocks to geological hash. Most rocks in the Franciscan have been 

metamorphosed to some degree, and some have been converted into phyllite and schist. 

In general, both the age and the degree of metamorphism increase eastward within 

the complex. In eastern parts of the Franciscan many of the rocks are blueschist that 

apparently formed at very high pressures but at moderate temperatures. The Franciscan 

is interleaved with sheets of dark mafic and ultramafic rocks that probably represent 

fragments of oceanic crust and that commonly lie along major thrust faults.

The contact between the Franciscan Complex and the Great Valley sequence is 

everywhere marked by faults. In many places the Franciscan seems to have been carried 

under the Great Valley rocks on a low angle thrust fault called the Coast Range thrust; 

in other places the two groups of rocks are separated by younger, steeply dipping faults 

along which there has probably been significant lateral movement.

The Franciscan is interpreted has having formed along an eastward-dipping 

subduction zone marked by a deep oceanic trench where tectonic plates of oceanic crust 

from the Pacific were being carried beneath the western edge of North America. The 

abundant low-angle faults, the melange, and the blueschist type of metamorphic rocks 

are thought to have formed as material eroded from the continent, fed into the trench, 

and was carried down the subduction zone. 

Beginning about 29 million years ago, plate motions along the California 

segment of the Pacific Coast began to change from convergence of the ocean floor 

plates beneath the Pacific with the North American plate to northwestward movement 

relative to North America. This change led to the development of a complex array of 

northwest-trending strike-slip faults along which rocks on the southwest side moved 

Figure 10. Mount Saint Helens, Washington. Mount Saint Helens is one of 

more than a dozen active volcanoes that lie along the crest of the Cascade 

Mountains between northern California and southern British Columbia. 

These volcanoes formed as a result of eastward subduction of the Juan 

de Fuca tectonic plate (which lies offshore beneath the Pacific Ocean) 

beneath the western edge of North America. 

On May 18, 1980, this volcano erupted explosively after several months of 

premonitory activity, including swarms of earthquakes and ash and stream 

eruptions. In the weeks preceding the main eruption an unstable bulge formed 

on the north face of the mountain and continued to grow at a rate of as much 

as 5 feet per day. At about 8:30 a.m. on May 18 a magnitude-5.1 earthquake 

caused the bulge to collapse catastrophically, triggering a huge rock debris 

avalanche that surged north into and across Spirit Lake and west down the 

North Fork of the Toutle River at speeds estimated at 110–155 miles per hour.

Within seconds after the avalanche, the sudden unloading of the 

mountain’s north flank released the pent-up pressure on the volcanic 

system in an explosive northward-directed lateral blast that was heard 

hundreds of miles away. The shock wave from the blast, which traveled at 

speeds of more than 600 miles per hour, overtook the debris avalanche and 

spread out to the north and northwest, obliterating forests for as much as 8 

miles from the volcano and flattening trees as far as 19 miles away.

Shortly after the lateral blast, a strong, vertically directed explosion 

spewed out a rising column of ash and steam that reached an altitude of 12 

miles in less than 10 minutes. Traces of ash spread by the jet stream were 

recorded as far away as eastern Oklahoma. At the same time, dense flows 

of incandescent volcanic ash spread northward out of the breached crater, 

burying parts of the debris avalanche.

Within minutes after the start of the eruption, water from melting snow 

and glacier ice on the upper slopes of the mountain triggered mud flows of 

volcanic debris down the South and North Forks of the Toutle River and other 

streams draining off the volcano. Some of these flows reached depths of 66 

feet and surged up valley walls as much as 350 feet. The eruption, which has 

been called one of the most significant geologic events in the United States 

during the twentieth century, resulted in 57 deaths, including that of David 

A. Johnston, a U.S. Geological Survey scientist who was monitoring the 

volcanic activity.

Annotations on the photograph show North Fork of the Toutle River 

(NF); South Fork of the Toutle River (SF), Spirit Lake (SL), and Coldwater 

Lake (CL). Numbers show volcanic deposits: debris avalanche deposits (1), 

volcanic ash flows (2), blast zone (3) and mud flows (4). Distance from the 

center of the crater to the northeast end of Coldwater Lake is about 8 miles.

Image courtesy of the Image Science & Analysis Laboratory, NASA Johnson 

Space Center; (photograph STS064-51-25; available from http://eol.jsc.nasa.

gov). Direction of view is vertical; altitude not given. Taken September 1994.
PACIFIC OCEAN

ATLANTIC OCEAN

 Geologic Provinces 25

1

NORTH

SF

NF

SL

CL

1

1

2

2

3

3

3

3

3

4

4

4
26 About the Geologic Map in the National Atlas of the United States of America

northwest with respect to those on the northeast side. The largest and most continuous 

of these faults is the San Andreas, which extends from the head of the Gulf of California 

to Cape Mendocino, more than 800 miles, about 200 miles of which is offshore between 

Point Reyes (fig. 11) and Cape Mendocino. The total offset along the San Andreas since 

22 million years ago has been about 200 miles, but this represents only a fraction of the 

total displacement of the Pacific ocean floor relative to North America. The remainder 

of the movement is being taken up by movement on the myriad other faults of the San 

Andreas system and by extension of the crust in the Basin and Range province. The 

movements along the San Andreas and the other faults related to it have dismembered 

and displaced the previously assembled collage of tectonic terranes, greatly 

complicating the pattern of the geology, to the frustration of generations of geologists!

Southwest of the San Andreas fault in the southern Coast Ranges and Peninsular

Ranges (I) is a diverse assemblage of tectonic terranes, some of which may have 

originated hundreds of kilometers south of their present positions. Much of the 

northward movement of these terranes apparently took place since the Paleogene but 

before the present San Andreas fault was established. Rocks in these terranes include 

metamorphosed sedimentary and volcanic rocks of Mesozoic age, Cretaceous granitic 

rocks similar to those in the Sierra Nevada batholith, and sedimentary rocks similar to 

those of the Great Valley sequence and the Franciscan Complex. Lateral movements 

along the northwest-trending San Andreas fault and its predecessors led to the 

development of a number of isolated basins in which a great thickness of Paleogene and 

Neogene sedimentary rocks (units pgT and nTg) accumulated. Many of these sediments 

have been complexly folded and displaced during continuing fault movements (fig. 12). 

Lava Plains and Plateaus of the Columbia Intermontane Region

North of the Basin and Range Province, between the volcanic chain of the Cascades 

and the Idaho batholith, lies a vast region of plains and plateaus formed by flows of basalt 

(unit nTv) that accumulated since about 17 million years ago. The lava flows and layers of 

volcanic ash and sediment cover an area that exceeds 86,000 square miles; their total volume 

probably approaches 100,000 cubic miles. This region can be divided into three principal 

parts: the Oregon Plateaus, the Columbia River Plateau, and the Snake River Plain.

The Oregon Plateaus comprise the parts of the region south of the Blue Mountains and 

east of the Snake River Plain. They encompass most of southeastern Oregon and extend into 

northern California, northern Nevada, and southwestern Idaho. The plateaus are underlain 

by a thick sequence of lava flows and associated sedimentary rocks that accumulated largely 

between 17 and 14 million years ago in a deep basin that was open to the south, toward 

the Basin and Range Province. The lava is chiefly basalt erupted from deep linear fissures 

that seem to have formed at about the same time as the regional extension in the Basin and 

Range Province. In many places the volcanic rocks are cut by faults similar to those formed 

by extension in the Basin and Range. Although basalt predominates, rhyolite, erupted 

chiefly as volcanic ash from calderas, is also widespread.

Figure 11. The San Andreas fault at Point Reyes, California. The long, 

straight, northwest-trending depression that extends diagonally across 

the area of the photograph marks the San Andreas fault. Here the 

fault separates Late Cretaceous granite and nearly flat lying Tertiary 

sedimentary rocks on the southwest from rocks of the Franciscan 

Complex of Mesozoic (Jurassic and Cretaceous) age. The Franciscan 

is a chaotic mixture of blocks of sandstone, chert, and dark igneous 

rocks embedded in dark shale and sandstone. It accumulated along the 

western edge of North America as parts of the Pacific sea floor were 

being subducted eastward beneath the continent.

During the 1906 San Francisco earthquake Point Reyes moved 

as much as 16 feet northwestward with respect to the rest of North 

America. The town of Petaluma (P) is near the eastern edge of the area 

shown in the photograph.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS073-726-004; available from 

http://eol.jsc.nasa.gov). Direction of view is vertical; altitude not given. 

Horizontal distance across the area shown about 36 miles.
PACIFIC OCEAN

ATLANTIC OCEAN

 Geologic Provinces 27

NORTH

P
28 About the Geologic Map in the National Atlas of the United States of America

The Columbia River Plateau lies in southeastern Washington and northern 

Oregon, between the Blue Mountains and the northern Cascades. It is also underlain 

by a deep basin in which thousands of feet of basalt lava flows and interleaved sedimentary rocks accumulated, mainly between 17 and 14 million years ago, but some 

as recently as 6 million years ago. Some of these flows spread westward down an 

ancestral valley of the Columbia River all the way to the sea. The basalt flows of the 

Columbia Plateau were also erupted from fissures perhaps related to Basin and Range 

extension. In south-central Washington the lava flows and the sediments interlayered 

with them are broken by young thrust faults and warped by folds that trend northwest–southeast.

The western part of the Snake River Plain is a northwest-trending, fault-bounded 

depression in southwestern Idaho that is filled with as much as 5,000 feet of interlayered river deposits, lake beds, and basalt of Neogene age capped by Quaternary basalt 

flows.

The eastern part of the Snake River Plain is a downwarp filled with volcanic 

rocks that trends northeastward from south-central Idaho into Yellowstone National 

Park, in northwestern Wyoming. The downwarp is largely floored by Quaternary 

basalt flows (unit Qv), but drilling shows that the basalt is underlain by great thicknesses of rhyolite volcanic ash erupted from calderas, some of which are buried 

beneath the younger basalt. The calderas range in age from 12 million years at the 

southwest end of the eastern plain to the still-active caldera in Yellowstone, which 

contains the world’s largest active geyser field. The regular progression in ages from 

the oldest at the southwest to the youngest at the northeast has led to speculation that 

the eastern Snake River Plain marks the path of the North American plate across a 

stationary deep-seated plume of hot material rising from beneath the Earth’s crust. 

Figure 12. The Los Angeles Basin, California. The city of Los Angeles 

sits atop a deep basin filled with as much as 30,000 feet of very young 

sedimentary rocks, most of which were deposited as sediments on 

the sea floor, but some of which were deposited on land. The basin 

developed as a result of strike-slip movement on the San Andreas Fault, 

which lies just beyond the upper right corner of the area shown.

Just north of Los Angeles the generally northwest-trending San 

Andreas fault bends abruptly to the west-northwest for almost 150 miles, 

before bending back to the northwest. Northwestward movement of 

coastal California southwest of the fault causes compression and folding 

of rocks as the tectonic plate encounters the bend. The complex array 

of faults in and around Los Angeles is the result. The faults sketched on 

this photograph include northwest-trending strike-slip faults and easttrending thrust faults caused by compression. The strike-slip faults, such 

as the San Gabriel (1), Whittier-Elsinore (2), Newport-Englewood (3), 

and Palos Verdes (4), are roughly parallel to the San Andreas. They are 

marked with pairs of arrows indicating the relative motion of the rocks 

on either side of the fault. The compression-related thrust faults, such 

as the Simi (5), Malibu Coast (6), Santa Monica (7), Sierra Madre (8), 

and Oak Ridge (9), are decorated with small triangles that point in the 

direction of inclination of the fault and are placed on the side of the fault 

that moved up. 

The Los Angeles Basin has been a prolific producer of oil, which was 

generated in the organic-rich sediments and trapped in folds and along 

faults. The active faults pose a significant hazard to lives and property. 

The approximate epicenters of the 1971 magnitude-6.7 San Fernando 

earthquake, the 1987 magnitude-5.9 Whittier Narrows earthquake, 

and the 1994 magnitude-6.7 Northridge earthquake are shown by stars 

labeled SF, WN, and NR. The focus of the San Fernando earthquake 

lay at a depth of about 5.2 miles along the San Fernando thrust fault. The 

focus of the Whittier Narrows earthquake was at a depth of 7.5 miles 

along a blind thrust related to the Whittier fault (a blind thrust is a thrust 

fault that does not reach the surface). The focus of the Northridge quake 

lay at a depth of about 4 miles along a south-dipping thrust fault.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS039-089-0062; available from 

http://eol.jsc.nasa.gov). Direction of view is vertical; altitude 141 nautical 

miles.
PACIFIC

O

CEA

N

ATLANTIC OCEAN

 Geologic Provinces 29

NR

SF

WN

NORTH 2

6

7

1

4

9

8

5

3
30 About the Geologic Map in the National Atlas of the United States of America

Figure 13. Index map showing geologic provinces and subprovinces 

in Alaska. Heavy red lines show boundaries between major provinces; 

medium-weight blue lines show boundaries of subprovinces and belts. 

In the central intermontane plateaus and basins province, circled 

numbers indicate general outcrop areas of some of the tectonic 

terranes mentioned in the text. Blue stipple shows late Tertiary and 

Quaternary volcanic provinces. In the Aleutian–Alaska Range mountain 

system province, circled letters show general positions of flysch basins 

mentioned in the text and circled numbers indicate outcrop areas of 

tectonic terranes.

 Alaska

The vast State of Alaska, about one-fifth the size of the entire conterminous United States, 

has some of the most complex and poorly understood geology in North America. The State 

comprises a complicated collage of tectonic terranes (fig. 13) similar to those already described 

in the western cordillera of the conterminous United States. These terranes include displaced 

and (or) rotated fragments of the continental margin of North America, accumulations of volcanic rocks that were erupted during subduction of oceanic plates beneath North America, chunks 

of oceanic crust carrying seamounts and oceanic plateaus that were accreted to the margins of 

the continent, and extensive belts of sedimentary rocks that were deposited on oceanic plates 

and scraped off as they were pushed against the continental margin during subduction. Assembly of these disparate tectonic terranes began in the early Paleozoic and is continuing today. 

During and after suturing of the various terranes, thick sequences of sedimentary rocks accumulated in local or regional basins, concealing extensive parts of the terrane collage. Both the 

mosaic of tectonic terranes and the overlying basin rocks have been broken by large strike-slip 

faults. Some of these displace rocks on one side relative to those on the other side by hundreds 

of miles. For purposes of brief geological description, Alaska can be roughly divided into four 

regions: (1) Brooks Range and Arctic slope, (2) central intermontane plateaus and basins, (3) 

Aleutian–Alaska Range mountain system, and (4) the volcanoes of the Aleutian Islands, Alaska 

Peninsula, and Wrangell Mountains, and the Bering Sea volcanic field.

Brooks Range and Arctic Slope

The Brooks Range is the broad mountain belt that extends westward across northern Alaska 

from the Canadian border to the Arctic Ocean north of the Seward Peninsula. The Arctic slope 

includes the northern foothills of the Brooks Range and the Arctic coastal plain.

The Brooks Range consists largely of sedimentary rocks including Late Proterozoic and 

Paleozoic (Z, Zg), lower Paleozoic (lg), and middle Paleozoic (mg) strata. These rocks are 

thought to have been part of the Arctic continental margin in western Canada that has been 

rotated 66° counterclockwise into their present position by sea-floor spreading in the Beaufort 

Sea during the Jurassic and Early Cretaceous. The rotation caused compression of the rocks 

on the leading edge of the rotating block, and this compression drove extensive sheets of rock 

northward along the thrust faults in the Brooks Range. Sediments shed northward from the rising mountains into the flanking basin (c, K) were themselves deformed into elongate east-west 

trending folds in the northern foothills. During this deformation many of the Paleozoic and Proterozoic rocks in the southern part of the range were complexly deformed and variably metamorphosed to gneiss, schist, slate, and phyllite.

The Arctic Slope north of the Brooks Range foothills is underlain by the Colville Basin, 

a deep basin filled with upper Cretaceous (K) and Tertiary rocks (pgT) that was depressed by 

loading of the Earth’s crust by the thrust sheets in the range and the weight of the sediments shed 

from them. Much of the oil and gas production on the Arctic Slope comes from Paleozoic rocks 

buried beneath the Colville Basin. On the Arctic Coastal Plain the deposits in the Colville Basin 

are mantled glacial drift of late Tertiary and Quaternary age and by Quaternary and younger 

swamp deposits.
PACIFIC OCEAN

A

BROOKS RANGE AND

ARCTIC

SLOPE

ARCTIC SLOPE

BROOKS RANGE

ALEUTIAN

VOLCANIC CHAIN

ALASKA PENINSULA

 VOLCANIC CHAIN

ALEUTIAN - ALASKA RANGE

MOUNTAIN

SYSTEM

CENTRAL

INTERMONTANE

PLATEAUS AND

BASINS

WRANGELL MOUNTAINS

 VOLCANIC FIELD

 BERING SEA

VOLCANIC FIELD

COAST MOUNTAINS

 BATHOLITH

DENALI FAULT

8

9 B

8

9

9

A

A A

8

7

1

1

1 3

5 5 5 2 5

5 2

5

2

6

KOBUK BELT

YUKON - KOYUKUK BELT

KUSKOWIM BELT

4

4

Geologic Provinces 31
32 About the Geologic Map in the National Atlas of the United States of America

Central Intermontane Plateaus and Basins

Interior Alaska, that part of the State that lies between the KobukMalamute strike-slip fault at the southern edge of the Brooks Range 

and the Denali strike-slip fault at the northern front of the Alaska 

Range, comprises a vast region of low hills or plateaus and broad, 

sediment-filled basins and valleys. The region includes more than a 

dozen disparate tectonic terranes. Some are believed to represent rotated 

and translated fragments of the northwest-trending continental margin 

of North America that existed in the early Paleozoic or of sediments that 

were eroded from continental fragments and deposited on their flanks. 

Others are interpreted as containing rocks formed in and near arcuate 

chains of volcanoes that were accreted to the margin of the continent 

by subduction of oceanic crust. Still others are thought to be sheets of 

ocean floor and crust that were thrust onto the edge of the continent 

during the docking of the island-arc terranes.

The continent-related terranes include the Yukon composite 

terrane (1), the Ruby terrane (2), the Porcupine terrane (3), and the 

Seward terrane (4). They consist largely of metamorphic rocks like 

those in adjacent parts of North America, including highly deformed 

sandstone, schist, and marble (unit Zg) and gneiss of Paleozoic age 

(unit gn). The rocks underlie metamorphosed Paleozoic and Mesozoic 

sedimentary rocks (units g and gc), and all of the rocks are invaded 

by large bodies of Cretaceous granite (unit Kg). The Porcupine terrane is 

apparently a fragment of the continental margin that lay near the hinge 

of the rotation that formed the Brooks Range and so escaped the major 

rotation. The Yukon composite terrane resembles the other continentrelated terranes but is composed mainly of Paleozoic limestone (unit 

g) and upper Paleozoic and Mesozoic limey sandstone, siltstone, and 

shale (unit gc) that formed from sediments deposited in a shallow sea, 

probably on continental shelves.

Rocks shown on the map as unit gcv, forming a discontinuous, 

northeast-trending belt across central Alaska from Goodnews Bay 

to the Canadian border south of the Brooks Range, consist of highly 

deformed ocean-floor basalt and deep ocean sedimentary rocks, in many 

places interleaved with ophiolites (unit lcu). These rocks are parts 

of the Angayucham terrane (5). They are believed to be dismembered 

fragments of ocean floor that was thrust onto the edge of the continent 

during closure of an ocean that lay to the west. They now overlie rocks 

of the Ruby terrane and Yukon composite terrane. Similar rocks also 

crop out in a narrow belt (too small to show on this map) along the 

southern edge of the Brooks Range and in thin sheets that have been 

shoved northward across the sedimentary strata of the range.

The Koyukuk (6) and Togiak (7) terranes comprise volcanic rocks and 

associated volcanic-rich sedimentary rocks (units cv and c). The volcanic 

rocks have compositions of rocks typically erupted along volcanic chains 

on the ocean floor (like the Aleutian Islands), and these terranes are thought 

to be parts of a volcanic arc that was accreted to North America during the 

closure of the ocean marked by the Angayucham terrane. 

The tectonic terranes are separated and in part overlain by belts of 

sedimentary rocks, chiefly thinly interbedded dark argillite, siltstone, dark 

sandstone, and impure limestone, that originated as deposits in basins and 

troughs between the tectonic terranes as they were being assembled. Much 

of the sandstone contains fragments of volcanic rocks, and volcanic rocks 

are interlayered with the sedimentary rocks. Rocks in many parts of these 

belts were deformed and metamorphosed during terrane amalgamation and 

were invaded by bodies of granite and other plutonic rocks. Three of the 

principal belts of this sort are indicated by name on the map:

•	 The Yukon-Koyukuk belt, which consists of strongly deformed and 

mildly to strongly metamorphosed rocks of Early and Late Cretaceous 

age.

•	 The Kobuk belt, which consists of strongly deformed and variably 

metamorphosed rocks containing scattered mid-Cretaceous fossils.

•	 The Kuskokwim belt, which contains rocks similar to the other belts, but 

also some conglomerate. The rocks are generally mildly to moderately 

deformed but are little metamorphosed.

Assembly of the tectonic terranes started in the Late Jurassic and was 

largely completed by the Late Cretaceous, although later movement along 

major right-lateral strike-slip faults has significantly modified their Late 

Cretaceous configuration. Broad basins filled with Tertiary and Quaternary 

rocks formed after terrane assembly and are relatively undeformed.
Aleutian–Alaska Range Mountain System

The Aleutian–Alaska Range mountain system is a great mountain belt 

that extends in a broad, convex-northward, arcuate belt from the Aleutian 

Islands through the Alaska Peninsula, across southern Alaska and eastward 

through the southwestern part of the Yukon Territory of Canada and into 

southeastern Alaska. It includes some of the highest and most rugged peaks 

in North America. The belt consists largely of two principal groups of 

tectonic terranes: a group north of the Border Ranges fault system known as 

the Wrangellia composite terrane (8), and a group south of the fault system 

known as the Southern Margin composite terrane (9).

The Wrangellia composite terrane (8) consists chiefly of volcanic and 

sedimentary rocks that accumulated in or near several belts of midocean 

volcanic islands, possibly beginning in the Proterozoic, but continuing 

through the Paleozoic and much of the Mesozoic. The location of these 

islands at the time of accumulation is unknown, but paleomagnetic studies 

suggest that they may have originated hundreds or thousands of miles south 

of their present positions and that they were carried northward on tectonic 

plates that were being subducted under North America.

The Southern Margin composite terrane (9) is made up largely of 

sedimentary and volcanic debris that was deposited on the sea floor along 

the southern margin of Wrangellia. In the Middle Triassic (about 230 

million years ago) subduction began to carry parts of the sea floor beneath 

Wrangellia. As a result, a trench in the sea floor developed and chains of 

volcanoes developed inland from the subduction zone. Great thicknesses 

of dark argillite and muddy sandstone, composed of material eroded from 

Wrangellia and from the volcanic chain that fringed it, accumulated in 

the trench, where they mixed with volcanic debris. These flysch deposits 

were then scraped off the oceanic plate and added to the southwest margin 

of Wrangellia as the oceanic plate was subducted beneath it. Much of 

the off-scraped material formed melange, a jumbled mixture of various 

sized blocks of rocks of all types, some from the sea floor and some from 

nearby continental areas, all enclosed in fine-grained sedimentary material. 

Generally, the melange is intensely deformed and weakly metamorphosed. 

At the same time that subduction was taking place along the southwest 

margin of Wrangellia, Wrangellia itself was moving northward. By the time 

the Wrangellia composite terrane arrived in its present position sometime 

between 120 and 84 million years ago, it was already carrying parts of the 

Southern Margin composite terrane piggyback along its trailing edge. After 

the Wrangellia composite terrane docked with North America, subduction 

continued along its southern margin and material is still being added to the 

Southern Margin composite terrane as the Pacific plate continues to move 

northward along the coast of southeastern Alaska (fig. 14) and beneath the 

Alaska Peninsula.

The arrival of Wrangellia against North America was accompanied 

by intense deformation and metamorphism of rocks in the flanking 

terranes that were already part of the continent. Flysch deposits that had 

accumulated in basins in the ever-narrowing seaway that lay between the 

continental margin and the approaching landmass of Wrangellia were 

crammed against the continent to form a discontinuous belt of highly 

deformed dark sandstone, shale, and volcanic rocks that marks the inner 

edge of the Wrangellia composite terrane. The belt includes the Kantishna 

belt (A) and the Gravina-Nutzotin belt (B).

During the arrival of the Wrangellia composite terrane adjacent parts 

of the continent inboard of Wrangellia were invaded by great batholiths 

of granite, including those that make up the Coast Mountains batholith in 

the northeastern part of southeastern Alaska and adjacent parts of Canada, 

the granite bodies that make up much of Mt. McKinley (fig. 15) and its 

neighbors in the central Alaska Range, and most (but not all) of the granitic 

bodies in the Aleutian Mountains northwest of Cook Inlet. 

Geologic Provinces 33
34 About the Geologic Map in the National Atlas of the United States of America

Volcanoes of the Aleutian Islands, Alaska Peninsula, 

and Wrangell Mountains, and the Bering Sea Volcanic Field

Alaska has several Tertiary and Quaternary volcanic chains and volcanic fields, 

and many active or recently active volcanoes. The volcanic belt that includes the 

Aleutian volcanic chain, the Alaska Peninsula volcanic chain, and the Wrangell 

Mountains volcanic field developed as a result of subduction of oceanic plates 

beneath the Alaskan margin of North America. The current rate of movement of the 

Pacific plate relative to North America is a little less than 3 inches per year. The 

direction of movement of the Pacific plate is perpendicular to the volcanic arc in 

the area of the Alaska Peninsula, but because of the curvature of the arc the motion 

becomes nearly parallel to the arc in the westernmost part of the Aleutian Islands. 

The Wrangell Mountains, in the southern part of eastern Alaska (northwest of the 

point where the Alaska Panhandle joins the main part of Alaska) are part of the same 

volcanic chain but are separated from the main belt by a gap of about 250 miles. By 

contrast, the Bering Sea volcanic field seems to be related to crustal extension in 

western Alaska and the Bering Sea shelf.

The Aleutian volcanic chain is the emergent part of a largely submerged volcanic 

mountain range. The volcanic peaks stand above the sea surface to form the Aleutian 

Islands. These undersea mountains were built by volcanic activity beginning about 50 

million years ago during the Eocene. The volcanic activity waned between about 35 

and 5.5 million years ago and then resumed. The many currently active volcanoes 

were all built during this last time interval.

The Alaska Peninsula volcanic chain differs from that in the Aleutians to the 

west in that it was built on continental crust rather than oceanic crust. In the Alaska 

Peninsula most of the volcanic rocks overlie rocks of the Peninsular terrane (part of 

the Southern Margin composite terrane) or on Mesozoic granite plutons that intrude 

them. Most of the older volcanic rocks were erupted between 48 and 22 million years 

ago. A second period of volcanism started about 15 million years ago, but most of the 

active volcanoes and calderas have formed in the last million years, many within the 

last few thousand years.

The Wrangell Mountains are a group of voluminous shield-like volcanoes that 

have eruption rates among the highest reported for plate-margin volcanic fields. The 

volcanic field includes several peaks with elevations more than 16,000 feet. The ages 

of volcanic activity generally decrease from 15–20 million years in the eastern part of 

the field to 1–5 million years in the central part, and to 1 million years in the western 

part. The volcanoes have no recorded historic activity, but a vent in the field was the 

source of two eruptions that spread ash over wide areas in Alaska and Canada 1,890 

and 1,250 years ago.

Figure 14. Glacier Bay and the St. Elias Range, Alaska. The St. Elias Range 

marks the boundary between Alaska and the Yukon Territory along the 

northwestern part of Alaska’s southeastern panhandle. The highest peak 

in the range, 18,000-foot Mount St. Elias, lies just north of the area shown. 

Glacier Bay is a multibranched fjord visible in the lower right part of the 

photograph, and Icy Strait is visible along the lower-right edge.

The Border Ranges thrust fault shown in the central part of the 

photograph marks the suture between the Wrangellia composite terrane on 

the east and the Southern Margin composite terrane on the west. The rocks 

east of the fault are chiefly middle and lower Paleozoic gray sandstone, 

argillite, limestone, and volcanic rocks, all of which are intruded by large, 

irregular bodies of Cretaceous and Tertiary granite. Those west of the fault 

are chiefly metamorphosed Cretaceous sandstone, argillite, and basalt cut 

by Tertiary granite and other igneous rocks.

The Fairweather fault shown in the left part of the photograph is an 

active strike-slip fault that has displaced part of the Wrangellia composite 

terrane northward relative to the rocks of the Southern Margin composite 

terrane. In 1958 an earthquake along the Fairweather fault triggered a huge 

rock slide that plunged about 3,000 feet into the head of Lituya Bay, causing 

a wave that surged up the opposite wall of the inlet to an altitude of 1,740 

feet!

Part of the Denali strike-slip fault, which separates the Wrangellia 

composite terrane from the Yukon composite terrane, is visible in the 

upper-right corner of the photograph.

When Vitus Bering explored Icy Strait in 1742, glaciers extended all 

the way to the mouth of Glacier Bay. Since that time the ice front has 

retreated more than 60 miles, and most of the glaciers are continuing 

to retreat. Strangely, however, a few seem to be advancing slightly, 

probably due to local shifts in storm tracks.

Annotations on the photograph show the Border Ranges fault (br); 

Fairweather fault (f); Denali fault (d); Lituya Bay (lb); and position of glacier 

terminus at mouth of Glacier Bay in 1742 (g).

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS068-229-7; available from http://eol.

jsc.nasa.gov). Direction of view is vertical; altitude not given. Horizontal 

distance across center of the area shown is about 110 miles.
PACIFIC OCEAN

A

NORTH

GULF

OF

ALASKA

Icy 

Strait

Glacier

Bay

br

br

f

f

g

lb

d

d

Geologic Provinces 35
36 About the Geologic Map in the National Atlas of the United States of America

The Bering Sea volcanic field differs from the other volcanic areas in that its 

volcanic rocks consist almost entirely of basalt, whereas the subduction-related 

volcanic rocks are largely andesite or rhyolite. Most of the volcanic rocks are less 

than 5 million years old, and some lava flows and cinder cones are less than 10,000 

years old, but none is active at present. Much of the lava was erupted from clusters 

of low shield volcanoes or from fissures in the underlying rocks.

Figure 15. Mount McKinley, Alaska. View from the south from over the 

Gulf of Alaska with an 800-mm lens. Mount McKinley, often referred to 

by its native name, Denali, is the highest mountain in North America. 

The South Peak, altitude 20,320 feet, is marked with an arrow on the 

photograph; the slightly lower North Peak is visible behind and slightly 

to the left. McKinley and its towering neighbors, including 17,400-foot 

Mount Foraker (F) and 13,200-foot Mount Silverthrone (S), are carved 

from large bodies of Tertiary granite that intruded intensely deformed 

dark sandstone, argillite, and volcanic rocks along the northern edge 

of the Wrangellia composite terrane. These rocks were uplifted by 

movements along the Denali fault, which lies just north of the high peaks 

but is out of sight in this photograph. Mount McKinley lies near the 

apex of a major bend in the fault where its trend changes from westnorthwest to southwest. Horizontal movement along the fault carried the 

rocks south of the fault westward with respect to the rocks to the north. 

As they encountered the bend, the rocks were forced up and northward. 

Erosion of these uplifted rocks, largely by glaciers, removed the softer, 

more easily eroded sedimentary rocks and left the massive granite 

bodies standing as some of the continent’s highest peaks.

In this photograph, taken in August 2005, the valleys are partly 

obscured by a pall of smoke from widespread forest fires. However, 

several of the large glaciers that flow southward from the Alaska 

Range toward the Chulitna River are visible, including the Kahiltna (K), 

Tokositna (T), and Ruth Glaciers (R). The Kahiltna Glacier is about 45 

miles long, making it the longest in the Alaska Range.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph ISS011-E-11805; available from 

http://eol.jsc.nasa.gov). Direction of view is north; altitude 188 nautical 

miles.
PACIFIC

O

CEA

N

A

F

S

K

T

R

R

Geologic Provinces 37
38 About the Geologic Map in the National Atlas of the United States of America

 Hawaii

The State of Hawaii consists of a chain of islands (fig. 16) that represent 

the emerged summits of a huge northwest-trending submerged volcanic ridge. 

The large islands at the southeastern end of the chain extend for about 400 miles 

northwestward from the Island of Hawai’i to Ni’ihau, but small islands and 

reefs included in the State extend another 930 miles westward, so that the total 

length of the island chain in the State is only slightly less than the distance from 

Washington, D.C., to Denver, Colo. However, the total land area of the State is 

only about 6,600 square miles, about the size of Connecticut and Rhode Island 

combined. The volcanic ridge continues northwestward to beyond Midway 

Island and then turns northward at the totally submerged Emperor Seamounts 

and extends almost to the western tip of the Aleutian Islands.

The sea floor on which the volcanic ridge is built ranges in depth from 

about 15,000 feet near the large islands to about 17,000 feet around the small 

islands to the northwest. The age of the sea floor on which the ridge stands is 

between 120 and 80 million years. However, the age of the basalt flows that 

make up the major part of the larger islands increases regularly from less than 

400,000 years on Hawai’i (where the volcanoes Kilauea, Mauna Loa, Mauna 

Kea, and Hualalai are still active) to about 5 million years on Ni’ihau and 

Kaua’i (the northwesternmost of the large islands). The same trend continues to 

the northwest—the lava on Midway Island was erupted almost 28 million years 

ago. The northwestward increase in the ages of the volcanoes that make up the 

island chain is thought to be related to passage of the Pacific 

plate northwestward across an area of high temperature 

caused by upwelling of hot material from a stationary source 

deep within the Earth. As the Pacific plate moves across the 

hotspot it is melted to form the lava that builds the volcanic 

ridge. The situation might be likened to slowly moving an 

old wax phonograph record across a blowtorch flame. If the 

record moves northwest across a stationary flame, an ant on 

the surface of the record would observe that from his point 

of view the zone of melting is progressing southeast. As a 

volcanic center is moved northwest away from the hotspot, 

volcanic activity ceases and streams and waves erode the 

volcanic edifice. At the same time, cooling of the volcanic 

material causes the rocks to contract and become denser, 

causing the volcanic pile and the surrounding ocean floor to 

subside. Thus, the islands become smaller and more deeply 

eroded toward the northwest along the island chain and 

eventually are reduced to coral atolls. 

Figure 16. Index map showing the chain of islands that makes up 

the State of Hawaii.

0 100 200 300 400 MILES

0 100 200 300 400 500 600 KILOMETERS

KAUA'I

OAHU MOLOKA'I

LÄNA'I

HAWAI'I

MAUI

KAHOOLAWE

NI'IHAU

Necker Island

Nihoa

French Frigate

 Shoals

Maro Reef Gardner Pinnacles

Laysan Island

Lisianski

 Island

Pearl and

Hermes Atoll

Kure Atoll

20 ̊

25 ̊

30 ̊

175 ̊ 170 ̊ 165 ̊ 160 ̊ 155 ̊

 Puerto Rico and the U.S. Virgin Islands

Puerto Rico and the U.S. Virgin Islands lie just south of the plate boundary 

between the Caribbean plate and the North American plate. The islands are largely 

composed of volcanic rocks and closely related sedimentary rocks (unit Kv). Most 

of these rocks are of Late Cretaceous age, but some are Early Cretaceous and some 

are as young as Paleogene (unit pgTv). All these rocks probably accumulated in and 

among arcuate chains of volcanic islands that developed along the plate boundary. 

Near the southwestern tip of Puerto Rico are several small areas of ultramafic rocks 

(unit u) thought to have been pieces of old ocean floor that were shoved onto piles 

of volcanic rock fragments as the plates were jostled into their present positions. 

The volcanic-sedimentary rock sequence has been partly dismembered by a complex series of west-northwest-trending faults along which rocks on the north side of 

the fault have moved west relative to those on the other side. The sequence has also 

been intruded by large plutons of granite that are shown as unit Kg on the map, but 

some of which may be of early Tertiary age. Along the northwestern and southwestern coasts of Puerto Rico the older rocks are covered by younger sedimentary rocks, 

chiefly limestone, shown as unit nT on the map. 

The Puerto Rico trench, which lies about 160 kilometers off the north coast 

of the island, marks the present southern boundary of the North American plate, 

and frequent earthquakes along the trench show that significant movement is still 

taking place along the plate boundary.
Faults

Faults are fractures or fracture zones along which rocks on one side have moved significantly 

with respect to those on the other side. Most faults shown on the geologic map layer have displacements of miles or even hundreds of miles, but some, such as those in the Gulf Coastal Plain, have 

displacements of only a few feet. Wherever possible, faults shown on the geologic map are classified on the basis of the inclination of the fault surface and the relative direction of offset of the 

rocks on either side. A normal fault is a steeply inclined fault along which the rocks above the fault 

have moved downward with respect to those below the fault. A thrust fault is a gently or moderately 

inclined fault along which the rocks above the fault have moved upward with respect to those below 

the fault. A low-angle detachment fault is a gently inclined or horizontal fault formed during the pulling apart of brittle rocks in the upper parts of the continental crust accompanied by rise of warmer, 

more plastic rocks beneath to form dome-shaped uplifts. The remaining faults on the geologic map 

(“Fault” in the legend) are not classified, either because the sense of displacement is unknown, or 

because they are too short or too closely spaced to allow placement of distinguishing map symbols. 

All strike-slip faults (steeply inclined faults along which the rocks on opposite sides are displaced 

horizontally) are included with the “unclassified” faults on the geologic map. Most of the faults 

shown on the geologic map are inactive, and many have not moved for millions or tens of millions of 

years.

The faults shown on the layer of the National Atlas entitled “Quaternary faults” are faults along 

which large-magnitude earthquakes have occurred within the past 1.8 million years. These faults 

may be capable of generating future earthquakes having magnitudes of 6 or greater. Large-magnitude 

earthquakes in the Western United States produce displacements during a single earthquake that are 

generally a few feet, but cumulative Quaternary displacement across a fault may be several miles for 

normal faults, such as the Wasatch fault near Salt Lake City, and as much as tens of miles for major 

plate-bounding faults, such as the San Andreas fault. Many Quaternary faults in the Western United 

States coincide with faults shown on the geologic map. However, large-magnitude earthquakes in the 

less seismically active Eastern and Central United States commonly do not produce surface deformation. Therefore, most historical major earthquakes do not coincide with faults shown on either the 

geologic map or the Quaternary fault layer. Additional information on potential earthquake faults is 

available at http://earthquake.usgs.gov/regional/qfaults. Currently (2007), the Quaternary fault database covers faults in the conterminous United States, but information on faults in Alaska, Hawaii, Puerto 

Rico, and the U.S. Virgin Islands will eventually be added.

Faults 39
40 About the Geologic Map in the National Atlas of the United States of America

Figure 17. Edge of the continental ice sheet in South Dakota. View 

is west-northwest over Pierre, S. Dak. (P). Lake Oahe is impounded by 

the Oahe Dam across the Missouri River just upstream from Pierre. The 

western arm of the lake that extends nearly to the top of the photograph 

is the drowned valley of the Cheyenne River; the rest of the lake is the 

drowned valley of the Missouri.

The checkerboard pattern marks farmlands that have been divided 

into tracts according to townships and sections. The green circles north 

of the dam mark areas of sprinkler irrigation fed by groundwater pumped 

from shallow wells at the center of each circle. Most of the circles are 

one-quarter mile in diameter.

About 12,000–13,000 years ago the western edge of a large lobe of 

the great glacier that covered most of the north-central states lay 5–10 

miles southwest (left) of the river. The approximate position of the ice 

front is marked with a dashed line on the photograph; ticks on the line 

point toward the ice. The farmlands west of the ice front are underlain 

by gravel carried by streams flowing off the melting glacier and by 

weathered, flat-lying Cretaceous shale and sandstone. The conspicuous 

streaks are ridges of windblown silt carried by northwest winds blowing 

off the ice-sheet farther north. The checkerboard of cultivated fields east 

and north of the river is underlain by silt, sand, and gravel deposited as 

the glacier stagnated and melted.

Image courtesy of the Image Science & Analysis Laboratory, NASA 

Johnson Space Center; (photograph STS028-076-056; available from 

http://eol.jsc.nasa.gov). Direction of view is west-northwest; altitude 163 

nautical miles.

Continental Ice Sheets

Continental ice sheets have advanced southward from Canada into the Central and Eastern 

United States a dozen or more times in the past 2 million years (fig. 17). The most recent of these, 

the Wisconsin glaciation, reached its maximum extent about 20,000 years ago and had retreated 

by about 10,000 years ago. The map shows the approximate maximum extent of the Wisconsin 

ice, but the ice may not have reached this limit everywhere at the same time. The map also shows 

the maximum southern extent of ice sheets older than the Wisconsin, but the margins shown on 

the map may represent ice advances of very different ages.

Although ice-laid surficial deposits are widespread north of these glacial limits, stream-laid 

and windblown deposits related to these glacial advances extended hundreds of miles farther 

south. None of these deposits is shown on the map, but they are shown in detail on the map 

of surficial deposits and materials in the Eastern and Central United States that is part of the 

National Atlas.

The limits of mountain glaciers in the Cordillera could not be depicted on the present map 

because their extreme complexity would obscure the bedrock geology, which is the primary focus 

of the map.
PACIFIC OCEAN

ATLANTIC OCEAN

NORTH

P

Continental Ice Sheets 41
Impact Structures

Impact structures are the records of the Earth’s encounters with objects from space, chiefly asteroids and 

cometary nuclei. It has been estimated that there are about 1,000 asteroidlike objects in Earth-crossing orbits and 

nearly as many comets. On average, about three asteroids more than half a mile in diameter strike the Earth every million years and one asteroid larger than 6 miles in diameter strikes the Earth every 40 million years or so. Most of the 

impact structures in the conterminous United States are in the Coastal Plain or Interior Lowlands because these regions 

have remained stable for hundreds of millions of years; elsewhere, many impact features may have been removed by 

erosion, destroyed by tectonic processes, or covered by younger lava or sediments. The size of the crater produced by 

an impact is many times the diameter of the projectile. For example, the 15-mile-diameter Ries crater in Germany was 

probably produced by a projectile a mile or two in diameter. Seventeen known or strongly suspected impact structures 

(table 1) are recorded in the continental United States and adjacent parts of the continental shelf. These range in diameter from a few hundred feet (Haviland, Kans.) to about 60 miles (Chesapeake Bay, Va.) and in age from about 50,000 

years (Meteor Crater, Ariz.; fig. 18) to about 360 million years (Flynn Creek, Tenn.). Only one, the Avak structure, has 

been recognized in Alaska.

Table 1. Impact structures in the United States.

[lat, latitude; long, longitude; ±, plus or minus]

Figure 18. Meteor Crater, Arizona. Meteor Crater is visible 

in the upper-left (northwestern) part of the photograph; 

it is about 4,000 feet in diameter and about 600 feet deep. 

It was produced by impact of a meteorite in the flat-lying 

sedimentary rocks of the Colorado Plateau about 50,000 

years ago. The target rocks were sandstone, limestone, and 

dolomite of the Coconino, Toroweap, and Kaibab Formations 

of Permian (late Paleozoic) age and sandstone and siltstone 

of the Moenkopi Formation of Triassic (early Mesozoic) age. 

The Permian rocks appear light gray in the photograph, 

while the Triassic (Moenkopi) rocks appear reddish brown. 

The light-colored fringe around the crater is composed 

of material ejected during impact. East and West Sunset 

Mountains, the two dark hills visible in the lower part of the 

photograph, are underlain by young basalt flows. Because 

of its proximity to the San Francisco Peak volcanic field, 

Meteor Crater was originally thought to be a volcanic crater, 

but its impact origin is now well established. Calculations 

suggest that it could have been caused by an object 100 feet 

in diameter traveling at a speed of 9 miles per second. The 

force of the impact is estimated to have been equivalent to 

the explosion of 4 or 5 megatons of TNT.

Winslow, Ariz. (W) is visible near the upper-right corner 

of the photograph. The highway parallel to the upper edge 

is Interstate Highway 40. The meandering stream that flows 

north through Winslow is the Little Colorado River.

Image courtesy of the Image Science & Analysis 

Laboratory, NASA Johnson Space Center; (photograph 

STS040-614-58; available from http://eol.jsc.nasa.gov). 

Direction of view is north; altitude is not given.

42 About the Geologic Map in the National Atlas of the United States of America

Name Lat Long Diameter

(miles) Age Remarks

1. Meteor Crater, Ariz. 35°02’ 111°01’ 0.3 50,000 years

Meteoric fragments 

identified. 2. Odessa Craters, Tex. 31°45’ 102°28’ .01 250,000 years

3. Haviland Crater, Kans. 37°35’ 99°10’ .68 less than1 million years

4.

Toms Canyon, beneath 

the ocean southeast 

of New Jersey

39°05’ 72°50’ 12.5 35 million years Buried; found by drilling and geophysics; 

impact in ocean. 5. Chesapeake Bay, Va. 37°16’ 76°01’ 60 35 million years

6. Bee Bluff, Tex. 29°02’ 99°51’ 1.5 less than 40 million years Shock features identified. 7. Hico, Tex. 32°00’ 98°02’ less than 60 million years

8. Manson, Iowa 42°35’ 94°31’ 22 74 million years Buried; found by drilling 

and geophysics.

9. Sierra Madera, Tex. 30°36’ 102°55’ 8 100 million years

10. Redwing Creek, N. Dak. 47°36’ 103°33’ 6 200 million years Shock features identified.

11. Wells Creek, Tenn. 36°23’ 87°40’ 9 200±100 million years

12. Decaturville, Mo. 37°54’ 92°43’ 3.5 less than 300 million years

Shock features identified

13. Kentland, Ind. 40°45’ 87°24’ 8 300 million years

14. Middlesboro, Ky. 36°37’ 83°44’ 3.5 300 million years

15. Serpent Mound, Ohio 39°02’ 83°24’ 4 300 million years

16. Crooked Creek, Mo. 37°50’ 91°23’ 3.5 320±80 million years

17. Flynn Creek, Tenn. 36°17’ 85°40’ 2.4 360±20 million years

18. Avak, Alaska 71°25’ 156°63’ 7.5 greater than 95 million 

years

Buried, found by drilling 

and geophysics.
PACIFIC OCEAN

ATLANTIC OCEAN

NORTH

W

Impact Structures 43
44 About the Geologic Map in the National Atlas of the United States of America

Selected References

Bally, A.W., and Palmer, A.R., eds., 1989, The geology of North America—An 

overview, v. A of The geology of North America: Boulder, Colo., Geological 

Society of America, 619 p.

Beikman, H.M., 1980, Geologic map of Alaska: U.S. Geological Survey Special 

Map, scale 1:2,500,000.

Buchfiel, B.C., Lipman, P.W., and Zoback, M.L., eds., 1992, The Cordilleran 

orogen, conterminous U.S., v. G–3 of The geology of North America: Boulder, 

Colo., Geological Society of America, 724 p.

Decker, R.W., Wright, T.J., and Stauffer, P.H., eds., 1987, Volcanism in Hawaii, 

v.1: U.S. Geological Survey Professional Paper 1350, 839 p.

Dengo, G., and Case, J.E., 1990, The Caribbean region, v. H of The geology of 

North America: Boulder, Colo., Geological Society of America, 528 p..

Hatcher, R.D., Jr., Viele, G.W., and Thomas, W.A., eds., 1989, Appalachian-Ouachita orogen in the United States, v. F–2 of The geology of North America: 

Boulder, Colo., Geological Society of America, 767 p.

Hazlett, R.W., and Hyndman, D.W., 1996, Roadside geology of Hawaii: Missoula, 

Mont., Mountain Press Publishing Co., 307 p.

King, P.B., and Beikman, H.M., 1974, Geologic map of the United States (exclusive of Alaska and Hawaii): U.S. Geological Survey Special Map, scale 

1:2,500,000.

Muehlberger, W.R., compiler, 1992, Tectonic map of North America: Tulsa, 

Okla., American Association of Petroleum Geologists, scale 1:5,000,000.

Palmer, A.R., and Geissman, John, compilers, 1999, Geologic time scale: Boulder, Colo., The Geological Society of America, Product Code CTS004.

Plafker, George, and Berg, H.C., eds., 1993, The geology of Alaska, v. G–1 of

The geology of North America: Boulder, Colo., Geological Society of America, 1,055 p.

Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., and 

Sims, P.K., eds., 1993, Precambrian, conterminous U.S., v. C–2 of The geology of North America: Boulder, Colo., Geological Society of America, 664 p.

Reed, J.C., Jr., Wheeler, J.O., and Tucholke, B.E., compilers, 2005, Geologic 

map of North America: Boulder, Colo., Geological Society of America, scale 

1:5,000,000.

Salvador, A., ed., 1992, The Gulf of Mexico Basin, v. J of The geology of North 

America: Boulder, Colo., Geological Society of America, 557 p.

Sheridan, R.E., and Grow, J.A., eds., 1988, The Atlantic continental margin, U.S., 

v. I–2 of The geology of North America: Boulder, Colo., Geological Society of 

America, 632 p.

Silver, L.T., and Schultz, P.H., 1982, Geological implications of impacts of large 

asteroids and comets on the Earth: Geological Society of America Special 

Paper 190, 548 p.

Sloss, L.L., ed., 1986, Sedimentary cover—North American craton, v. D–2 of The 

geology of North America: Boulder, Colo., Geological Society of America, 

506 p.
accreted terrane A package of rocks that has arrived in its present position by 

plate-tectonic movements, either by subduction of intervening oceanic crust 

or by lateral movement along the margin of the continent.

alluvium A surficial deposit of silt, clay, sand, or gravel laid down by running 

water.

andesite A gray volcanic rock composed of subequal amounts of calcium and 

sodium-rich feldspar and dark iron- and magnesium-rich minerals, and 

minor amounts of quartz.

anorthosite A plutonic rock composed almost entirely of calcium-rich feldspar.

anticline A fold in which the rock layers have been bowed upward into an arch.

Archean In the stratigraphic time scale, the first eon of the Precambrian. It 

extended from about 3.8 billion to 2.5 billion years ago.

argillite A hard, massive sedimentary rock formed by compaction of clay and silt.

basalt A black or dark-gray volcanic rock composed of subequal amounts of 

calcium-rich feldspar and various dark iron- and magnesium-rich minerals 

and little or no quartz.

basement rocks A collective term for metamorphic and plutonic rocks that 

underlie sedimentary and volcanic rocks. In many cases it refers to 

Precambrian rocks that underlie Paleozoic and younger rocks in an area.

batholith A pluton that is exposed over an area of at least 40 square miles at the 

surface.

bedrock A general term for the rock, generally solid, that underlies soil or other 

unconsolidated material at the land surface.

blueschist A schistose metamorphic rock formed at very high pressure and 

moderate temperature that contains blue, sodium-rich minerals.

breccia A rock composed of a jumble of angular rock fragments cemented by 

finer grained materials. Breccia can be formed by sedimentary or volcanic 

processes or by faulting.

caldera A large, more or less circular depression formed by explosive volcanic 

eruptions.

Cenozoic The last eon of the stratigraphic time scale. It extends from about 65 

million years ago to the present.

chert An extremely hard sedimentary rock made up of tiny interlocking crystals 

of silica (SiO2). It may form by chemical precipitation of silica from sea 

water, by deposition by microscopic organisms, or by chemical replacement 

of limestone. It is also called flint.

collisional episode In plate tectonics, sometimes used to describe an event 

when two or more plates impinge on one another. The word “collision” is 

somewhat misleading because it implies a rapid encounter that involves 

momentum whereas plates are moving at only a few centimeters per year 

when they “collide.”

complex An assemblage or mixture of rocks of different origins, commonly 

having complicated and obscure relations to one another.

conglomerate A sedimentary rock that contains abundant pebbles, cobbles, or 

boulders of various older rocks.

continental crust The type of Earth’s crust that underlies continents. It 

is composed chiefly of granitic and intermediate plutonic rocks and 

metamorphic rocks. Continental crust is much thicker and lighter than 

oceanic crust, so it is rarely subducted when two tectonic plates collide.

convergent plate margin A boundary between tectonic plates along which one 

plate is being subducted beneath the other.

craton A large, stable area of continental crust, such as the interior of North 

America.

Cretaceous In the stratigraphic time scale, the last period of the Mesozoic Era. It 

lasted from about 144 to 65 million years ago.

delta An accumulation of sediment deposited where a stream or river enters a lake 

or the ocean. So named because most such accumulations have the shape of 

the Greek letter delta (∆).

depositional age The time at which a sedimentary or volcanic rock was formed.

dip The inclination of a plane measured perpendicular to a horizontal line on the 

plane.

docking In plate tectonics, the joining together of tectonic plates or fragments of 

plates.

dolomite A mineral composed of calcium-magnesium carbonate (CaMgCO3); also 

a sedimentary rock composed of the mineral dolomite.

downwarp A large, more or less circular depression in rock layers caused by 

subsidence.

eolian Pertaining to the wind. Commonly used to describe materials deposited by 

the wind, such as dune sand and loess.

evaporite A sedimentary rock composed of minerals deposited during evaporation 

of salty water, either sea water or lake water. The minerals include rock salt, 

gypsum, and various sulfate, borate, and nitrate minerals.

fault scarp An approximately linear break in the land surface caused by offset 

across a fault.

Glossary

Glossary 45
faulting The formation of approximately planar breaks in rocks along which rocks 

on one side are moved with respect to those on the other.

flysch A thick sequence of sedimentary rocks, chiefly shale, dark sandstone, and 

conglomerate deposited in a submarine basin in front of a rising mountain 

range.

folding The formation of approximately linear wrinkles in rock layers.

fore-arc basin An elongate basin that lies between the volcanic arc and the trench 

along the margin of a tectonic plate where a plate is being subducted beneath 

it at a convergent plate margin.

geologic age The time at which a rock formed or when some other geologic event 

occurred. Geologic ages are stated in years before the present (based on age 

determinations from decay of radioactive elements) or with reference to the 

stratigraphic time scale.

geophysical studies Studies of the physical properties of the Earth and its various 

layers. These include surveys of gravity, magnetism, and travel speeds of 

shock waves from earthquakes or explosions, all of which give clues to the 

density and composition of rocks in and beneath the crust.

gneiss A metamorphic rock characterized by layers or aligned streaks of mineral 

grains. Gneiss can be formed from sedimentary, volcanic, or plutonic rocks 

by intense metamorphism and deformation. Generally, most traces of 

features that allow identification of the rock from which it has been derived 

are obliterated during metamorphism.

granitic Said of light-colored plutonic rocks composed chiefly of quartz and 

feldspar and small amounts of mica, hornblende, and other minerals.

gravity anomaly The amount that the pull of gravity at the Earth’s surface differs 

from the value predicted by a mathematical model based on a uniform Earth. 

Positive gravity anomalies indicate bodies of heavier than average rocks at 

depth; negative anomalies indicate lighter than average rocks.

growth fault A fault in sedimentary rocks that forms by slow creep of wet 

sediments down a slope at the same time they are being deposited. Such 

faults are common in the Gulf Coastal Plain.

intermediate Said of medium- to dark-gray plutonic or volcanic rocks composed 

of roughly equal amounts of quartz, feldspar, mica, and hornblende.

Jurassic In the stratigraphic time scale, the first period of the Mesozoic Era. It 

lasted from about 206 to 144 million years ago.

Laramide Formed during, or pertaining to, the Laramide orogeny.

limestone A sedimentary rock composed of calcium carbonate (CaCO3) deposited 

by precipitation from sea or lake water or by the accumulation of shells and 

other parts of various aquatic organisms.

mafic Said of dark-colored plutonic or volcanic rocks composed chiefly of 

feldspar and dark minerals rich in iron and magnesium, such as hornblende, 

pyroxene, and olivine, and containing little or no quartz.

magnetic anomaly The amount by which the pull of the Earth’s magnetic field 

measured at the surface or from aircraft or satellites differs from the value 

predicted by a mathematical model based on a uniform Earth. Positive 

anomalies indicate magnetic rocks in the upper part of the Earth’s crust; 

negative anomalies indicate less magnetic rocks.

melange A mappable body of rock containing blocks of all sizes of a variety 

of rocks in a fine-grained matrix and characterized by lack of internal 

continuity. Melange is commonly formed in subduction zones where bodies 

of sedimentary rocks deposited along the leading edge of a plate are carried 

beneath the plate during subduction.

Mesozoic The era in the stratigraphic time scale that extended from about 248 to 

65 million years ago.

metamorphic rock Rocks formed by recrystallization of mineral grains in older 

sedimentary, volcanic, or plutonic rocks during metamorphism.

metamorphism Change in the chemical composition, physical properties, and 

component minerals in a rock in response to changes in temperature, 

pressure, and (or) chemical conditions.

migmatite A rock made up of a mixture of metamorphic and plutonic rock. 

Migmatite forms either by partial melting of metamorphic rock at high 

temperatures or by injection of igneous material into the metamorphic rock.

Mississippian In the stratigraphic time scale, the period in the Paleozoic Era that 

lasted from about 354 to 323 million years ago.

moraine A mound, ridge, or other accumulation of glacial debris (till).

mudflow deposits A deposit laid down by flows of fine-grained, water-saturated 

material commonly carrying angular pieces of rock. When consolidated, 

these materials form one type of breccia.

Neogene In the stratigraphic time scale, the last major subdivision of the Tertiary 

Period. It lasted from about 24 to 1.8 million years ago. It includes the 

Miocene and Pliocene Epochs.

novaculite A very hard, even-textured, light-colored type of chert, commonly 

used as whetstone.

oceanic crust The type of Earth’s crust that typically underlies ocean basins. 

Oceanic crust is made up largely of rocks having the composition of basalt 

and is much thinner and denser than continental crust, so that it is commonly 

subducted when two plates “collide.”

oceanic plateau A region of abnormally thick oceanic crust formed by eruption of 

large volumes of basalt on older oceanic crust.

ophiolite Pieces of oceanic crust that have been carried up onto the leading edge 

of a plate during closure of an ocean. Ophiolites typically consist of basalt 

in pillowlike masses formed during eruption in deep water; closely spaced, 

nearly vertical sheets of basalt; and gabbro (a dark plutonic rock having the 

composition of basalt but coarser grained).

46 About the Geologic Map in the National Atlas of the United States of America
orogeny An episode of uplift and mountain building.

Paleogene The first major subdivision of the Tertiary Period in the stratigraphic 

time scale. It lasted from about 65 to 24 million years ago. It includes the 

Paleocene, Eocene, and Oligocene Epochs.

Paleozoic The era in the stratigraphic time scale that lasted from about 540 to 248 

million years ago.

paleomagnetic measurement Measurement of the alignment of magnetic 

minerals in a rock. The magnetic grains become aligned with the magnetic 

field that existed at the time a sedimentary rock was deposited or a volcanic 

rock cooled. Such measurements allow us to estimate the latitude at which 

the rock formed and the amount it has since been rotated.

Pennsylvanian In the stratigraphic time scale, the period in the Paleozoic Era that 

lasted from about 323 to 290 million years ago.

Permian In the stratigraphic time scale, the last period in the Paleozoic Era. It 

lasted from about 290 to 248 million years ago.

phyllite A fine-grained, lustrous, mica-rich metamorphic rock formed from 

siltstone or shale at relatively low temperature and pressure. Schist is 

formed from the same rocks but at higher temperature and pressure.

plate tectonics The theory accepted by most earth scientists that the brittle outer 

parts of the Earth’s crust and mantle are divided into a number of moving, 

semirigid plates and that the interaction of these plates drives many geologic 

phenomena, including mountain building, volcanoes, and earthquakes.

pluton A sizable body of generally coarse-grained igneous rock that has 

crystallized at depth below the Earth’s surface

plutonic rocks Rocks formed by cooling and crystallization of molten igneous 

material at depth below the Earth’s surface. Plutonic rocks are generally 

more coarsely crystalline than volcanic rocks.

Precambrian In the stratigraphic time scale, the interval of geologic time prior 

to the beginning of the Paleozoic Era. It lasted from the origin of the Earth 

(about 4.5 billion years ago) to about 540 million years ago.

Proterozoic In the stratigraphic time scale, the last eon in the Precambrian. It 

lasted from 2.5 billion years ago to the beginning of the Cambrian (the first 

period in the Paleozoic era) about 540 million years ago.

pyroclastic rock A rock formed of grains or blocks of material ejected from a 

volcanic vent.

Quaternary In the stratigraphic time scale, the last period of the Cenozoic Era. It 

began about 1.8 million years ago and extends to the present.

residual soils Soil produced by weathering of bedrock in place without 

significant movement of material.

rift An approximately linear zone of faulting formed where a tectonic plate is being 

pulled apart. In continental crust a rift is generally marked by a linear valley 

filled with young sediments and flanked by older rocks.

rhyolite A light-colored volcanic rock composed of sodium- and calcium-rich 

feldspar, quartz, and small amounts of dark minerals. Most rhyolite contains 

significant amounts of volcanic glass.

sandstone A sedimentary rock formed by the cementation of grains of sand. The 

cementing material is commonly silica, iron oxide, or calcium carbonate.

sea mount An undersea volcano.

sedimentary rocks Rocks composed of material derived from weathering or 

disintegration of older rocks that was transported and deposited by water, 

air, or ice, or of material that accumulates by other natural agents, such as 

chemical precipitation from solution or secretion by organisms. Sedimentary 

rocks are generally deposited in more or less continuous layers.

sequence An informal name for a group of related sedimentary rock layers.

shale A fine-grained sedimentary rock formed by consolidation of clay, silt, or 

mud. It generally has a finely laminated structure that causes it to break into 

thin chips or plates.

siltstone A fine-grained sedimentary rock formed by consolidation of silt. Siltstone 

generally lacks the laminated structure of shale and tends to break into small 

angular blocks.

strata A collective term for layered sedimentary rocks.

stratigraphic time scale A time scale established from the study of sedimentary 

rock layers and based on the physical relations of the layers to one 

another and on the fossils they contain. Stratigraphic ages of igneous and 

metamorphic rocks are determined by their relations to sedimentary rocks 

that have been placed in the stratigraphic time scale <http://ngmdb.usgs.

gov/Geolex/geolex_home.html>. Note that when we describe the age of a 

sequence of sedimentary layers we describe them as “lower,” “middle,” or 

“upper,” because that is the order in which they were deposited, but when we 

describe intervals of time we describe them as “early,” “middle,” or “late.” 

Thus, Upper Cambrian rocks were deposited during Late Cambrian time.

strike-slip fault A fault along which the movement of one side with respect to the 

other is horizontal. Strike-slip faults are described as “right-lateral” or “leftlateral” depending on whether the side opposite you moves to your right or 

your left as you stand facing the fault.

subduction The process by which one tectonic plate sinks beneath another plate as 

the two approach one another.

subduction zone The zone along a plate boundary where subduction is taking 

place.

Glossary 47
surficial deposits Unconsolidated materials that lie at or near the land surface. 

The term includes soil, stream, glacial, and lake deposits, and windblown 

materials.

suture A line or zone along which two tectonic plates have joined together.

syncline A fold in which rock layers have been bent downward into a trough.

tectonic plate One of the large segments of the Earth’s crust that is moving 

in relation to other parts of the crust. The plates may be made up of 

continental crust or oceanic crust, or partly of each.

terrane A fault-bounded body of rock of regional extent that has a geologic 

history distinctly different from the adjoining rocks. Not to be confused 

with “terrain,” which refers to the physical features of the land surface.

Tertiary In the stratigraphic time scale, the first period of the Cenozoic Era. It 

lasted from about 65 to 1.8 million years ago.

thrust fault A flat or gently inclined fault along which the rocks above have 

moved relative to the rocks below. Thrust faults are generally formed by 

compression.

till Material deposited by or beneath glacial ice. It generally consists of 

unstratified clay, silt, and sand studded with pebbles, cobbles, and boulders 

having a wide variety of sizes and shapes.

Triassic In the stratigraphic time scale, the first period in the Mesozoic Era. It 

lasted from about 248 to 206 million years ago.

tuff A consolidated deposit of volcanic ash and larger fragments explosively 

ejected from a volcano.

volcanic arc An arcuate chain of volcanoes formed along the leading edge of 

a tectonic plate as another plate is subducted beneath it. Where the plate 

collision takes place beneath the ocean the volcanoes commonly form a 

chain of islands commonly described as an island arc. The Aleutian Islands 

are an example.

volcanic ash Fine material explosively ejected during a volcanic eruption. It is 

generally composed of tiny shards of volcanic glass.

volcanic breccia A rock composed of relatively large, angular fragments of 

volcanic rock embedded in finer grained volcanic material. Such breccias 

are formed either from materials ejected directly from a volcanic vent, or 

from mudflows generated by heavy rains, or by meltwater from snowfields 

or glaciers suddenly heated by volcanic eruptions.

volcanic rocks Finely crystalline or glassy igneous rocks that form by volcanic 

action at or near the surface. They are composed of material that has either 

been ejected explosively or that flowed out onto the surface as lava.

welded tuff A tuff deposit composed chiefly of fragments of volcanic glass that 

were erupted at such a high temperature that they welded together when 

they came to rest to form a dense volcanic rock. Most welded tuff has the 

composition of rhyolite or closely related rocks.

48 About the Geologic Map in the National Atlas of the United States of America
Geologic Provinces 49

Manuscript approved for publication August 23, 2006

Prepared by the U.S. Geological Survey Publishing Service Center, Denver, Colorado

USGS Publishing Staff

Edited by Diane Lane and Jon W. Raese

Designed and prepared by Amber Hess

Technical Reviews

William R. Page, U.S. Geological Survey

Dwight L. Schmidt, U.S. Geological Survey

Jeremiah B. Workman, U.S. Geological Survey

John C. Reed and Charles A. Bush

Earth Surface Processes Team

U.S. Geological Survey

Bldg. 25, Room 1512, MS 980

Federal Center

Lakewood, CO 80225

Telephone: 303-236-4723

Website: http://nationalatlas.gov/

For more information concerning the research in this report, contact:
Reed and Bush—About the Geologic Map in the National Atlas of the United States of America—Circular 1300

For more information concerning the research in this report, contact:

Telephone: 303-236-4723

Website: http://nationalatlas.gov/

 Earth Surface Processes Team

 U.S. Geological Survey

 Bldg. 25, Room 1512, MS 980

 Federal Center

 Lakewood, CO 80225

John C. Reed and Charles A. Bush