During the Pleistocene Epoch, all continents of the Northern Hemisphere
and some of the Southern Hemisphere were partly covered at least four
times by huge glaciers. Each glacial advance in Europe and North America
was ended by a warmer interval during which the glaciers melted and
retreated northward; then, vegetation and soil had time to become
re-established. Thus, the Pleistocene has properly been called the ice
age.
None of the continental glaciers reached the Monument or the Uncompahgre
Plateau, or arch, but small alpine glaciers grew in the high Rocky
Mountains to the east, sculpturing sharp-crested peaks and ridges and
forming beautiful valleys and lakes. Many of the beautiful lakes on
Grand Mesa were formed by glaciation, but some near the edges were
formed by landslides.
The increased streamflow from the greater precipitation and from melting
alpine glaciers in the Rockies, particularly during times of glacial
retreat, helped the Colorado River cut through the rocks faster, thus
assisting in the formation of Colorado National Monument as we see it
today. The river carried thousands of cubic miles of sediment to the
Gulf of California, including a lot of rock that once covered the
Monument, and the river is still actively at work on this immense
earthmoving project.
If the ancestral Colorado River carried sediment at about the same rate
as the present river since the building of Hoover Dam, it may have
carried about 3 cubic miles of sediment each century. Now most of the
rock debris is being dumped into Lake Powell—the new reservoir behind
Glen Canyon Dam. When this, Lake Mead, and other reservoirs ultimately
become filled with sediment, the Gulf of California will again be the
burial ground.
But other events during the Pleistocene also played a role in shaping
the area. The Uncompahgre arch was again uplifted and deformed in the
Pleistocene soon after the abandonment of Unaweep Canyon. This caused
added tilting of the strata and more bending and breaking along some of
the folds and faults in the Monument.