The western quarter of Colorado is a region of flat-lying Paleozoic,
Mesozoic, and Cenozoic sedimentary rocks which have not been bent up
into mountains except in a few isolated instances. This area lies more
than a mile above sea level, however, and because of the gradient such
an elevation affords, it is deeply sculptured. The Colorado River and
its tributaries have sliced into the plateau surface, separating it into
many isolated tablelands or mesas. Some are capped with sedimentary
rock, others with Tertiary basalt.
[Illustration: The Grand Hogback is a good example of the type of
geologic structure known as a _monocline_. The hogback ridge is formed
by differential erosion, where soft layers wear away more easily than
hard layers.]
Simple folds and faults have given the mesas different elevations. Thus
the average elevation of the White River Plateau is 11,000 feet, that of
the Roan Plateau 9,500 feet, and that of Mesa Verde only 7,000 feet.
West of Durango the plateaus dip gently southward, as can be seen at
Mesa Verde. Igneous intrusions and extrusions have altered plateau
topography in some areas. West of Mesa Verde, for instance, an intrusive
stock forms a prominent dome in the Southern Ute Indian Reservation.
West of the northern Colorado mountains, and north and west of the White
River Plateau, a rolling upland extends from Colorado into Utah and
Wyoming. It is interrupted by the Uinta Mountains and a number of
smaller related uplifts such as Juniper Mountain and Cross Mountain.
South of the Uinta axis the area is known as the Uinta Basin.
The northern part of this area is structurally the south edge of the
Green River or Washakie Basin in Wyoming. The Rangely anticline, in the
northeastern corner of the Uinta Basin, is one of Colorado’s richest oil
fields; it is discussed in Chapter III.
Although surfaced with much younger sediments than the rest of the
Plateau Province, this area is structurally similar. On the whole,
sedimentary layers are relatively flat-lying, and where they are
uplifted they are deeply sculptured by streams and rivers. The
sedimentary rocks in this region contain uranium and placer gold in
addition to great oil and gas deposits. The southeastern part of the
Uinta Basin, usually called the Piceance Basin, is the site of a great
deposit of oil shale (see Chapter III). The term “basin” may here seem
unusual to the casual observer, for the oil shales occur on the Roan
Plateau at places well over 10,000 feet in elevation. However, the
entire region was basin-like—lower than the surrounding ranges—for many
millions of years, and during Tertiary time thousands of feet of valley
and lake deposits were laid down in it.
The White River Plateau, north of Glenwood Springs, is composed of
almost horizontal Paleozoic sedimentary rocks that fold downward sharply
along its south and west edges. The fold is 135 miles long and is
clearly marked by the Grand Hogback, the eroded edge of hard Cretaceous
and early Cenozoic rock layers. Shale and coaly layers involved in the
same fold have eroded more readily, leaving the resistant sandstone as a
prominent ridge.
The Uncompahgre Plateau, southwest of Grand Junction, is structurally
very like the White River Plateau. Its features can be well observed in
Colorado National Monument. It has been elevated several thousand feet
more than the Book Cliffs and Grand Valley areas to the north. Sharp
folding and faulting near the Colorado River at the north boundary of
the National Monument show that differential movement between the two
regions was sharp and localized.
A series of northwest-trending anticlines along the Utah border in
southwestern Colorado are of special geologic interest. They represent
peculiar structures in which salt and gypsum have played a major part.
These minerals were deposited in thick layers late in Paleozoic time;
subsequently they were covered by thousands of feet of sand, shale, and
limestone. Because of their low density and high plasticity they have
since crept upward along weak spots in the overlying sediments, often
contorting these rocks as they moved. Breaking through to the surface,
the salt and some of the gypsum washed away more rapidly than the
surrounding rock, leaving long faulted troughs such as Gypsum Valley and
Paradox Valley. In most of these structures the gypsum can still be
seen, although the more soluble salt has eroded away. Oil wells in this
part of Colorado and in adjacent parts of southeast Utah have penetrated
thousands of feet of evaporites, including pure salt, gypsum, and
potassium salts.
[Illustration: 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.
II
Geologic History of Colorado
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.