View to the west from
Dotsero to Glenwood Springs, Colorado. Contour intervals are 100 feet.
The major highway is I-70. The Colorado River cuts through the south
side of the White River Plateau to form Glenwood Canyon. Cottonwood
Pass Road (Near electric transmission lines to the left) crosses
Cottonwood Divide some 1,000 feet lower than the average height of the
Glenwood Canyon is an interesting study in how rivers get
into canyons. Cottonwood Divide (lower route to the left around the
plateau) is capped with Tertiary lava. Hence it has never been much
higher than it is now. The rims of Glenwood Canyon are capped by
Mississippi and Pennsylvanian Limestone and would be higher yet if you
travel back in time to restore layers that have been eroded away. Even
more interesting, upstream from here, the Colorado River could escape
to the north near Yampa, Colorado and still stay 1,000 feet lower than
the current rims of Glenwood Canyon. Why
did the river establish a path across the higher surface?
We go back to our rule that rivers will always take the
easiest available path when they first become established. When the
Colorado River turned westward by early Miocene time, the White River
Plateau had not risen yet. Then after the river had established its
course, the plateau rose during the rest of the Miocene. (Recent
research indicates much of this uplift has occurred in the last 5
million years.) By the time the plateau started to rise, the river was
stuck in a rut. As the plateau rose, the "rut" just got deeper.
It is probable the plateau is still slowly rising with the
river cutting ever deeper. The sheer sides of the canyon and steep
gradient of the river through it are classic signs of recent activity.
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