Rocky Mountain National Park, Colorado

by Edwin C. Alberts

THE MOUNTAINS ARE MADE

The geological story of Rocky Mountain National Park is a long one. Most of its details are lost in the passage of hundreds of millions of years. Some of the story has been put together by scientists from bits of evidence scattered here and there. The evidence strongly indicates a certain chain of events, but no eyewitnesses are available to confirm the deductions. Few of these events can be proved to everyone’s satisfaction; we can but pass on to you some determinations that geologists have made.

Most of the rocks which you see in the park are crystalline and very ancient. The gneiss and schist were, in part, once sediments formed in seas, perhaps a billion years ago, under conditions about which there is little knowledge or general agreement. These sediments were buried beneath thousands of feet of other sediments, cemented and hardened into layers of sedimentary rock and later squeezed, crushed, and elevated by slow, ceaselessly working earth forces that produced mountains even in that ancient time. During this period the sedimentary rocks were changed to harder metamorphic rocks, probably because of deep burial under tremendous pressure and considerable heat. Masses of molten rock welled up into these earlier deposits and hardened under the earth’s surface. This later (though still very ancient) intrusive material is now exposed granite in many parts of Rocky Mountain National Park.

Rocks once buried miles deep are now exposed on Longs Peak, at an altitude of more than 14,000 feet.

These ancient mountains were gradually worn away by wind, rain, and other agents of erosion, which must have attacked the surface of the earth as vigorously then as now. With the passage of millions of years, these mountains were reduced to a lowland. Another sea gradually lapped over the land where mountains had been, and once again sediments were dropped in its bottom. This new invasion of the ocean affected the park region during many millions of years in which the dinosaurs dominated the earth.

In response to little-understood rhythms of the earth’s crust, which have lifted mountains ever so slowly at great intervals all over the world, the seas drained away as the crust rose again, and the rising land once more became subject to the ceaseless attack of erosion. This uplift—which began about 60 million years ago—originated the system of mountain ranges and basins that today give Colorado its spectacular scenery and much of its climate. This great period of mountain-making is called the Laramide Revolution, from its early recognition in the Laramie Basin region of Wyoming.

The Front Range, of which this park preserves a choice sample, was buckled in the fashion of a great long wrinkle in a carpet. This “roll” of rock was about 200 miles long and some 40 miles across. In its earlier stages it was covered by the arched-up sediments, but, as time passed and erosion continued, the inner core of earlier crystalline rocks was exposed once again. Today, all traces of the former thick mantle of sedimentary beds are gone from the park. They are still present beneath the plains to the east and the basins to the west, and the cut-off ends of some of them now lie exposed in a tilted position against both east and west flanks of the mountains. The sandstones of some of the hogback ridges crossed by the approach roads from Lyons and Loveland are a part of this once continuous overburden.

Uplift continued intermittently during many millions of years. In the western section of the park, volcanic eruptions took place. Specimen Mountain is the remnant of a volcano; some of its flows are seen today as the cliffs behind Iceberg Lake, on Trail Ridge Road. Great sheets of lava and other volcanic rocks piled in layers now make up much of the Never Summer Range. Eventually, these rocks, too, will be stripped away by the relentless work of erosion; this will require millions of years.

An unusual feature of the landscape here is the rolling, sometimes flattened character of many mountain summits. Trail Ridge Road crosses several miles of one of these summits—a gently rolling upland above 11,000 feet. These mountaintops appear to be all that is left of an old land surface that once may have been continuous far eastward over the area occupied today by the Great Plains. Such surfaces, of which the mountains in the park show many good remnants, are called peneplains. Their presence atop the mountains is a part of the evidence suggesting that the range had been worn down by erosion to a fairly flat upland a few million years ago. Then renewed uplifting occurred, and streams draining the highland gradually cut canyons two or three thousand feet into the elevated surface.