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.
[Illustration: 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.