The Ice Age
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.