Raising the Roof of the Rockies
A Geologic History of the Mountains and of the Ice Age in Rocky Mountain National Park

. . . And Sudden Death

Amelioration of the climate and the beginning of glacial recession probably set in about 13,000 years ago. The glaciers did not retreat continuously at first, but pulsated back and forth across short distances, leaving behind one or more recessional moraines. During one of these minor readvances in the valley of Fall River, a small glacier tongue pushed through the outermost end moraine and left a narrow hairpin-shaped moraine which extends through the outer moraine nearly to the Aspenglen Campground. In Glacier Basin, melt-waters flowing along the edge of the ice built a great gravel terrace at the site of Glacier Basin Campground. Later, minor readvances of the glacier constructed two small moraines across the valley above the basin. West of the mountains, the large moraine which encloses Grand Lake was built by a readvance of the glacier in the canyon of East Inlet. A highway cut through the moraine at one time showed that the moraine overlies layers of soft silt deposited in a lake into which the ice readvanced.

Balanced boulders, remaining after deeply weathered rock has been stripped away. On hills northeast of Estes Park. (Fig. 40) (Wayne B Alcorn)

Once the recession really set in, the ice wasted rapidly, and glaciers several miles long and 1,500 feet thick nearly disappeared in less than 1,000 years. During this time meltwaters flowed downvalley from the ice in much greater volume than during the glacial advance. Large lakes existed in Horseshoe Park, Glacier Basin, and in the valley of the Colorado River north of the Park entrance. Moraine Park may have contained a shallow lake, though this is not certain. Soon, however, the heavily laden glacial streams filled the lakes with sand and gravel until they became mere swampy flats. Only Grand Lake survived.

Moraine of a readvance of the last glaciation (Pinedale) enclosing Grand Lake, to the left, and bordered to the right by Shadow Mountain Lake, a reservoir. Distant islands in Shadow Mountain Lake form terminal moraine of maximum advance of the last glaciation (Pinedale). (Fig. 41) (Wayne B Alcorn)

Attempts at Revival

By about 12,000 years ago, the glaciers had retreated to the valley heads. Shortly thereafter, the forest spread rapidly back into the mountains. A little more than 11,000 years ago, man made his way into the region from Alaska. Folsom and Yuma Points attest his presence in many places in Colorado; among them, the archaeologically famous Lindenmeier Site on the plains east of the mountains near the Wyoming State line. Curiously, the large mammals of the Ice Age, excepting the bison, died out coincidently. Man may have played a part in their extinction!

Brief cold periods at different times between 11,000 and 8,000 years ago induced a succession of minor readvances of the glaciers which left at least two small moraines in many of the upper valleys. These are low ridges, usually covered with large blocks. Most are forested. The Loch Vale Trail crosses a lateral moraine formed by one of these advances in a burned area about half a mile west of the Trail head, and follows the crest of another just above Alberta Falls. The forested moraine enclosing Fern Lake was also formed by one of these advances. Finally, however, the glaciers receded into the cirques and, about 7,500 years ago, disappeared entirely.

Layered lake sediments of lake overrun by moraine of Pinedale readvance that encloses Grand Lake. Formerly exposed along highway west of Grand Lake village. (Fig. 42) (Gerald M. Richmond)

A Warm Dry Time

From about 7,500 years ago to about 3,800 years ago, snowline was above the summits and the climate was at times warmer than today. Treeline was higher and summers were probably somewhat longer. Blocks fell less frequently from the cliffs; movement of debris was rare on summit uplands. Slopes became stable under the protective cover of vegetation. Erosion was minimal in the mountains.

During this time paleo-Indians penetrated the region of the Park in summer in search of game. Possibly it was these people who first developed the Ute Trail, named for its use by a much later tribe from west of the mountains.

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Last Updated: 8-May-2007
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