In the arid climate of the Colorado Plateaus, ledges of well-cemented sandstone stand out sharply from slopes of shale or mudstone. The Mesa Verde and Mancos Formations, Cretaceous in age, form the slopes and top of Mt. Garfield near Grand Junction (Jack Rathbone photo)
The peculiar weathering characteristics of flat-lying sedimentary rocks in an arid climate are well demonstrated in Colorado National Monument, Mesa Verde National Park, and elsewhere in the Plateau Province. Those fortunate enough to make a river trip through the Yampa or Green River Canyons in northwestern Colorado or on the rivers of eastern Utah and northern Arizona will have an unusually fine opportunity to observe close at hand the weathering and erosion in this area. Resistant sandstone and limestone layers break into sheer cliffs, often many hundreds of feet high, while the softer layers of mudstone and shale form gentle slopes and terraces. Vast arching caves often develop where resistant layers are undermined—caves sometimes containing ancient Indian dwellings.
Astronomical and geologic evidence indicates that the earth was probably formed as an immense blob of molten rock, held together and shaped into spherical form by its own gravity. It may even have been gaseous at first, cooling gradually to a molten state. After hundreds of millions of years it became cool enough to begin to harden.
As the surface cooled, a crust formed, and lay like a blanket over the liquid mass beneath. Convection currents—large-scale boiling movements—stirred the molten interior, thrust portions of the crust upward, and sucked other portions downward to be remelted. Some of the lighter components, such as compounds of silicon and oxygen and hydrogen, accumulated on the surface like froth on a kettle: the continents were born. With further cooling the atmosphere and oceans came into being.
Something can be told of the age of the continents. Measurements of radioactivity in the most ancient rocks exposed at the surface today indicate that the oldest known continental rock is between three and four billion years old. Since the continents were formed, they have been bent and shifted and broken by the pressures exerted against them by convection in the interior. Parts of the continents at times have been submerged below the level of the sea, even as they are today. Other portions, lifted above sea level, were immediately attacked by the wearing-down processes of erosion. The battle between mountain-building forces and erosion has been a continuous one ever since the crust was formed. Even now earthquakes give testimony to continued crustal movement, storms still sweep across the continents and wash mud and frost-loosened rocks into churning torrents, rivers still deposit great floodplains and deltas, sediments accumulate slowly but persistently upon the bottoms of the seas.
Only part of the earth’s very early history is represented in Colorado, where the oldest known rocks are the gneisses and schists of the Idaho Springs Formation, at least 1,800,000,000 years old. These rocks appear to be the remains of ancient sediments, folded and metamorphosed into vast mountain areas long before recognizable life inhabited the earth.
Precambrian rocks in Colorado are on the whole very poorly known. They have, however, been studied in detail in the Front Range west of Denver and Boulder, where they have been intensively explored for valuable minerals. The lack of fossils in the oldest rocks makes their close correlation difficult, but from studies of radioactive minerals contained in these rocks, and of the relationships of the rock units themselves, we can list them in order of their relative ages.
Note that the rock sequence given below reads chronologically from bottom to top—a logical pattern in geology since younger rocks, especially those of sedimentary origin, normally lie above older ones. Recent studies indicate that the sequence may be much more complex than shown here.
| (youngest) Silver Plume Granite: | light pinkish gray, fine-grained granite. |
| Pikes Peak Granite: | pink, coarse-grained granite. |
| Boulder Creek Granite: | dark gray, faintly banded granodiorite. |
| Coal Creek Quartzite: | light gray quartzite with grains ranging in size from fine sand to boulders, with some interbedded schist. |
| Swandyke Hornblende Gneiss: | dark gray to black, strongly banded gneiss. |
| (oldest) Idaho Springs Formation: | gray to black schist and gneiss. |
From a sequence such as this, it is possible to reconstruct some features of Colorado’s early history. The first chapter of which we have a record is the deposition of the Idaho Springs Formation, probably as an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke deposition followed—the sediments were iron-rich, perhaps derived from ancient volcanic materials. The original Coal Creek sediments were sands and gravels, some of them quite coarse and therefore indicative of near-shore deposition. The schist layers suggest that muds must have been deposited also.
South of Ouray, Cambrian sandstones of the Sawatch Formation lie almost horizontally across the vertical Precambrian metamorphic rocks. (Jack Rathbone photo)
Together these three formations represent some 40,000 feet of sedimentary layers. Deposition of such a great thickness of mud, sand, and lime must have taken a very long period of time. Details of the geography of the continent during that period have of course been obscured by later events, when these rocks were subjected to repeated uplift, crumpling, folding, various degrees of remelting and recrystallization, and erosion. But the ancient sediments must have been derived from even more ancient highlands, either folded and faulted mountains or volcanoes, and probably were deposited under water in broad expanses of sea that covered portions of the continent.
Geologic map of Colorado. Geologic maps show the age of rocks appearing at the surface, disregarding soil cover. A more detailed geologic map of Colorado may be obtained from the U.S. Geological Survey at the Federal Center in Denver.
The Boulder Creek, Pikes Peak, and Silver Plume Granites cut through the metamorphic rocks, and are therefore younger. They represent pulses of molten rock forced upward from deep within the crust, probably during three separate episodes of mountain building. As each set of mountains was formed, it was worn down, perhaps to low rolling hills, perhaps to flat plains almost at sea level, and partially or entirely covered with thick layers of sediment. Each time, another mountain building episode followed, with new intrusions of granite and new metamorphism of the pre-existing rocks.
Each succeeding period of metamorphism and mountain building further changed the nature of the rocks involved, complicating the patterns of folding and faulting, adding recrystallization to recrystallization, until the oldest of the rocks bore little trace of their original sedimentary nature. In general, the rocks that are oldest were most altered by the repeated metamorphism, while the younger rocks were less altered.
The Black Canyon of the Gunnison River is one of the state’s deep and spectacular chasms. Canyon walls are of Precambrian gneiss intruded by many dikes and highly fractured by later uplifts. The flat upper surface of the Precambrian rocks represents an ancient plain on which, during Jurassic time, the dinosaur-bearing Morrison Formation was deposited. (John Chronic photo)
The Precambrian Era ended with a long period of erosion, a period known to geologists as the Lipalian Interval. During this time, over almost the entire world there was no mountain building. The land lay sleeping, subject only to the forces of erosion. The last mountains were flattened nearly to sea level. Slow, sluggish streams and rivers carried sand and mud toward the oceans—oceans in which perhaps primitive, soft-bodied organisms, with no hard parts to be preserved as fossils, were beginning to evolve.
On the continents, the time of intense metamorphism was over; most rocks of later eras are preserved today in pretty much their original state. The boundary between the Precambrian and later rocks is normally well defined. It is visible at many places in Colorado: in Williams Canyon near Colorado Springs, in Glenwood Canyon, near Red Rocks west of Denver, just west of La Veta Pass, at the top of Royal Gorge and the Black Canyon of the Gunnison. At most of these localities it is a smoothly beveled surface, with highly contorted Precambrian rocks below it and flat-lying Paleozoic sediments above it. Near Red Rocks and La Veta Pass, the same relationship prevails, but the entire contact, and the rocks above and below it, have been steeply tilted by the uplift of the present mountains.
In portions of western North America, deposition late in Precambrian time has left a series of flat-lying rocks between the contorted Precambrian and later Paleozoic sediments. These rocks can be seen in northwestern Colorado, where they form the dark red sedimentary core of the Uinta Mountains.
Geologists have divided the second great era of geologic time into units called Periods. The rocks deposited during a Period are called Systems, but more often than not it is convenient to discuss them in terms of easily recognized units of rock, called Formations. Formations are named after areas in which they are well exposed.
The stratigraphic column given in Chapter I shows the Periods and Systems in their correct order, and gives the age in years for each, as determined by radioactivity methods. As you read, refer as often as necessary to this column.
The geologic map on page 35 will help you locate areas where the rocks discussed in the text are exposed, and will greatly facilitate your understanding of the geology of the state.
The Cambrian Sawatch Sandstone lies almost horizontally on Precambrian granite in Glenwood Canyon. In the foreground is the Colorado River. (Jack Rathbone photo)
The first fossiliferous rocks in Colorado were deposited during the Cambrian Period, at a time when over much of the world the seas were creeping in across wide, level plains formed during the Lipalian Interval. Colorado was not covered by these seas until quite late in the Cambrian Period. Beach deposits progressively younger in age suggest that the sea invaded from the west, and spread slowly eastward, inundating most of the central part of the state but not the extreme north or south.
The beach deposits, now called the Sawatch Sandstone because they are well exposed in the Sawatch Range, are composed mostly of fine quartz sand. They are colored with glauconite, a green mineral, and hematite, a dark red mineral, so that the rock has a variegated appearance. The post office at Manitou is built of this red and green rock, and good exposures of it exist in Williams Canyon near Manitou, along U. S. Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in Glenwood Canyon.
The sea which crept over Colorado at this time contained small conical-shelled mollusks, brachiopods, and trilobites. Their shells can occasionally be found in Cambrian rocks in Williams Canyon and in the Sawatch and Mosquito Ranges. At two localities unusual fossils called graptolites have been found in thin Upper Cambrian shales overlying the Sawatch Sandstone.
These fossils can occasionally be found in Cambrian rocks in central Colorado.
The sea deepened and widened as the Ordovician Period began, and a series of limestones and dolomites was deposited, either on top of the Sawatch Sandstone or, where the Sawatch had not been deposited, directly on the Precambrian. These rocks are now called the Manitou Formation.
The fossils in these rocks are much more varied than those in the Sawatch Sandstone: snails, echinoderms, sponges, cephalopods, brachiopods, and trilobites are common. The Ordovician sea must have teemed with life, as many rocks deposited at this time are more than half composed of animal remains. In addition to hard-shelled animals which formed fossils, there were probably abundant soft-bodied animals such as jellyfish and worms, which left no record of their presence.
After deposition of the Manitou Formation, the seas receded slightly. A new series of sands was deposited above the Manitou in central Colorado. These now form the Harding Sandstone, a formation of unusual interest because it contains remains of the earth’s earliest known vertebrates, primitive jawless fish called Agnathids. In places in the Harding Sandstone there are dense accumulations of the tiny polygonal armor plates from these fish. Although no whole fish have been found, we can reconstruct their appearance by comparing individual plates or groups of plates with later, better known relatives.
Also present in great quantities in the Harding Sandstone are conodonts, peculiar tiny brown tooth-like fossils. Relationships of the conodonts are unknown; they may be parts of the Agnathids, or perhaps they represent some entirely different group of animals, with no living relatives.
After deposition of the sands of the Harding Sandstone, the sea deepened locally and the Fremont Limestone, a massive gray crystalline limestone containing many marine fossils, was deposited. Mollusks (some quite large), brachiopods, and corals contributed their shells to the Fremont Limestone. The chain coral Catenipora and the horn coral Streptelasma may often be used to identify the formation.
The Fremont Limestone was deposited very late in the Ordovician Period. Probably the seas were much more extensive then than present deposits indicate; subsequent erosion has at several times erased the evidence in uplifted areas.
These Ordovician fossils can be found in the Manitou Formation in the Colorado Springs area.
The earliest known fish remains come from the Ordovician Harding Sandstone of central Colorado. These fragments of the protective plates have been magnified about five times.
Corals and coral-like organisms occur in the Ordovician Fremont Limestone.
Until very recently, no Silurian rocks or fossils were known in Colorado, and it was thought that seas did not extend into the state during this period. However, a few years ago good Silurian corals and brachiopods were discovered near the northern edge of the state. They occur in broken blocks and patches of Silurian limestone, mingled with blocks of other sedimentary rocks and, oddly enough, with volcanic material.
What seems to have happened here is that sedimentary layers of Silurian age were present over northern Colorado at one time. During some subsequent period of volcanism, volcanic lavas penetrated these sediments from below. Near the volcanic tubes, broken, angular fragments of the surrounding sedimentary rocks were sometimes carried upward or downward by the motion of the lava.
Much later, both the volcanic outpourings (if the lavas ever reached the surface) and the sediments were stripped away by erosion, probably at a time when mountains were rising in the area. Only the deep portions of the tubes that fed the volcanoes were preserved. These tubes are called diatremes, and thanks to the blocks of sedimentary rock in them we know that there were indeed seas in Colorado during Silurian time, seas containing the abundant life of a shallow marine environment very much like that existing at the same time in Illinois, Iowa, and Indiana.
As far as we know now, Colorado was just a little above sea level during most of Devonian time. Early and Middle Devonian deposits are lacking. Late in the period, however, Colorado was widely inundated once more. Embayments of a western sea covered most of the central part of the state and an area in southwestern Colorado around Ouray.
Deposits formed in these embayments have been given several names. Chaffee Formation is the name most commonly used in central Colorado; Ouray Formation identifies rocks of the same age in southwest Colorado. The Chaffee Formation has been subdivided into two well defined units, the Parting Sandstone or Quartzite, and the Dyer Dolomite or Limestone. Many ore deposits are associated with these rock units—notably deposits of lead and zinc. The Parting Sandstone is frequently so well cemented with silica that it is actually a quartzite; thin shale beds or “partings” make it easy to recognize. It frequently contains remains of fossil fish and distinctive beds of algae.
The Dyer Dolomite contains brachiopods and bryozoans, mollusks and corals. Some of the best fossil hunting in Colorado is in Dyer beds around the White River Plateau, where the fossils frequently weather out of the rock as almost perfect specimens.
These Devonian brachiopods come from the White River Plateau in western Colorado.
The sea continued to cover most of Colorado after the end of the Devonian Period, well into Mississippian time. Mississippian rocks are characteristically thick, massive gray limestones collectively called the Leadville Limestone. This unit is well known as the host rock for many Colorado ore deposits, notably those around the town of Leadville.
During Mississippian time the western sea, warm and rich in organisms, covered much of North America. Brachiopods and corals flourished, as did many other forms of life. The seas during part of this time extended completely across Colorado to merge with seas that covered the midwestern part of the United States.
Over all this vast area, as well as southwest into Arizona, the gray, massive, fossiliferous Mississippian limestone is remarkably uniform and easily recognized, although it is called by different names in different areas.
Late in Mississippian time, the Colorado area rose slightly and the sea in which the Leadville Limestone was deposited receded. An interval of erosion followed. The surface of the limestone was dissolved and pitted, tunnels and caves formed where running water etched deep into the rock, and a reddish soil formed on the surface and in the hollows. This portion of the limestone, which in some places also contains pebbles of chert, is named the Molas Formation. Part of the Molas may be Pennsylvanian in age.
Mississippian fossils from western Colorado show that seas covered much of the state about 330 million years ago.
As the Pennsylvanian Period began, the Colorado area continued to rise. Earliest deposits of this age are fine-grained black shales and sands—the Glen Eyrie Formation along the southern Front Range and the Belden Formation in west central Colorado. Then, through millions of years, mountain-building took place. Some areas rose more than others, so that formerly flat-lying marine sediments were bent and broken, and a series of high mountain ridges and deep basins were formed. Geologists sometimes call these the Ancestral Rocky Mountains.
Although the pattern of the mountains changed repeatedly, the Ancestral Rockies consisted principally of two large ranges. One range roughly paralleled the present Front Range, but lay thirty to fifty miles further west. The other extended from the San Luis Valley northwest toward Colorado National Monument, including the area around the Black Canyon of the Gunnison and the present Uncompahgre Plateau. Coarse sediments washed off both sides of both ranges, and accumulated as alluvial fans and valley fill along the mountain margins. These exist today as the Fountain Formation of the eastern Front Range, the Minturn Formation between the ancient uplifts, and the Hermosa Formation west of the western uplift.
This paleogeographic map reflects the distribution of land and sea during the early part of the Pennsylvanian Period and shows where coarse sediments derived from the Ancestral Rockies were deposited.
West of Denver, the main line of the Denver & Rio Grande Railroad tunnels beneath steeply dipping sandstones and conglomerates of the Fountain Formation. (Jack Rathbone photo)
Corals, brachiopods, and fusulinid Foraminifurida can be found in the Pennsylvanian Minturn Formation at many places in the Mountain and Plateau Provinces.
In western Colorado, where vegetation is sparse, rock structures are clearly defined. This photograph shows beds of the Pennsylvanian Minturn Formation sharply folded, probably as a result of the deformation of gypsum in underlying layers. (Jack Rathbone photo)
In the Flatirons near Boulder, Red Rocks Park near Denver, and the Garden of the Gods near Colorado Springs we see well exposed examples of the Fountain Formation. The Minturn Formation is visible along the Eagle River west of Wolcott, and along Gore Creek near Vail. The Hermosa Formation forms striking red cliffs north of Durango. In the Sangre de Cristo Mountains area, exceptionally great and rapid deposition took place, and the Minturn Formation is very thick.
In west central Colorado, near the towns of Eagle and Gypsum, a large basin formed. In it, gypsum and other salts were deposited as arms of the sea were cut off from the main marine area. The unusual appearance of the hills along the Eagle River, especially north of U. S. Highway 24, is caused by the presence of gypsum in the bedrock.
In a similar manner, the Paradox Basin was formed in southwestern Colorado. Thousands of feet of gypsum, salt, and potash were deposited here, probably also precipitated in restricted arms of the sea. These minerals, the so-called evaporites, have since significantly controlled development of the landscape in Gypsum Valley and other parts of this region. (See The Plateaus in Chapter I and the section on Gypsum in Chapter III).
Between the mountain masses and their surrounding alluvial deposits, shallow seas repeatedly invaded the lowland areas of the state. Marine fossils in some parts of the Minturn Formation bear witness to as many as twenty marine cycles. Strangely, the Pennsylvanian Period appears to have been cyclical in other parts of the United States as well, for marine sediments are found alternating with nonmarine sediments in Pennsylvania, Illinois, Kansas, Nebraska, and New Mexico. In middle Pennsylvanian time, general uplift occurred in Colorado, and almost the entire state was above sea level for the rest of the period.
By the end of the Pennsylvanian Period, the mountains of the Ancestral Rockies had been almost entirely removed by erosion, and the deep basins were filled with sediments. Colorado was once more a great plain, sloping gently to the northeast. In eastern Colorado, a shallow sea gradually dried up, leaving some thin limestone and gypsum beds along its margin. The western shore of this sea was edged with beaches and sand dunes, preserved as the Lyons Sandstone. The buildings of the University of Colorado, as well as many homes and other structures in the Boulder-Denver area, are faced with this beautiful salmon-colored sandstone.
Balanced Rock, in the Garden of the Gods northwest of Colorado Springs, is an erosional remnant of iron-rich conglomerate and sandstone. It remains while the rest of the surrounding layers are gone because it is harder and more completely cemented together by silica. The rock is part of the Late Paleozoic Fountain Formation. (John Chronic photo)
In the western part of the state, Permian deposits consist mostly of shales and sandstones. The red color of these rocks, and the complete absence of fossils in them, suggest that the environment in which they were deposited was not marine, but was a vast, level mudflat subject to alternating wet and dry periods. The shales and sandstones collectively are called the Maroon Formation, named for Maroon Bells, near Aspen, where they are dramatically exposed in the mountain cliffs.
Tracks of Permian reptiles called Laoporus coloradoensis occur in the Lyons Sandstone near Lyons. These are about life size.
During part of Permian time, a shallow sea extended from Idaho, Utah, and Wyoming into the northwest corner of Colorado. In this sea was deposited the Phosphoria Formation, a highly phosphatic limestone containing only rare, poorly preserved molluscan fossils.
As the Paleozoic Era ended, Colorado was still flat and low-lying. By this time land plants and animals had evolved, but if vegetation grew in the Colorado area, or animals roamed it, they left few fossil remains. Tracks of early reptiles have been found in the Lyons Sandstone. Dune sandstones here and in adjacent areas suggest that desert conditions may have prevailed, in which case Colorado would have been very similar, scenically and climatically, to Sahara regions today.
Dark red Pennsylvanian and Permian conglomerates form the Flatirons that overlook the University of Colorado campus at Boulder. University buildings are faced with Permian Lyons Sandstone quarried along the foothills of the northern Front Range. (University of Colorado photo)
The Mesozoic Era, popularly known as the Age of Reptiles or Age of Dinosaurs, is divided into three periods. The climate of the entire earth appears to have been warmer then than it is at present, perhaps because of a different distribution of land and sea areas, or because continental areas were not as high and mountainous as they are just now. Colorado was a rather low land area for most of the first two Mesozoic periods; then a vast sea covered the entire state for the remainder of the era.
The pink cliffs of Colorado National Monument are made of Wingate and Entrada Sandstones. Underlying them, in the valley bottom, Chinle shales form steep red slopes. (William C. Bradley photo)
Saharan conditions continued to prevail in western North America during the early part of the Mesozoic Era. In central Colorado, the lowest Mesozoic deposits are the Triassic Lykins Formation, a series of soft, bright red sandstones and shales. Where the Lykins is exposed along the Front Range, its bright red color identifies it. Because of its softness, it is often less prominent than adjacent rock layers in the mountain foothills. The Lykins Formation includes some evaporites, apparently derived from Permian evaporites washed into the Triassic ponds and lakes which existed occasionally in this region.
Over almost the entire state, the rocks deposited at this time are very similar. Formation names may differ—Lykins, Moenkopi, Chinle, Ankareh, Wingate—but the rocks are almost universally fine-grained sandstones and shales with a red or pink color. They represent ancient coastal plain, dune, or delta deposits. Toward the western edge of the state they coarsen, and contain layers of conglomerates similar to the Triassic conglomerates of northern Arizona and Utah. These suggest that mountain-building was taking place west of here at that time.
There are virtually no fossils known from Triassic rocks in Colorado, although some fossil palm fronds have been found west of the San Juan Mountains, in the southwestern corner of the state.
During the Jurassic Period, Colorado was still a low, flat desert area with intermittent streams flowing eastward over the surface of older sediments. The Navajo Sandstone, formed from dune sands, was deposited in the western part of the state. Streams flowing eastward from Utah brought fine sediments—silts and muds—to western Colorado, forming what is now the Carmel Formation. Near Canon City, coarse gravels bear witness to local uplift in Jurassic time. Both these gravels and the Carmel Formation were overlain by more dune sands, now hardened into the Entrada Sandstone.
In Late Jurassic time the Colorado area, which had been predominantly desert since Permian time, appears finally to have been submerged once more. Fine calcareous muds of the Curtis Formation, containing ammonites, belemnites, and other marine shellfish, show us that a shallow sea transgressed from the west over the wind-blown sands. This sea was, geologically speaking, of short duration—only a few million years. Bounded on almost all sides by desert, it seems to have dried up, depositing the gypsum that is now present in a thin layer along the Front Range between Denver and Canon City in the Ralston Formation.
At about this time, however, the climate underwent a major change. Deposits above the Ralston indicate an increasingly moist environment, the environment in which the Morrison Formation was deposited over most of Colorado and parts of the adjacent states of Kansas, Arizona, Utah, and Wyoming. The Morrison Formation is exposed in many places, and is characteristically composed of layers of fine, limy mud, brightly colored in streaks of red, brown, green, and blue. In most areas it is so soft that it becomes soil-covered; it is well exposed only in roadcuts or where it is protected from erosion by a “caprock” of harder sediments or lava. Spectacular outcrops can be seen in new roadcuts along U. S. Interstate highway 70 just west of Denver.
In this roadcut along U. S. Interstate 70 west of Denver, Jurassic and Cretaceous rocks are unusually well exposed in the Dakota hogback. Green and purple shales represent the dinosaur-bearing Morrison Formation. The Cretaceous Dakota Group forms the eastern, higher half of the cut. Black layers are carbon-rich clays of the South Platte Formation, frequently quarried locally for ceramic uses. (John Chronic photo)
Fossil dinosaur bones occur in great numbers in the Morrison Formation near the towns of Morrison and Canon City and at several other places in Colorado. Those at Canon City have been quarried extensively, and are now mounted in a number of museums in the United States. At Dinosaur National Monument, in eastern Utah and northwestern Colorado, many excellent remains have been found; those in Utah can be seen in place in the rock in a striking exhibit at the National Monument.
In an old painting, a paleontologist contemplates fossil bones found near Morrison. The date is 1877. The bones are those of the 70-foot dinosaur Apatosaurus, more commonly known as Brontosaurus, shown below in reconstruction.
Some of the dinosaurs known from the Morrison Formation reached 80 feet in length. Both plant-eating and meat-eating types are known. In addition to the bones themselves, gastroliths or gizzard stones can frequently be found; these highly polished stones were as essential to dinosaur digestion as gravel is to a chicken or a caged canary.
Along with the dinosaur fossils are found abundant remains of water plants called charophytes. These plants formed tiny spiralled balls of calcite as part of their reproductive activities; both the little balls and the stalks of the plants themselves occur in many parts of the state. In western Colorado, near Grand Junction, silicified shells of freshwater snails can also be found in the Morrison.
Early in the 1900s vanadium, radium, and uranium were discovered in Jurassic sandstones and mudstones of western Colorado. Extensive mining in this area has revealed that these elements often become concentrated by groundwater in organic material such as fossil plant stems or dinosaur bones. The search for radioactive minerals has thus brought to light many ancient fossil accumulations.
Early in Cretaceous time, marine conditions once more prevailed in Colorado. This is indicated by a marked change in rock types from beach and near-shore deposits to true marine sediments.