The eastern end of Utah’s Uinta Mountains extends into Colorado. Unlike
other ranges in Colorado, these mountains trend east-west. Structurally,
the range is a faulted anticline. It is quite asymmetrical, however, and
is tilted and folded upward on the south, and overturned or
thrust-faulted on the north. Steeply dipping Mesozoic and Paleozoic
sediments on the south side of the range, sparsely vegetated and often
thrown into spectacular folds, are a prominent feature of northwest
Colorado scenery.
In Colorado the crest of the Uintas reaches an elevation of about 8,500
feet. It consists of Precambrian rocks, but these are not the igneous
and metamorphic rocks that characterize the Precambrian core of other
Colorado mountains. They are easily recognized as sediments—dark red
conglomerates, sandstones, and mudstones—virtually unmetamorphosed
though they were deposited nearly a billion years ago. Called the Uinta
Mountain Formation, these rocks are found only in this part of Colorado
and adjacent areas of Utah. They are probably related to similar
Precambrian rocks found in Montana and Canada.
At the east end of the Uintas two isolated uplifts, Cross Mountain and
Juniper Mountain, are faulted blocks of Paleozoic rocks standing like
islands in a sea of Cenozoic valley fill. They are the last outposts of
the Uinta anticlinal pattern as it wanes toward the southeast.
Dinosaur National Monument, a Uinta Mountain tourist attraction,
encompasses a vast area of wilderness on both sides of the Yampa River
in Colorado. Here many of the features of the east end of the Uinta
Mountain structure can be seen. A unique display of the world’s largest
fossils can be visited in the Utah portion of the Monument.
[Illustration: At their confluence in Dinosaur National Monument, the
Yampa and Green Rivers have carved Late Paleozoic sandstone into the
precipitous cliffs of Steamboat Rock. (William C. Bradley photo)]