Geologic Maps of the 50 United States

Volcanoes, Geysers and Water Falls Engraving
Volcanoes, Geysers and Water Falls Engraving. bauhaus1000 / Getty Images

Below you'll find geologic maps for each state, ordered alphabetically, plus details on each state's unique geologic structure.

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Alabama Geologic Map

Alabama's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy). ​

Alabama rises from the coastline, its gently dipping rock layers exposing deeper and older formations in majestic order as one moves north.

The yellow and gold stripes nearest the Gulf of Mexico coast represent rocks of Cenozoic age, younger than 65 million years. The southernmost green stripe labeled uK4 marks the Selma Group. The rocks between it and the dark green stripe of the Tuscaloosa Group, labeled uK1, all date from Late Cretaceous time, starting at about 95 million years ago.

The more resistant layers in this sequence crop out as long low ridges, steep on the north and gentle on the south, called cuestas. This part of Alabama formed in the shallow waters that have covered most of the central continent throughout geologic history.

The Tuscaloosa Group gives way to the compressed, folded rocks of the southernmost Appalachian Mountains to the northeast and the flat-lying limestones of the interior basins to the north. These different geologic elements give rise to a great variety of landscapes and plant communities, in what outsiders might consider a flat and uninteresting region.

The Geological Survey of Alabama has much more information on the state's rocks, mineral resources, and geologic hazards.

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Alaska Geologic Map

Alaska's rocks
Geologic Maps of the 50 United States. Map courtesy Alaska Department of Natural Resources (fair use policy)

Alaska is a colossal state that contains some of the world's most notable geologic features. Click the image for a larger version.

The long Aleutian Island chain sweeping to the west (cut off in this miniature version) is a volcanic arc that is fed magma from the subduction of the Pacific plate beneath the North American plate. 

Much of the rest of the state is built of chunks of continental crust carried there from the south, then plastered there where they compress the land into the highest mountains in North America. Two ranges right next to each other can have rocks that are totally different, formed thousands of kilometers away and millions of years apart. The ranges of Alaska are all part of a great mountain chain, or cordillera, that stretches from the tip of South America all the way up the west coast, then over into eastern Russia. The mountains, the glaciers upon them and the wildlife they support are enormous scenic resources; the minerals, metals and petroleum resources of Alaska are equally significant.

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Arizona Geologic Map

Arizona's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Arizona is divided roughly equally between the Colorado Plateau in the north and the Basin and Range province in the south. (more below)

The Colorado Plateau displays great expanses of flat-lying bedrock dating from the late Paleozoic Era through the Late Cretaceous Epoch. (Specifically, dark blue is late Paleozoic, lighter blue is Permian, and the greens signify Triassic, Jurassic and Cretaceous—see the time scale.) A great winding gash in the western part of the plateau is where the Grand Canyon exposes deeper rocks from the Precambrian. Scientists are far from a settled theory of the Grand Canyon. The edge of the Colorado Plateau, marked by the ribbon of darkest blue running from northwest to southeast, is the Mogollon Rim.

The Basin and Range is a wide zone where plate-tectonic motions have stretched apart the crust as much as 50 percent in the last 15 million years or so. The uppermost, brittle rocks have cracked like breadcrust into long blocks that have foundered and tilted upon the softer crust beneath. These ranges shed sediment into the basins between them, marked in light gray. At the same time, magma burst up from below in widespread eruptions, leaving lavas marked in red and orange. The yellow areas are continental sedimentary rocks of the same age.

The dark gray areas are Proterozoic rocks, some 2 billion years old, that mark the eastern part of Mojavia, a large block of continental crust that was attached to North America and broken off during the breakup of the supercontinent Rodinia, about a billion years ago. Mojavia may have been part of Antarctica or part of Australia—those are the two leading theories, but there are other proposals as well. Arizona will provide rocks and problems for many generations of geologists to come.

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Arkansas Geologic Map

Arkansas's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Arkansas encompasses a great variety of geology within its borders, even a public diamond mine.

Arkansas stretches from the Mississippi River on its eastern edge, where historic movement of the riverbed has left behind the original state borderlines, to the more settled Paleozoic rocks of the Ouachita Mountains (the broad tan and gray lobes) on the west and the Boston Mountains to their north.

The striking diagonal boundary across the heart of the state is the edge of the Mississippi Embayment, a wide trough in the North American craton where once, long ago, the continent tried to split. The crack has remained seismically active ever since. Just north of the state line along the Mississippi River is where the great New Madrid earthquakes of 1811–12 occurred. The gray streaks crossing the embayment represent the recent sediments of (from left to right) the Red, Ouachita, Saline, Arkansas, and White rivers.

The Ouachita Mountains are actually part of the same foldbelt as the Appalachian range, separated from it by the Mississippi Embayment. Like the Appalachians, these rocks produce coal and natural gas as well as various metals. The southwestern corner of the state yields petroleum from its early Cenozoic strata. And just on the border of the embayment, a rare body of lamproite (the largest of the red spots) is the only diamond-producing locality in the United States, open for public digging as Crater of Diamonds State Park.

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California Geologic Map

California's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from US Geological Survey Map I-512 ( fair use policy).

California offers a lifetime's worth of geologic sights and localities; the Sierra Nevada and San Andreas fault are the barest beginning. 

This is a reproduction a U.S. Geological Survey map published in 1966. Our ideas of geology have come a long way since then, but the rocks are still the same.

Between the red swath signifying the Sierra Nevada granites and the western greenish-yellow band of folded and faulted Coast Ranges lies the great sedimentary trough of the Central Valley. Elsewhere this simplicity is broken: in the north, the blue-and-red Klamath Mountains are torn from the Sierra and moved westward while the dotted pink is where young, widespread lavas of the Cascade Range bury all older rocks. In the south, the crust is fractured on all scales as the continent is being actively reassembled; deep-seated granites marked by red, rising as their cover erodes away, are surrounded by vast aprons of recent sediment in the deserts and rangelands from the Sierra to the Mexican border. Large islands off the southern coast rise from sunken crustal fragments, part of the same vigorous tectonic setting.

Volcanoes, many of them recently active, dot California from the northeast corner down the eastern side of the Sierra to its southern end. Earthquakes affect the whole state, but especially in the faulted zone along the coast, and south and east of the Sierra. Mineral resources of every kind occur in California, as well as geological attractions.

The California Geological Survey has a PDF of the latest state geologic map.

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Colorado Geologic Map

Colorado's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Colorado has parts of the Great Plains, the Colorado Plateau and the Rocky Mountains within its four border lines. (more below)

The Great Plains lie on the east, the Colorado Plateau on the west, the San Juan Volcanic Field with its circular calderas in the south-center marking the northern end of the Rio Grande Rift, and running in a wide band down the middle is the Rocky Mountains. This complex zone of multiple folding and uplift exposes rocks of the ancient North American craton while cradling Cenozoic lake beds full of delicate fossil fish, plants, and insects.

Once a mining superpower, Colorado is now a major destination for tourism and recreation as well as agriculture. It's also a powerful draw for geologists of all kinds, who gather by the thousands in Denver every three years for the Geological Society of America's national meeting.

I have also prepared a scan of a very large and much more detailed geologic map of Colorado compiled in 1979 by Ogden Tweto of the U.S. Geological Survey, a classic of geologic mapmaking. The paper copy measures about 150 by 200 centimeters and is at 1:500,000 scale. Unfortunately it is so detailed that it's of little use at anything less than full size, in which all place names and formation labels are legible. 

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Connecticut Geologic Map

Connecticut's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Rocks of many ages and types crop out in Connecticut, evidence of a long and eventful history. 

Connecticut's rocks divide into three belts. On the west are the state's highest hills, bearing rocks largely dating from the Taconic orogeny, when an ancient island arc collided with the North American plate in Ordovician time about 450 million years ago. On the east are the deeply eroded roots of another island arc that arrived some 50 million years later in the Acadian orogeny, of Devonian age. In the middle is a large trough of volcanic rocks from Triassic times (about 200 million years ago), an abortive opening related to the birth of the Atlantic Ocean. Their dinosaur tracks are preserved in a state park.

 

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Delaware Geologic Map

Delaware's rocks
Geologic Maps of the 50 United States Map courtesy Delaware Geological Survey ( fair use policy).

A very small and flat-lying state, Delaware still packs something like a billion years of time in its rocks.

Most of Delaware's rocks are not really rocks, but sediments—loose and poorly consolidated materials that go all the way back to the Cretaceous. Only in the extreme north are there ancient marbles, gneisses, and schists belonging to the Piedmont province of the Appalachian Mountains, but even so the highest point in the state is barely a hundred meters above sea level.

Delaware's history for the last 100 million years or so has consisted of being gently bathed by the sea as it rose and fell over the eons, thin layers of sand and silt being draped over it like sheets on a sleeping child. The sediments have never had a reason (like deep burial or subterranean heat) to become rocks. But from such subtle records geologists can reconstruct how the slight rises and falls of land and sea reflect events on faraway crustal plates and deep in the mantle below. More active regions erase this kind of data.

Still, it must be admitted that the map is not full of detail. There's room on it to depict several of the state's important aquifers, or groundwater zones. Hard-rock geologists may turn up their noses and go swing their hammers in the far northern rises, but ordinary people and cities base their existence on their water supply, and Delaware's Geological Survey rightly focuses a great deal of attention on aquifers.

 

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Florida Geologic Map

Florida's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Florida is a platform of young rocks draped over a hidden ancient continental core. 

Florida was once in the heart of the tectonic action, nestled between North and South America and Africa when all three continents were part of Pangea. When the supercontinent broke up in late Triassic time (about 200 million years ago), the part with Florida on it slowly subsided into a low continental platform. The ancient rocks from this time are now deep underground and accessible only by drilling.

Since then Florida has had a long and placid history, most of it under warm waters where limestone deposits built up over millions of years. Almost every geologic unit on this map is very fine grained shale, mudstone, and limestone, but there are some sandy layers, especially in the north, and a couple of phosphate layers that are extensively mined by the chemical and fertilizer industries. No surface rock in Florida is older than Eocene, about 40 million years old.

In more recent times, Florida has been covered and uncovered many times by the sea as the ice-age polar caps released and withdrew water from the ocean. Each time, the waves carried sediments over the peninsula.

Florida is famous for sinkholes and caves that have formed in the limestone, and of course for its fine beaches and coral reefs. See a gallery of Florida geological attractions.

This map gives only a general impression of Florida's rocks, which are very poorly exposed and difficult to map. A recent map from the Florida Department of Environmental Protection is reproduced here in an 800x800 version (330KB) and a 1300x1300 version (500 KB). It shows many more rock units and gives a good idea of what you might find in a big building excavation or sinkhole. The largest versions of this map, which reach 5000 pixels, are available from the U.S. Geological Survey and the state of Florida.

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Georgia Geologic Map

Georgia's rocks
Geologic Maps of the 50 United States Base data from U.S. Geological Survey/Georgia Department of Natural Resources ( fair use policy).

Georgia extends from the Appalachian Mountains on the north and west to the Atlantic Coastal Plain and is wealthy in mineral resources. (more below)

In northern Georgia, the ancient folded rocks of the Blue Ridge, Piedmont, and Valley-and-Ridge provinces contain Georgia's coal, gold, and ore resources. (Georgia had one of America's first gold rushes in 1828.) These give way in the middle of the state to the flat-lying sediments of Cretaceous and younger age. Here are the great kaolin clay beds that support the state's largest mining industry. See a gallery of Georgia's geological attractions.

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Hawaii Geologic Map

Hawaii's rocks
Geologic Maps of the 50 United States Based on U.S. Geological Survey Miscellaneous Investigations Map I-1091-G ( fair use policy).

Hawaii is entirely built of young volcanoes, so this geologic map doesn't have much variety in color. But it's a world-class geologic attraction. 

Basically, all of the islands of the Hawaiian chain are less than 10 million years old, with the Big Island the youngest and the oldest being Nihoa (which is part of the islands but not part of the state), off the map to the northwest. The map color refers to the composition of the lava, not its age. The magenta and blue colors represent basalt and the brown and green (just a smidgen on Maui) are rocks higher in silica.

All of these islands are the product of a single source of hot material rising from the mantle—a hotspot. Whether that hotspot is a deep-seated plume of mantle material or a slow-growing crack in the Pacific plate is still being discussed. To the southeast of Hawaii island is a seamount named Loihi. Over the next hundred thousand years or so, it will emerge as Hawaii's newest island. The voluminous basaltic lavas build very large shield volcanoes with gently sloping flanks.

Most of the islands have irregular shapes, not like the round volcanoes you find on continents. This is because their sides tend to collapse in gigantic landslides, leaving chunks the size of cities scattered around the deep seafloor near Hawaii. If such a landslide happened today it would be devastating to the islands and, thanks to tsunamis, the entire coast of the Pacific Ocean.

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Idaho Geologic Map

Idaho's rocks
Geologic Maps of the 50 United States Modified from Idaho Geological Survey image. ( fair use policy).

Idaho is an igneous state, built from many different episodes of volcanism and intrusion, plus vigorous uplift and erosion by ice and water.

The two largest features on this simplified geologic map are the great Idaho batholith (dark pink), a huge emplacement of plutonic rock of Mesozoic age, and the swath of lava beds along the west and across the south that marks the path of the Yellowstone hotspot.

The hotspot first arose farther west, in Washington and Oregon, during the Miocene Epoch some 20 million years ago. The first thing it did was to produce a gigantic volume of highly fluid lava, the Columbia River basalt, some of which is present in western Idaho (blue). As time went on the hotspot moved east, pouring more lava upon the Snake River plain (yellow), and now lies just over the eastern border in Wyoming beneath Yellowstone National Park.

To the south of the Snake River plain is part of the extensional Great Basin, broken like nearby Nevada into downdropped basins and tilted ranges. This region is also profusely volcanic (brown and dark gray).

The southwestern corner of Idaho is highly productive farmland where fine volcanic sediment, ground into dust by the Ice Age glaciers, was blown into Idaho by the wind. The resulting thick beds of loess support deep and fertile soils.

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Illinois Geologic Map

Illinois's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Illinois has almost no bedrock exposed at the surface, only a little at its south end, northwest corner, and on the west by the Mississippi River. 

Like the rest of the upper Midwest states, Illinois is covered with glacial deposits from the Pleistocene ice ages. (For that aspect of the state's geology, see the Quaternary map of Illinois page on this site.) The thick green lines represent the southern limits of continental glaciation during the most recent ice age episodes.

Beneath that recent veneer, Illinois is dominated by limestone and shale, deposited in shallow-water and coastal environments during the middle of the Paleozoic Era. The whole southern end of the state is a structural basin, the Illinois Basin, in which the youngest rocks, of Pennsylvanian age (gray), occupy the center and successively older beds around the rim dip downward beneath them; these represent Mississippian (blue) and Devonian (blue-gray). In the northern part of Illinois these rocks are eroded away to expose older deposits of Silurian (dove-gray) and Ordovician (salmon) age.

The bedrock of Illinois is richly fossiliferous. Besides the abundant trilobites found throughout the state, there are many other classic Paleozoic life forms represented, which you can see on the fossils page at the Illinois State Geological Survey site. See a gallery of Illinois geological attractions.

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Indiana Geologic Map

Indiana's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Indiana's bedrock, mostly hidden, is a grand procession through Paleozoic time raised up by two arches between two basins. 

Bedrock in Indiana is at or near the surface only in the central south end of the state. Elsewhere it's buried by much younger sediment carried down by the glaciers during the ice ages. The thick green lines show the southern limits of two of those glaciations.

This map shows the sedimentary rocks, all of Paleozoic age, that lie between the glacial deposits and the extremely old (Precambrian) basement rocks making up the heart of the North American continent. They are mostly known from boreholes, mines and excavations rather than outcrops.

The Paleozoic rocks are draped over four underlying tectonic structures: the Illinois Basin to the southwest, the Michigan Basin to the northeast, and an arch running northwest to southeast that is called the Kankakee Arch on the north and the Cincinnati Arch on the south. The arches have lifted the layer-cake of rocks so that the younger beds have eroded away to reveal the older rocks beneath: Ordovician (about 440 million years old) in the Cincinnati Arch and Silurian, not quite so old, in the Kankakee Arch. The two basins preserve rocks as young as Mississippian in the Michigan Basin and Pennsylvanian, youngest of all at about 290 million years, in the Illinois Basin. All of these rocks represent shallow seas and, in the youngest rocks, coal swamps.

Indiana produces coal, petroleum, gypsum and huge amounts of stone. Indiana limestone is widely used in buildings, for instance in Washington DC's landmarks. Its limestone is also used in cement production and its dolostone (dolomite rock) for crushed stone. See a gallery of Indiana geological attractions.

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Iowa Geologic Map

Iowa's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Iowa's gentle landscape and deep soils hide almost all of its bedrock, but drillholes and excavations will reveal rocks like these.

Only in Iowa's far northeast, in the "Paleozoic Plateau" along the Mississippi River, do you find bedrock and fossils and the other delights of the eastern and western states. There's also a tiny bit of ancient Precambrian quartzite in the extreme northwest. For the rest of the state, this map has been constructed from outcrops along riverbanks and many boreholes.

Iowa's bedrock ranges in age from Cambrian (tan) in the northeast corner through Ordovician (peach), Silurian (lilac), Devonian (blue-gray), Mississippian (light blue) and Pennsylvanian (gray), a period of some 250 million years. Much younger rocks of Cretaceous age (green) date from the days when a wide seaway stretched from here into Colorado.

Iowa is solidly in the midst of the continental platform, where shallow seas and gentle floodplains usually lie, laying down limestone and shale. Today's conditions are definitely an exception, thanks to all the water drawn out of the sea to build the polar ice caps. But for many millions of years, Iowa looked much like Louisiana or Florida does today.

One notable interruption in that peaceable history occurred about 74 million years ago when a large comet or asteroid struck, leaving behind a 35-kilometer feature in Calhoun and Pocahontas counties called the Manson Impact Structure. It's invisible at the surface—only gravity surveys and subsurface drilling have confirmed its presence. For a while, the Manson impact was a candidate for the event that ended the Cretaceous Period, but now we believe that the Yucatan crater is the real culprit.

The wide green line marks the southern limit of continental glaciation during the late Pleistocene. The map of surface deposits in Iowa shows a far different picture of this state.

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Kansas Geologic Map

Kansas's rocks
Geologic Maps of the 50 United States Image courtesy Kansas Geological Survey.

Kansas is largely flat, but it straddles a wide variety of geology.

In The Wizard of Oz, L. Frank Baum chose Kansas as the symbol of dry, flat dreariness (except for the tornado of course). But dry and flat are only part of this quintessential Great Plains state. River beds, forested plateaus, coal country, cactus-covered buttes, and stony glacial moraines can also be found around Kansas.

Kansas bedrock is old in the east (blue and purple) and young in the west (green and gold), with a long gap in age between them. The eastern section is late Paleozoic, beginning with a small portion of the Ozark Plateau where rocks date from Mississippian times, about 345 million years old. Rocks of Pennsylvanian (purple) and Permian (light blue) age overlie them, reaching to about 260 million years ago. They are a thick set of limestones, shales and sandstones typical of Paleozoic sections all across the middle of North America, with beds of rock salt as well.

The western section begins with Cretaceous rocks (green), some 140 to 80 million years old. They consist of sandstone, limestone and chalk. Younger rocks of Tertiary age (red-brown) represent a huge blanket of coarse sediment washing down from the rising Rocky Mountains, punctuated by beds of widespread volcanic ash. This wedge of sedimentary rocks was subsequently eroded in the last few million years; these sediments are shown in yellow. The light tan areas represent large fields of sand dunes that are grass-covered and inactive today. In the northeast, continental glaciers left behind thick deposits of gravel and sediment that they carried down from the north; the dashed line represent the glacier's limit.

Every part of Kansas is full of fossils. It's a great place to learn geology. The GeoKansas site of the Kansas Geological Survey has excellent resources for more detail, photos and destination notes.

I have made a version of this map (1200x1250 pixels, 360 KB) that includes the key to the rock units and a profile across the state.

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Kentucky Geologic Map

Kentucky's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Kentucky extends from the inland side of the Appalachian Mountains on the east to the Mississippi River bed on the west.

Kentucky's coverage of geologic time is spotty, having gaps in the Permian, Triassic, and Jurassic periods, and no rocks older than Ordovician (dark rose) are exposed anywhere in the state. Its rocks are mostly sedimentary, laid down in warm, shallow seas that have covered the central North American plate throughout most of its history.

Kentucky's oldest rocks crop out in a wide, gentle uplift in the north called the Jessamine Dome, a particularly high part of the Cincinnati Arch. Younger rocks, including thick deposits of coal laid down during later periods, have been eroded away, but Silurian and Devonian rocks (lilac) persist around the edges of the dome.

The coal measures of the American Midwest are so thick that the rocks known as the Carboniferous Series elsewhere in the world are subdivided by American geologists into the Mississippian (blue) and Pennsylvanian (dun and gray). In Kentucky, these coal-bearing rocks are thickest in the gentle downwarps of the Appalachian Basin on the east and the Illinois Basin on the west.

Younger sediments (yellow and green), starting from the late Cretaceous, occupy the Mississippi River valley and the banks of the Ohio River along the northwestern border. The west end of Kentucky is in the New Madrid seismic zone and has a significant earthquake hazard.

The Kentucky Geological Survey Web site has much more detail, including a simplified, clickable version of the state geologic map.

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Louisiana Geologic Map

Louisiana's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Louisiana is entirely made of Mississippi mud, and its surface rocks go back some 50 million years. (more below)

As the seas rose and fell over Louisiana, some version of the Mississippi River was carrying vast sediment loads here from the core of the North American continent and piling it on the rim of the Gulf of Mexico. Organic matter from highly productive marine waters has been deeply buried under the whole state and far offshore, turning into petroleum. During other dry periods, large beds of salt were laid down through evaporation. As a result of oil company exploration, Louisiana may be better known underground than on its surface, which is closely guarded by swamp vegetation, kudzu, and fire ants.

The oldest deposits in Louisiana date from the Eocene Epoch, marked by the darkest gold color. Narrow strips of younger rocks crop out along their southern edge, dating from Oligocene (light tan) and Miocene (dark tan) times. The speckled yellow pattern marks areas of Pliocene rocks of terrestrial origin, older versions of the wide Pleistocene terraces (lightest yellow) that cover southern Louisiana.

The older outcrops dip downward toward the sea owing to the steady subsidence of the land, and the coast is very young indeed. You can see how much the Holocene alluvium of the Mississippi River (gray) covers the state. The Holocene represents only the latest 10,000 years of Earth history, and in the 2 million years of Pleistocene time before that the river has wandered over the whole coastal region many times.

Human engineering has temporarily tamed the river, most of the time, and it's no longer dumping its sediment all over the place. As a result, coastal Louisiana is sinking out of sight, starved of fresh material. This is not permanent country.

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Maine Geologic Map

Maine's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Aside from its mountains, Maine reveals its enigmatic bedrock only along the rock-bound coast.

The bedrock of Maine is hard to find, except along the coast and in the mountains. Almost all of the state is covered with glacial deposits of recent age (here's the surface geologic map). And the rock beneath has been deeply buried and metamorphosed, bearing almost no details of the time when it first formed. Like a badly worn coin, only gross outlines are clear.

There are a few very old Precambrian rocks in Maine, but the state's history basically begins with activity in the Iapetus Ocean, where the Atlantic lies today, during the Late Proterozoic Era. Plate-tectonic activity similar to what's occurring in southern Alaska today pushed microplates onto the Maine shore, deforming the region into mountain ranges and spawning volcanic activity. This happened in three major pulses or orogenies during Cambrian to Devonian times. The two belts of brown and salmon, one at the extreme tip and the other starting at the northwest corner, represent rocks of the Penobscottian orogeny. Nearly all the rest represents the combined Taconic and Acadian orogenies. At the same time as these mountain-building episodes, bodies of granite and similar plutonic rocks rose from below, shown as light-colored blobs with random patterns.

The Acadian orogeny, in Devonian time, marks the closing of the Iapetus Ocean as Europe/Africa collided with North America. The whole eastern American seaboard must have resembled today's Himalaya. Surface sediments from the Acadian event occur as the great fossil-bearing shales and limestones of upstate New York to the west. The 350 million years since then have mainly been a time of erosion.

Around 250 million years ago, the Atlantic Ocean opened up. Stretch marks from that event occur in Connecticut and New Jersey to the southwest. In Maine only more plutons remain from that time.

As the land of Maine eroded, the rocks beneath continued to rise in response. So today the bedrock of Maine represents conditions at great depths, up to 15 kilometers, and the state is noteworthy among collectors for its high-grade metamorphic minerals.

More details of Maine's geologic history can be found in this overview page by the Maine Geological Survey.

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Maryland Geologic Map

Maryland's rocks
Geologic Maps of the 50 United States Image courtesy Maryland Geological Survey ( fair use policy).

Maryland is a small state whose surprising variety of geology encompasses all the major geologic zones of the eastern United States. 

Maryland's territory stretches from the Atlantic coastal plain on the east, just recently emerged from the sea, to the Allegheny Plateau on the west, the far side of the Appalachian Mountains. In between, going west, are the Piedmont, Blue Ridge, Great Valley, and Valley and Ridge provinces, distinct geologic regions that extend from Alabama to Newfoundland. Parts of the British Isles have these same rocks, because before the Atlantic Ocean opened in the Triassic Period, it and North America were part of one continent.

Chesapeake Bay, the large arm of the sea in eastern Maryland, is a classic drowned river valley and one of the nation's preeminent wetlands. You can learn more detail about Maryland geology at the state geological survey site, where this map is presented in county-sized chunks at full fidelity.

This map was published by the Maryland Geological Survey in 1968.

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Massachusetts Geologic Map

Massachusetts' rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

The Massachusetts region has been ridden hard over the course of the ages, from continental collisions to glacial overrides. (

Massachusetts consists of several terranes, large packages of crust with the rocks that accompany them—that have been carried here from different places by the interactions of ancient continents.

The westernmost part is the least disturbed. It contains limestone and mudstone from the seas near the ancient Taconic mountain-building episode (orogeny), crumpled and uplifted by later events but not appreciably metamorphosed. Its eastern edge is a major fault called Cameron's Line.

The middle of the state is the Iapetus terrane, oceanic volcanic rocks that erupted during the opening of a pre-Atlantic ocean in the early Paleozoic. The rest, to the east of a line running from roughly the west corner of Rhode Island to the northeastern coast, is the Avalonian terrane. It is a former chunk of Gondwanaland. Both the Taconian and Iapetus terranes are shown with dotted patterns that signify significant "overprints" of later metamorphism.

Both terranes were sutured to North America during a collision with Baltica, which closed the Iapetus ocean during the Devonian. Large bodies of granite (random pattern) represent magmas that once fed great volcano chains. At that time Massachusetts probably resembled southern Europe, which is undergoing a similar collision with Africa. Today we are looking at rocks that were once deeply buried, and most traces of their original nature, including any fossils, have been wiped out by metamorphism.

During the Triassic the ocean we know today as the Atlantic opened up. One of the initial cracks ran through Massachusetts and Connecticut, filling with lava flows and redbeds (dark green). Dinosaur tracks occur in these rocks. Another Triassic rift zone is in New Jersey.

For more than 200 million years after that, little happened here. During the Pleistocene ice ages, the state was scrubbed by a continental ice sheet. The sand and gravel created and carried off by the glaciers formed Cap Cod and the islands Nantucket and Martha's Vineyard. See a gallery of Massachusetts geological attractions.

Many local geologic maps in Massachusetts are available for free download from the Office of the Massachusetts State Geologist.

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Michigan Geologic Map

Michigan's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Michigan's bedrock is not very widely exposed, so you should take this bedrock map with a grain of salt. (more below)

Much of Michigan is covered with glacial drift—ground-up Canadian rocks bulldozed onto Michigan and much of the rest of the northern United States by several Ice Age continental glaciers, like the ones that rest on Antarctica and Greenland today. Those glaciers also excavated and filled the Great Lakes that today make Michigan two peninsulas.

Beneath that blanket of sediment, the Lower Peninsula is a geologic basin, the Michigan Basin, that has been occupied by shallow seas for most of the last 500 million years as it slowly warped downward under the weight of its sediments. The central part filled in last, its shale and limestone dating from the Late Jurassic Period around 155 million years ago. Its outer rim exposes successively older rocks going back to the Cambrian (540 million years ago) and beyond on the Upper Peninsula.

The rest of the Upper Peninsula is a cratonic upland of very ancient rocks from as long ago as Archean times, nearly 3 billion years ago. These rocks include the iron formations that have supported the American steel industry for many decades and continue to be the nation's second-largest producer of iron ore. 

 

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Minnesota Geologic Map

A Map of Minnesota's geology and rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Minnesota is America's premier state for exposures of extremely old Precambrian rocks. 

North America's heart, between the Appalachians and the great western cordillera, is a great thickness of very old highly metamorphosed rock, called the craton. In most of this part of the United States the craton is hidden by a blanket of younger sedimentary rocks, accessible only by drilling. In Minnesota, as in much of neighboring Canada, that blanket is gone and the craton is considered exposed as part of the Canadian Shield. However, actual bedrock outcrops are few because Minnesota has a young veneer of ice-age sediment laid down by continental glaciers in Pleistocene times.

North of its waist, Minnesota is almost entirely cratonic rock of Precambrian age. The very oldest rocks are in the southwest (purple) and date back some 3.5 billion years. Next come the large Superior Province in the north (tan and red-brown), the Anamikie Group in the center (blue-gray), the Sioux Quartzite in the southwest (brown) and the Keweenawan Province, a rift zone, in the northeast (tan and green). The activities that built and arranged these rocks are ancient history indeed.

Lapping onto the edges of the shield on the northwest and southeast are sedimentary rocks of Cambrian (beige), Ordovician (salmon) and Devonian age (gray). A later rise of the sea left more sedimentary rocks of Cretaceous age (green) in the southwest. But the map also shows the traces of the underlying Precambrian units. Above all of this lie glacial deposits.

The Minnesota Geological Survey has many, many more-detailed geologic maps available in scans.

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Mississippi Geologic Map

A Map of Mississippi's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

, Before the state of Mississippi there was the Mississippi River, but before the river was a great geologic structure, the Mississippi Embayment. 

Geologically, the state of Mississippi is dominated by the Mississippi Embayment along its western sidenot the Mississippi River. This is a deep trough or thin spot in the North American continent where a new ocean tried to form once upon a time, cracking the crustal plate and leaving it weakened ever since. Such a structure is also called an aulacogen ("aw-LACK-o-gen"). The Mississippi River has run down the embayment ever since.

As the seas have risen and fallen over geologic time, the river and the sea have combined to fill the trough with sediment, and the trough has sagged under the weight. Thus the rocks that line the Mississipi Embayment are bent downward in its midsection and exposed along its edges, older the farther east you go.

In only two places are there deposits not related to the embayment: along the Gulf coast, where short-lived sandbars and lagoons are regularly swept away and sculpted by hurricanes, and in the extreme northeast where a tiny edge is exposed of the continental platform deposits that dominate the Midwest.

The most distinctive landforms in Mississippi arise along the stripes of rocks. Gently dipping strata that are harder than the rest are left by erosion as low, level ridges, broken off steeply on one face and ramping gently into the ground on the other. These are called cuestas.

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Missouri Geologic Map

Missouri's rocks
Geologic Maps of the 50 United States Image courtesy Missouri Department of Natural Resources ( fair use policy).

Missouri is a gentle state with a terrifying earthquake in its history. (more below)

Missouri contains the largest of the gentle arches in the American midcontinent—the Ozark Plateau. It has the the largest outcrop area of Ordovician-age rocks in the country (beige). Younger rocks of Mississippian and Pennsylvanian age (blue and light green) occur to the north and west. On a small dome at the east end of the plateau, rocks of Precambrian age are exposed in the St. Francois Mountains.

The southeast corner of the state lies in the Mississippi Embayment, an ancient zone of weakness in the North American plate where once a rift valley threatened to turn into a young ocean. Here, in the winter of 1811–12, a terrible series of earthquakes rolled through the thinly inhabited country around New Madrid County. The New Madrid quakes are thought to be the most severe seismic event in American history, and research into their cause and effects continues today.

Northern Missouri is carpeted with Ice Age deposits of Pleistocene age. These consist mostly of till, the mixed debris lifted and dropped by glaciers, and loess, thick deposits of windblown dust that are known around the world as excellent farming soils.

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Montana Geologic Map

Montana's rocks
Geologic Maps of the 50 United States Image courtesy Montana State University. Map by Robert L. Taylor, Joseph M. Ashley, R. A. Chadwick, S. G. Custer, D. R. Lageson, W. W. Locke, D. W. Mogk, and J. G. Schmitt. ( fair use policy).

Montana includes the high Northern Rockies, the gentle Great Plains and part of Yellowstone National Park.

Montana is an enormous state; luckily this map, produced by the Department of Earth Sciences at Montana State University from the official map of 1955, is simplified enough to be presentable on a monitor. And with the larger versions of this map you get Yellowstone National Park thrown in as a bonus, a unique area where an active hot spot is pushing fresh magma through a thick continental plate. Just to its north is the famous Stillwater Complex, a thick body of platinum-bearing plutonic rocks.

Other notable features in Montana are the glaciated country in the north, from Glacier International Park in the west to the windswept plains in the east, and the great Precambrian Belt complex in the Rockies.

 

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Nebraska Geologic Map

Nebraska's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Nebraska is old in the east and young in the west.

Along the eastern edge of Nebraska, defined by the Missouri River, is ancient sedimentary rock of Pennsylvanian (gray) and Permian (blue) age. The famous coals of Pennsylvanian rocks are almost absent here. Cretaceous rocks (green) occur mainly in the east, but also are exposed in the valleys of the Missouri and Niobrara rivers in the north, the White River in the extreme northwest and the Republican River in the south. Almost all of these are marine rocks, laid down in shallow seas.

The majority of the state is of Tertiary (Cenozoic) age and terrigenous origin. A few slivers of Oligocene rocks crop out in the west, as do larger areas of Miocene (pale tan), but most is of Pliocene age (yellow). The Oligocene and Miocene rocks are freshwater lake beds ranging from limestone to sandstone, the sediment derived from the rising Rockies to the west. They include large volcanic ash beds from eruptions in present-day Nevada and Idaho. The Pliocene rocks are sandy and limy deposits; the Sand Hills in the west-central part of the state derive from these.

The thick green lines in the east mark the western limit of the great Pleistocene glaciers. In these areas glacial till overlies the old rock: blue clay, then thick beds of loose gravel and boulders, with occasional buried soils where once forests grew.

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Nevada Geologic Map

Nevada's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Nevada is almost entirely within the Great Basin, the heart of the Basin and Range province of North America. (more below)

Nevada is unique. Consider the Himalaya region, where two continents are colliding and creating an area of very thick crust. Nevada is the opposite, where a continent is stretching apart and leaving the crust exceptionally thin.

Between the Sierra Nevada to the west in California and the Wasatch Range in Utah to the east, the crust has been extended by some 50 percent over the last 40 million years. In the upper crust, the brittle surface rocks broke into long blocks, while in the hotter, softer lower crust there was more plastic deformation, allowing these blocks to tilt. The upward-tilting parts of the blocks are mountain ranges and the downward-tilting parts are basins. These filled with sediments, topped with dry lake beds and playas in the arid climate.

The mantle responded to the crustal extension by melting and expanding and lifted Nevada into a plateau more than a kilometer high. Volcanism and magma intrusions covered the state deep in lava and ash, also injecting hot fluids in many places to leave metal ores behind. All this, coupled with spectacular rock exposures, makes Nevada a hard-rock geologist's paradise.

Northern Nevada's young volcanic deposits are associated with the Yellowstone hotspot track, running from Washington to Wyoming. Southwestern Nevada is where the most crustal extension is occurring these days, along with recent volcanism. The Walker Lane, a wide zone of tectonic activity, parallels the diagonal border with southern California.

Before this period of extension, Nevada was a convergent zone similar to South America or Kamchatka today with an oceanic plate sweeping in from the west and being subducted. Exotic terranes rode in on this plate and slowly built the land of California. In Nevada, large bodies of rock moved eastward in great thrust sheets on several occasions during Paleozoic and Mesozoic time.

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New Hampshire Geologic Map

New Hampshire's rocks
Geologic Maps of the 50 United States Courtesy New Hampshire Department of Environmental Services.

New Hampshire was once like the Alps, thick sediment sequences, volcanic deposits, bodies of granitic rocks pushed up by plate collisions. (more below)

Half a billion years ago, New Hampshire lay on the edge of the continent as a new ocean basin opened and then closed nearby. That ocean was not today's Atlantic but an ancestor named Iapetus, and as it closed the volcanic and sedimentary rocks of New Hampshire were thrusted and kneaded and heated until they became schist, gneiss, phyllite, and quartzite. The heat came from intrusions of granite and its cousin diorite.

All this history took place in the Paleozoic Era from 500 to 250 million years ago, which accounts for the traditional dense, saturated colors used on the map. The green, blue, and purplish areas are the metamorphic rocks, and the warm colors are the granites. The general fabric of the state runs parallel to the rest of the mountain ranges of the eastern United States. The yellow blobs are later intrusions related to the opening of the Atlantic, mostly during the Triassic, around 200 million years ago.

From then until nearly the present, the history of the state was one of erosion. The Pleistocene ice ages brought deep glaciers to the whole state. A surface geologic map, showing the glacial deposits and landforms, would look very different from this one.

I have two apologies. First, I left off the tiny Isles of Shoals, which sit offshore past the lower right corner of the state. They look like dirt specks, and they're too small to show any color. Second, I apologize to my old professor Wally Bothner, the map's first author, for the mistakes I've surely made interpreting this map.

 You can get your own copy from the state Department of Environmental Services as a free PDF.

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New Jersey Geologic Map

New Jersey's rocks
Geologic Maps of the 50 United States Courtesy New Jersey Geological Survey.

New Jersey is sharply divided on this geologic map, but it's an accident of geography.

New Jersey has two rather different regions. The south half of the state is on the low, flat-lying Atlantic coastal plain, and the north half is in the ancient folded Appalachian mountain chain. In fact they fit together very well, but the course of the Delaware River, which establishes the state border, cuts across and along the grain of the rocks giving the state its chunky shape. At New Jersey's northwest edge in Warren County, the river makes an especially impressive water gap, cutting through a high ridge of tough conglomerate. Geologists have shown that the river once took the same course in a flat landscape high above today's, with older mountains buried in a thick layer of younger sediment. As erosion removed this sediment layer the river cut down across the buried mountains, not through them.

The state is rich in fossils, and the thick basalt intrusions (bright red) of Jurassic age are well known among mineral collectors. The state contains coal and metal ores that were extensively exploited from colonial times until the early 20th century.

The green-and-red oval marks a region where the crust split during the initial opening of the Atlantic Ocean. A similar feature is in Connecticut and Massachusetts.

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New Mexico Geologic Map

New Mexico's rocks
Geologic Maps of the 50 United States Image courtesy NM Bureau Mines & Mineral Resources.

New Mexico extends over several different geologic provinces, ensuring it a great variety of rocks. 

New Mexico is a large state with a wide variety of geologic and tectonic features, fairly easy to read from this map if you know the traditional map colors and a bit of regional geology. The Mesozoic rocks in the northwest (green) mark the Colorado Plateau, topped by some younger strata indicated with orange. Yellow and cream areas on the east are young sediments washed off the Southern Rockies.

Similar young sedimentary rocks fill the Rio Grande Rift, a failed spreading center or aulacogen. This narrow would-be ocean basin runs up the left-center of the state with the Rio Grande flowing down its middle, exposing the Paleozoic (blues) and Precambrian (dark brown) rocks on its uplifted flanks. The reds and tan indicate younger volcanic rocks associated with the rifting.

The large swath of light blue-violet marks where the great Permian Basin of Texas continues into the state. Younger sediments of the Great Plains cover the whole eastern edge. And a bit of basin-and-range terrain appears in the extreme southwest, wide dry basins choked with coarse sediments eroded from the blocks of uplifted older rocks.

Also,. The state geologic bureau publishes a giant state geologic map and also has virtual tours for deeper detail about New Mexico.

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New York Geologic Map

New York's rocks
Geologic Maps of the 50 United States (c) 2001 Andrew Alden, licensed to About.com, Inc. ( fair use policy).

New York is full of interest for all kinds of geologists.

This thumb-sized version of New York is from a 1986 publication by several state government agencies (click it for a much larger version). At this scale only the gross features are apparent: the grand sweep of the western state's classic Paleozoic section, the gnarled ancient rocks of the northern mountains, the north-south stripe of folded Appalachian strata along the eastern border, and the huge glacial sediment deposit of Long Island. The New York Geological Survey issued this map, along with much explanatory text and two cross sections.

The Adirondack Mountains in the north are part of the ancient Canadian Shield. The wide set of flat-lying sedimentary rocks in western and central New York are part of the North American heartland, laid down in shallow seas between Cambrian (blue) and Pennsylvanian (dark red) times (500 to 300 million years ago). They grow in thickness toward the east, where high mountains raised during plate collisions were eroded. The remnants of these alpine chains remain as the Taconic Mountains and Hudson Highlands along the eastern border. The entire state was glaciated during the ice ages, and rock debris was piled up forming Long Island.

See a gallery of New York geological attractions.

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North Carolina Geologic Map

North Carolina's rocks
Geologic Maps of the 50 United States Courtesy North Carolina Geological Survey.

North Carolina runs from young eastern sediments to western rocks a billion years old. In between is a rich diversity of rocks and resources.

North Carolina's oldest rocks are the metamorphic rocks of the Blue Ridge belt in the west (tan and olive), cut off abruptly at the Brevard Fault Zone. They are strongly altered by several episodes of folding and disruption. This region yields some industrial minerals.

In the Coastal Plain in the east, younger sediments are denoted by beige or orange (Tertiary, 65 to 2 million years) and light yellow (Quaternary, less than 2 m.y.). In the southeast is a large area of older sedimentary rocks of Cretaceous age (140 to 65 m.y.). All of these are little disturbed. This region is mined for sand and phosphate minerals. The Coastal Plain is home to hundreds, perhaps thousands, of the mysterious oval basins called Carolina bays.

Between the Blue Ridge and Coastal Plain is a complex set of mostly metamorphosed, mostly Paleozoic rocks (550 to 200 m.y.) called the Piedmont. Granite, gneiss, schist and slate are the typical rocks here. North Carolina's famous gem mines and gold district, America's first, are in the Piedmont. Exactly in the middle is a former rift valley of Triassic age (200 to 180 m.y.), marked olive-gray, filled with mudstone and conglomerate. Similar Triassic basins exist in states to the north, all of them made during the initial opening of the Atlantic Ocean.

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North Dakota Geologic Map

North Dakota's rocks
Geologic Maps of the 50 United States Image courtesy North Dakota Geological Survey.

This is North Dakota without its surface blanket of glacial sand and gravel, which covers three-fourths of the state. 

The outlines of the broad Williston basin in the west are clear; these rocks (brown and purple) all date from Tertiary times (younger than 65 million years). The rest, starting with the light blue, make up a thick Cretaceous section (140 to 65 million years) covering the eastern half of the state. A narrow strip of Archean basement, billions of years old, with a few stray blobs of much younger Ordovician (pink) and Jurassic (green) rocks, spills across the border from Minnesota.

Also, You can also buy a printed 8-1/2 x 11 copy from the state; order publication MM-36.

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Ohio Geologic Map

Ohio's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Ohio is rich in rocks and fossils, just not at the surface.

Underneath a widespread cover of young glacial sediment laid down in the last million years, Ohio is underlain by sedimentary rocks older than 250 million years: mostly limestone and shale, laid down in gentle, shallow seas. The oldest rocks are of Ordovician age (about 450 million years), in the southwest; overlying them in a sweep over to the southeast border are (in order) Silurian, Devonian, Mississippian, Pennsylvanian and Permian rocks. All are rich in fossils. 

Deep beneath these rocks is the far more ancient core of the North American continent, sloping away to the Illinois Basin to the southwest, the Michigan basin to the northwest, and the Appalachian Basin to the east. The part that isn't sloping, in the western half of the state, is the Ohio Platform, buried some 2 kilometers deep.

The thick green lines mark the southern limit of continental glaciation during the Pleistocene ice ages. On the north side, very little bedrock is exposed at the surface, and our knowledge is based on boreholes, excavations and geophysical evidence.

Ohio produces a great deal of coal and petroleum as well as other mineral products such as gypsum and aggregate.

Find more geologic maps of Ohio at the Ohio Geological Survey website.

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Oklahoma Geologic Map

Oklahoma's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Oklahoma is a Great Plains state, but its geology is anything but plain. 

Oklahoma resembles other Midwestern states in having Paleozoic sedimentary rocks folded up against the ancient Appalachian mountain belt, only the mountain belt runs east-west. The small colorful areas in the south and the deeply folded area in the southeast are, from west to east, the Wichita, Arbuckle and Ouachita Mountains. These represent a western extension of the Appalachians that also appears in Texas.

The westward sweep of gray to blue represents sedimentary rocks of Pennsylvanian to Permian age, most of them laid down in shallow seas. In the northeast is part of the uplifted Ozark Plateau, which preserves older rocks of Mississippian down to Devonian age.

The strip of green in southernmost Oklahoma represents Cretaceous-age rocks from a later incursion of the sea. And in the western panhandle are still younger layers of rock debris that were shed from the rising Rockies in Tertiary time, after 50 million years ago. These have been eroded in more recent time to reveal deep-seated older rocks in the farthest west end of the state in the High Plains.

Learn much more about Oklahoma's geology at the Oklahoma Geological Survey site.

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Oregon Geologic Map

Oregon's rocks
Geologic Maps of the 50 United States U.S. Geological Survey.

Oregon is the most volcanic state in the continental United States, but that's not all. 

Oregon is a mostly volcanic state, thanks to its position at the edge of the North American crustal plate where a small oceanic plate, the Juan de Fuca plate (and others before it), is being subducted beneath it from the west. This activity creates fresh magma which rises and erupts in the Cascade Range, represented by the stripe of medium-red in the western part of Oregon. To its west are more volcanics plus marine sediments from episodes when the crust was lower and the sea higher. Older rocks not quite covered by volcanic deposits are found in the Blue Hills of northeastern Oregon and in the northern Klamath Mountains in the extreme southwest, a continuation of the California Coast Ranges.

Eastern Oregon is divided between two large features. The southern part is in the Basin and Range province, where the continent has stretched in the east-west direction, breaking up into great blocks with intervening valleys, like the rocks of Nevada. This high lonesome place is known as the Oregon Outback. The northern part is a vast expanse of lava, the Columbia River Basalt. These rocks were emplaced in fearsome fissure eruptions as the continent overrode the Yellowstone hotspot, during Miocene time some 15 million years ago. The hotspot has torched its way across southern Idaho and now sits at the corner of Wyoming and Montana beneath the geysers of Yellowstone National Park, far from dead. At the same time, another trend of volcanism led westward (the darkest red) and now sits at Newberry Caldera, south of Bend at the center of Oregon.

See a gallery of Oregon geological attractions.

This is a scanned copy of U.S. Geological Survey Map I-595 by George Walker and Philip B. King, published in 1969. 

Visit the Oregon Department of Geology and Mineral Industries to find more information and published products. "Oregon: A Geologic History," is an excellent place to learn more detail.

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Pennsylvania Geologic Map

Pennsylvania's rocks
Geologic Maps of the 50 United States Image courtesy Pennsylvania Department of Conservation and Natural Resources.

Pennsylvania may be the quintessential Appalachian state. 

Pennsylvania straddles the entire Appalachian range, starting from the Atlantic coastal plain on the extreme southeast corner, where young sediments are shown in dark green (Tertiary) and yellow (recent). The oldest rocks (Cambrian and older) at the core of the Appalachians are depicted in orange, tan and pink. The collisions between the North American and Europe/African continents pushed these rocks into steep folds. (The green-gold strip represents a crustal trough where today's Atlantic Ocean began to open much later, in Triassic and Jurassic time. The red is thick intrusions of basalt.)

To the west, the rocks grow progressively younger and less folded as the full range of the Paleozoic Era is represented from the orange Cambrian through the Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian, to the greenish-blue Permian basin in the southwest corner. All these rocks are full of fossils, and rich coal beds occur in western Pennsylvania.

The American petroleum industry began in western Pennsylvania, where natural oil seeps were exploited for many years in the Devonian rocks of the Allegheny River valley. The first well in the United States drilled specifically for oil was in Titusville, in Crawford County near the northwest corner of the state, in 1859. Soon afterward began America's first oil boom, and the region is littered with historic sites.

See a gallery of Pennsylvania geological attractions.

Also, You can also get that map and many others from the state Department of Conservation and Natural Resources.

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Rhode Island Geologic Map

Rhode Island's rocks
Geologic Maps of the 50 United States Click the image for a 1000 x 1450 version. Rhode Island Geological Survey

Rhode Island is part of an ancient island, Avalonia, that joined North America long ago. 

The smallest state, Rhode Island has been lovingly mapped at 1:100,000 scale. If you live there, this inexpensive map is well worth buying from the Rhode Island Geological Survey.

Like the rest of New England, Rhode Island is largely covered by sand and gravel dating from the latest ice age. Bedrock is found in scattered outcrops or in roadcuts and building foundations and mines. This map ignores the surface coating for the living rock beneath, except on the coast and on Block Island, in Long Island Sound.

The whole state lies in the Avalon terrane, a block of crustal rocks that once lay off the North American continent more than 550 million years ago. Two chunks of that terrane are separated by a major shear zone running down the west edge of the state. The Hope Valley subterrane is on the west (in light brown) and the Esmond-Dedham subterrane is on the right covering the rest of the state. It in turn is broken in two by the light-toned Narragansett basin.

These subterranes have been intruded by igneous rocks in two main orogenies, or mountain-building episodes. The first was the Avalonian orogeny in the Late Proterozoic, and the second includes the Alleghenian orogeny, from Devonian through Permian time (about 400 to 290 million years ago). The heat and forces of those orogenies left most of the state's rocks metamorphosed. The colored lines in the Narragansett basin are contours of metamorphic grade where this can be mapped.

The Narragansett basin formed during this second orogeny and is filled with largely sedimentary rocks, now metamorphosed. Here is where Rhode Island's few fossils and coal beds are found. The green strip on the south shore represents a later Permian intrusion of granites near the end of the Alleghenian orogeny. The next 250 million years are years of erosion and uplift, exposing the deeply buried layers that now lie on the surface.

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South Carolina Geologic Map

South Carolina's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

South Carolina extends from the young sediments of the Atlantic coast to the ancient folded Precambrian metasediments of the deepest Appalachians.

Since the nation's first gold rush in the early 1800s, geologists have explored South Carolina's rocks for resources and for science. This is a good place to learn geology—indeed, the 1886 Charleston earthquake makes South Carolina of interest to seismologists as well as petrologists.

South Carolina's rocks represent the Appalachian foldbelt starting at the western border with a thin sliver of its deep, contorted heart, the Blue Ridge province. The rest of northwestern South Carolina, left of the dark green strip, is in the Piedmont belt, which is a series of rocks that have been piled up here by ancient plate collisions throughout Paleozoic time. The beige stripe across the eastern edge of the Piedmont is the Carolina slate belt, site of gold mining in the early 1800s and again today. It also coincides with the famous Fall Line, where rivers rushing down to the Coastal Plain afforded water power for the early settlers.

The Coastal Plain includes all of South Carolina from the sea to the dark green strip of Cretaceous-age rocks. The rocks generally get older with distance from the coast, and all of them were laid down under the Atlantic at times when it was much higher than today.

South Carolina is rich in mineral resources, starting with crushed stone, limestone for cement production, and sand and gravel. Other notable minerals include kaolinite clay in the Coastal Plain and vermiculite in the Piedmont. The metamorphic mountain rocks are also known for gemstones.

The South Carolina Geological Survey has a free geologic map that shows these rock units labeled as packages, or terranes.

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South Dakota Geologic Map

A Map of South Dakota's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

South Dakota's rocks are a carpet of Cretaceous seabed deposits, punctuated by areas of extremely old rock on east and west.

South Dakota occupies a large area of the North American craton or continental core; this map shows the younger sedimentary rocks that are draped upon its ancient flattened surface. Cratonal rocks appear uncovered at both ends of the state. In the east, the Sioux Quartzite of Proterozoic age in the south corner and the Milbank Granite of Archean age in the north corner. In the west is the Black Hills uplift, which began rising late in Cretaceous time (about 70 million years ago) and was eroded to expose its Precambrian core. It is ringed with younger marine sedimentary rocks of Paleozoic (blue) and Triassic (blue-green) age that were laid down when ocean lay to the west.

Soon afterward the ancestor of today's Rockies erased that sea. During the Cretaceous the ocean was so high that this part of the mid-continent was flooded with a great seaway, and that's when the swath of sedimentary rocks shown in green was laid down. Afterward in Tertiary time, the Rockies rose again, shedding thick aprons of debris upon the plains. Within the last 10 million years much of that apron was eroded away leaving remnants shown in yellow and tan.

The thick green line marks the western limit of the ice age continental glaciers. If you visit eastern South Dakota, the surface is almost totally covered with glacial deposits. So a map of South Dakota's surface geology, like the clickable map from the South Dakota Geological Survey, looks rather different from this bedrock map.

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Tennessee Geologic Map

Tennessee's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Tennessee's length extends from ancient granites in the Appalachian east to modern sediment of the Mississippi River valley in the west. (more below)

Tennessee is warped at both ends. Its western end is in the Mississippi Embayment, a very old break in North America's continental core in which rocks from modern to Cretaceous age (about 70 million years) are exposed in age order from gray to green. Its eastern end is in the Appalachian foldbelt, a mass of rocks wrinkled by plate-tectonic clashes during early Paleozoic time. The easternmost strip of brown is in the central Blue Ridge province, where the oldest rocks of Precambrian age have been pushed up and exposed by long erosion. To its west is the Valley and Ridge province of tightly folded sedimentary rocks that date from Cambrian (orange) through Ordovician (pink) and Silurian (purple) age.

In central Tennessee is a wide zone of fairly flat-lying sedimentary rocks on the Interior Platform that includes the Cumberland Plateau on the east. A low structural arch related to the Cincinnati Arch of Ohio and Indiana, called the Nashville Dome, exposes a large area of Ordovician rocks from which all overlying younger rocks have been removed by erosion. Around the dome are rocks of Mississippian (blue) and Pennsylvanian (tan) age. These yield most of Tennessee's coal, oil and gas. Zinc is mined in the Valley and Ridge, and ball clay, used in common ceramics, is a mineral product in which Tennessee leads the nation.

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Texas Geologic Map

Texas's rocks
Geologic Maps of the 50 United States Courtesy Texas Bureau of Economic Geology.

Texas contains elements of nearly all the United States in its rocks.

Texas is a microcosm of the American south, plains, Gulf, and Rockies. The Llano Uplift in the center of Texas, exposing ancient rocks of Precambrian age (red), is an outlier of the Appalachian Mountains (along with small ranges in Oklahoma and Arkansas); the Marathon range in west Texas is another. The great exposures of Paleozoic strata shown in blue in north-central Texas were laid down in a shallow sea that retreated westward, ending with the deposition of rocks in the Permian Basin in north and west Texas. Mesozoic strata, covering the middle of the map with their green and blue-green colors, were laid down in another gentle sea that extended from New York to Montana for many millions of years.

The vast thicknesses of more recent sediments in the Texas coastal plain are riddled with salt domes and petroleum deposits, just like Mexico to the south and the Deep South states to the east. Their weight pushed the crust downward along the Gulf of Mexico throughout the Cenozoic Era, tipping their landward edges up in gentle cuestas that march inland in ever-older succession.

At the same time Texas was undergoing mountain-building, including continental rifting with attendant volcanism (shown in pink), in its far west. Great sheets of sand and gravel (shown in brown) washed down over the northern plains from the rising Rockies, to be eroded by streams and reworked by winds as the climate grew colder and drier. And the most recent period has built the world-class barrier islands and lagoons along the Texas Gulf coast.

Each period of Texas's geologic history is displayed in large areas—appropriate for this enormous state. The University of Texas library has an online summary of the geologic history of Texas as shown on this map.

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Utah Geologic Map

Utah's rocks
Geologic Maps of the 50 United States Image courtesy Brigham Young University.

Utah contains some of America's most spectacular geology. (more below)

The western part of Utah is in the Basin and Range province. Due to plate movements on the far-off west coast during late Tertiary time, this part of the state and all of Nevada to its west have been stretched by some 50 percent. The upper crust split into strips, which tilted upward into ranges and downward into basins, while the hot rocks beneath rose up to elevate this region by nearly 2 kilometers. The ranges, shown in various colors for their rocks of many different ages, shed huge amounts of sediment into the basins, shown in white. Some basins contain salt flats, most notably the floor of former Lake Bonneville, now a world-famous test track for ultrafast automobiles. Widespread volcanism at this time left deposits of ash and lava, shown in pink or purple.

The southeastern part of the state is part of the Colorado Plateau, where the mostly flat-lying sedimentary rocks laid down in shallow Paleozoic and Mesozoic seas were slowly raised and gently folded. The plateaus, mesa, canyons, and arches of this region make it a world-class destination for geologists as well as wilderness lovers.

In the northeast, the Uinta Mountains expose Precambrian rocks, shown in dark brown. The Uinta range is part of the Rockies, but almost alone among American ranges, it runs east-west.

The Utah Geological Survey has an interactive geologic map to provide all the detail you can get.

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Vermont Geologic Map

Vermont's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Vermont is a land of compression and sutures as well as marble and slate.

Vermont's geologic structure parallels the Appalachian chain, which runs from Alabama to Newfoundland. Its oldest rocks, of Precambrian age (brown), are in the Green Mountains. To its west, starting with the orange band of Cambrian rocks, is a belt of sedimentary rocks that formed near shore on the western shore of the ancient Iapetus Ocean. In the southwest is a large sheet of rocks that were thrusted over this belt from the east during the Taconian orogeny some 450 million years ago, when an island arc arrived from the east.

The thin purple strip running up the center of Vermont marks the boundary between two terranes or microplates, a former subduction zone. The body of rocks to the east formed on a separate continent across the Iapetus Ocean, which closed for good during the Devonian about 400 million years ago.

Vermont produces granite, marble and slate from these various rocks as well as talc and soapstone from its metamorphosed lavas. The quality of its stone makes Vermont a producer of dimension stone out of proportion to its size.

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Virginia Geologic Map

Virginia's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Virginia is blessed with a great cross-section of the Appalachian chain. 

Virginia is one of only three states that include all five classic provinces of the Appalachian Mountains. From west to east these are the Appalachian Plateau (tan-gray), Valley and Ridge, Blue Ridge (brown), Piedmont (beige to green) and Coastal Plain (tan and yellow).

The Blue Ridge and Piedmont have the oldest rocks (about 1 billion years), and the Piedmont also includes younger rocks of Paleozoic age (Cambrian to Pennsylvanian, 550-300 million years). The Plateau and the Valley and Ridge are entirely Paleozoic. These rocks were laid down and disrupted during the opening and closing of at least one ocean where the Atlantic is today. These tectonic events led to widespread faulting and thrusting that has placed older rocks above younger ones in many places.

The Atlantic began to open during the Triassic (about 200 m.y.), and the teal-and-orange blobs in the Piedmont are stretch marks in the continent from that time, filled with volcanic rocks and coarse sediments. As the ocean widened the land settled down, and the young rocks of the Coastal Plain were laid down in the shallow offshore waters. These rocks are exposed today because ice caps hold water out of the ocean, leaving sea level unusually low.

Virginia is full of geologic resources, from coal in the Plateau to iron and limestone in the mountains to sand deposits in the Coastal Plain. It also has notable fossil and mineral localities. See a gallery of Virginia geological attractions.

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Washington Geologic Map

Washington's rocks
Geologic Maps of the 50 United States Washington State Department of Natural Resources.

Washington is a rugged, glaciated, volcanic patchwork on the edge of the North American continental plate.

Washington's geology can be discussed in four tidy pieces.

Southeastern Washington is covered with volcanic deposits from the last 20 million years or so. The reddish-brown areas are the Columbia River Basalt, a gigantic lava pile marking the path of the Yellowstone hotspot.

Western Washington, the edge of the North American plate, has been sliding over oceanic plates like the Pacific, Gorda, and Juna de Fuca plates. The coastline rises and falls from that subduction activity, and the friction of the plates produces rare, very large earthquakes. The pale blue and green areas near shore are young sedimentary rocks, laid down by streams or deposited during high stands of sea level. The subducted rocks heat up and release upwellings of magma that emerge as arcs of volcanoes, shown by the brown and tan areas of the Cascade Range and Olympic Mountains.

In the more distant past, islands and microcontinents have been carried from the west against the continental edge. Northern Washington shows them well. The purple, green, magenta, and gray areas are terranes of Paleozoic and Mesozoic age that began their existence thousands of kilometers to the south and west. Light-pink areas are more recent intrusions of granitic rocks.

The Pleistocene ice ages covered northern Washington deep in glaciers. The ice dammed some of the rivers that flow through here, creating large lakes. When the dams burst, gigantic floods burst across the whole southeastern part of the state. The floods stripped sediments off of the underlying basalt and laid them down elsewhere in the cream-colored regions, accounting for the streaky patterns on the map. That region is the famous Channeled Scablands. Glaciers also left thick beds of unconsolidated sediments (yellow-olive) filling the basin where Seattle sits.

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West Virginia Geologic Map

West Virginia's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

West Virginia occupies the heart of the Appalachian Plateau and its mineral wealth. 

West Virginia lies in three of the major provinces of the Appalachian Mountains. Its easternmost part is in the Valley and Ridge province, except for the very tip which is in the Blue Ridge province, and the rest is in the Appalachian Plateau.

The area of West Virginia was part of a shallow sea throughout most of the Paleozoic Era. It was mildly disturbed by tectonic developments that raised mountains to its east, along the continental edge, but mainly it accepted sediments from those mountains from Cambrian time (more than 500 million years ago) into the Permian (about 270 million years ago).

The older rocks in this series are of largely marine origin: sandstone, siltstone, limestone and shale with some salt beds during Silurian time. During the Pennsylvanian and Permian, starting about 315 million years ago, a long series of coal swamps produced seams of coal across most of West Virginia. The Appalachian orogeny interrupted this situation, folding the rocks in the Valley and Ridge to their present state and raising the deep, ancient rocks of the Blue Ridge where erosion has exposed them today.

West Virginia is a major producer of coal, limestone, glass sand and sandstone. It also produces salt and clay. Learn more about the state from the West Virginia Geological and Economic Survey.

 

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Wisconsin Geologic Map

Wisconsin's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

On the whole, Wisconsin has America's oldest rocks beneath its glacial cover of sand and gravel.

Wisconsin, like its neighbor Minnesota, is geologically part of the Canadian Shield, the ancient nucleus of the North American continent. This basement rock occurs throughout the American Midwest and plains states, but only here are large areas of it not covered by younger rocks.

The oldest rocks in Wisconsin are in a relatively small area (orange and light tan) just left of the upper center. They are between 2 and 3 billion years old, about half the age of the Earth. The neighboring rocks in northern and central Wisconsin are all older than 1 billion years and consist mostly of gneiss, granite and strongly metamorphosed sedimentary rocks.

Younger rocks of Paleozoic age surround this Precambrian core, chiefly dolomite and sandstone with some shale and limestone. They start with rocks of Cambrian (beige), then Ordovician (pink) and Silurian (lilac) age. A small area of even younger Devonian rocks (blue-gray) crops out near Milwaukee, but even these are a third of a billion years old.

There is nothing younger in the whole state—except for the ice-age sand and gravel, left behind by the Pleistocene continental glaciers, that completely hides most of this bedrock. The thick green lines mark the limits of glaciation. An unusual feature of Wisconsin's geology is the Driftless Area outlined by the green lines in the southwest, a region that the glaciers never covered. The landscape there is quite rugged and deeply weathered.

Learn much more about Wisconsin's geology from the Wisconsin Geological and Natural History Survey. It serves another annotated version of the state bedrock map.

 

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Wyoming Geologic Map

Wyoming's rocks
Geologic Maps of the 50 United States Created by Andrew Alden from the U.S. Geological Survey's Geologic Map of the United States, 1974, by Philip King and Helen Beikman ( fair use policy).

Wyoming is the second-highest American state after Colorado, rich in minerals and scenery alike. 

Wyoming's mountain ranges are all part of the Rockies, mostly the Middle Rockies. Most of them have very old rocks of Archean age in their cores, shown here by brownish colors, and Paleozoic rocks (blue and blue-green) on their flanks. The two exceptions are the Absaroka Range (upper left), which is young volcanic rocks related to the Yellowstone hotspot, and the Wyoming Range (left edge), which is faulted strata of Phanerozoic age. Other major ranges are the Bighorn Mountains (top center), Black Hills (top right), Wind River Range (left center), Granite Mountains (center), Laramie Mountains (right center) and Medicine Bow Mountains (bottom right center).

Between the mountains lie large sedimentary basins (yellow and green), which have large resources of coal, oil and gas as well as abundant fossils. These include the Bighorn (top center), Powder River (top right), Shoshone (center), Green River (lower left and center) and Denver Basin (lower right). The Green River basin is especially noted for its fossil fish, common in rock shops around the world.

Among the 50 states, Wyoming ranks first in coal production, second in natural gas and seventh in oil. Wyoming is also a major uranium producer. Other prominent resources produced in Wyoming are trona or soda ash (sodium carbonate) and bentonite, a clay mineral used in drilling muds. All of these come from the sedimentary basins.

In Wyoming's northwest corner is Yellowstone, a dormant supervolcano that hosts the world's largest assemblage of geysers and other geothermal features. Yellowstone was the world's first national park, although California's Yosemite Valley was reserved a few years earlier. Yellowstone remains one of the world's premier geological attractions for both tourists and professionals.

The University of Wyoming has the much more detailed 1985 state map by J. D. Love and Ann Christianson.