THE PRAIRIES
Beneath the flat prairies of eastern Colorado, [sedimentary rocks] form a series of layers. Those near the surface are among the youngest rocks in Colorado. We know this from the [fossils] they bear, fossils of large mammals such as the hairy mammoth, which lived in early Quaternary time, the bison, and many smaller mammals living today.
The layers below—sandstones, shales, and limestones—become progressively older as one goes deeper. Most of them were formed originally on the bottoms of shallow seas that covered this part of North America several times during the history of the continent. In most places the layers are horizontal or nearly so, but westward, as they approach the mountains, they bend upward, gently at first and then more steeply. At the very edge of the mountains, where they were dragged upward when the mountains rose, their eroded edges appear at the surface.
The entire sequence of flat-lying rocks can be studied where they are exposed along the mountain front or where streams and rivers have dissected them. They are also known from cuttings and cores of oil and water wells. Some parts of Colorado’s eastern plains have been drilled so intensively in the search for oil and gas that we know a great deal about the subsurface sedimentary rock and can even make maps showing the distribution and character of the individual rock layers. From such maps, the history of the region can be deduced. We know, for example, that the area around Denver has subsided more in the past than has the area near La Junta or Lamar; it is called the Denver Basin because of its past history and not because it is a basin at present.
Although the plains of Colorado appear flat, they really slope gently eastward. The rock layers near the surface slope eastward also, but the deeper rock layers may not.
Near the western edge of the Plains Province, hills and valleys are formed by differential erosion of hard and soft rock layers. Some hills, such as Castle Rock, are topped with resistant sandstone; others, like [Mesa] de Maya south of Trinidad and Table Mountain near Golden, are capped with layers of [basalt]. Close to the mountains flat-topped foothills result from partial dissection of former erosion surfaces as the mountains, stabilized for a time, rose again, or as climatic cycles changed. Examples of these dissected erosion surfaces can be seen north and south of Boulder.
Far east of the mountain front, near the northern border of Colorado, remnants of another, higher prairie surface stand as Pawnee Buttes. Torrential erosion—spring floods and summer thunderstorms—has deeply furrowed the prairie surface here and left these buttes as lonely sentinels.
This map shows the distribution, character, and thickness of certain Jurassic rocks in Colorado. These rocks are deeply buried beneath the plains and are known there only from well samples. They have been eroded from most mountain areas. They come to the surface along the edges of the mountains and in the deeply incised canyons of the [Plateau] Province.
PRECAMBRIAN ROCKS PALEOZOIC ROCKS JURASSIC ROCKS SANDSTONE SHALY SANDSTONE SANDY SHALE SHALE JURASSIC ROCKS COVERED WITH VOLCANICS OR NEVER DEPOSITED.
What lies below the sedimentary layers of the plains? The [sedimentary rocks] are 5,000 to 10,000 feet thick. They lie on an almost horizontal surface of much, much older rock, the Precambrian or “[basement]” rock. This is igneous and metamorphic rock, much crumpled and folded, the roots of long gone mountains which were beveled and leveled to an almost flat surface or [peneplain] perhaps a billion years ago.
We know little of the ancient [basement] rocks below the sedimentary layers of the plains, for few wells penetrate this deep. What we do know indicates that they are similar to rocks of the mountain masses to the west, and are composed of [granite], [schist], and [gneiss]. They probably are not rich in valuable minerals, however, for the mineral-rich [veins] of the mountains came about as a result of uplift of the mountain areas.