ROCKY MOUNTAIN A Guide to the Geology of Rocky Mountain National Park, Colorado

AGES OF deposition were followed, near the close of the pre-Cambrian (fig. 1), by uplift of the ocean floor caused by forces within the earth which are but little understood. The sea receded and mountains began to rise where it had been. They were not the Rockies but ranges vastly older. The uplift took place very slowly, so slowly in fact that the only indication of it was an occasional earthquake as some rock bed cracked under the irresistible pressure of the folding. There may have been volcanic eruptions when molten rock from some reservoir deep within the laboring earth found its way to the surface, but there was no life upon the land to be conscious of such disturbances.

That molten granite was injected into the ancient sediments is known, for it can be seen today in the light-colored bands and stringers which penetrate the dark schist. In some places the granite has pushed aside the older rock or has enveloped fragments of it in its mass until it appears, to the casual observer, as if the granite were the original rock and the schist the intruder. There are excellent exposures of both rocks on Longs Peak, and the granite has been called Longs Peak granite (fig. 2). These ancient rocks form the bedrock over most of Rocky Mountain National Park, the lava flows of Specimen Mountain being the one exception.

Figure 2.—Longs Peak from the east. At the foot of the 1,500-foot cliff Chasm Lake occupies the bed of a glacial cirque, or collecting basin of a former glacier. Another cirque lies in the valley to the left. The white bands of rock on the walls of this valley are granite which, as molten rock, was forced between the layers of the darker beds. These dark rocks may be almost one billion years old. Photograph by Carroll H. Wegemann

THE DESTRUCTION OF THE FIRST MOUNTAINS

Uplift and intrusion were followed by a long period in which, through millions of years, the solid rock of the mountains was slowly disintegrated and washed away by the agents of weathering and erosion. Streams cut deep mountain canyons which were widened into valleys, and the mountains between slowly lowered until their towering summits were reduced to a rolling plain. The formation by erosion of such a plain must have taken place at comparatively low elevation, otherwise the streams would have had sufficient fall to continue the deepening of their valleys and to produce a rough hilly country rather than a plain.

There is good evidence that a rolling plain was produced. In the foothills of the mountains the rock beds, originally horizontal, have been turned up on edge by the mountain folding so that their edges can be examined. The top of the pre-Cambrian rocks on which the younger sedimentary rocks lie is here seen to be a plane surface which cuts across the old schists and granites and is obviously erosional.