The Work of Glaciers

These canyons were filled by glaciers at intervals during the million years of the Ice Age. This period saw the formation of vast ice fields over much of northern North America. The causes of the Ice Age are complex, but its effects on our landscape are marked and convincing.

Every large high-altitude canyon in what is now Rocky Mountain National Park became filled with snow, much of which, under pressure, turned to ice. Thus, the glaciers by their own great weight moved with slow but tremendous power—broadening, deepening, and straightening the twists and turns of the original river-cut valleys. Great bowls, or cirques, were scooped out bit by bit at the glacier sources. These glaciers quarried and removed untold millions of tons of rocks from the upper reaches. Many of the cliffs and lakes of the park are the results of excavating done by the mountain glaciers.

SANDSTONE HOGBACKS FLANK THE MOUNTAINS ON THE EAST. SCENE NEAR MOUTH OF BIG THOMPSON CANYON, WEST OF LOVELAND.

REMNANTS OF AN EROSION SURFACE ON PEAKS SOUTH OF TRAIL RIDGE ROAD.

These glaciers were entirely local; they did not extend down to the plains in this region. At what is now an altitude of about 8,200 feet—just below our present Moraine Park, Horseshoe Park, Wild Basin, and a few miles below Grand Lake in the Colorado River valley—the glacier fronts melted as fast as the ice advanced. It was there that most of the rock debris plucked from higher up was dropped. Piles of rock are scattered over most of the meadowlands of this general altitude. These rock deposits are called moraines—ridges. and heaps, or scattered masses, of unsorted rock debris dumped where they settled from the melting ice. Classic examples of moraines may be seen in Moraine Park, named for these special features. You can learn more about them at the Moraine Park Museum.

When climatic changes caused the glaciers to melt back faster than they advanced, the moraines, like modern dams, formed lakes behind them whenever stream drainage from the shrinking glaciers was checked. Several such lakes, now silted in and changed to green meadows, occupied lower regions in the park. In Horseshoe Park, lake-shore terraces of an Ice Age lake still remain. Near Wild Basin Ranger Station is Copeland Lake, occupying a basin believed to have been left by the melting of a huge block of ice buried in a mass of glacial debris. The pastoral beauty of the lower mountains, then, owes most of its quality to these now vanished glaciers.

THE HIGH MOUNTAIN LAKES ARE SET AMONG THE SCARS OF GLACIER EXCAVATION. ARROWHEAD LAKE, IN GORGE LAKES CANYON.

MORAINES EXTENDING INTO THE MEADOWS, SEEN FROM MANY PARKS CURVE ON THE TRAIL RIDGE ROAD.

The glaciers invaded the park valleys at least twice during the Ice Age. Usually, two distinguishable ages of moraines can be seen; the older and more extensive one is made up of well-weathered, "rotten" boulders and finer material, while the newer ones are scarcely altered at all. It is thought that the last great glaciers retreated only some 12,000 years ago. Indians lived on the plains at that time!

It is by no means certain that the Glacier Age is entirely a thing of the past. Five small "glacierets" of geologically recent origin—Taylor, Andrews, Tyndall, Rowe, and Sprague's Glaciers—exist today in the park but are trivial when compared to the "kingsized" earlier glaciers. Since they are ice and they are moving (although very slowly), they are glaciers by definition. They are accessible only by arduous foot travel, but the first three are visible from heavily traveled roads and trails of the park.

BIERSTADT RIDGE, A HUGE GLACIAL MORAINE, SEEN EN ROUTE TO BEAR LAKE.

ANDREWS GLACIER, LATE IN SEASON. NOTE CREVASSES IN UPPER LEFT.

The story of the geological events, as we have seen, is long but interesting. The landscape of today, we now realize, is transient. It is the contemporary product of processes which have been working day and night throughout geologic time. These processes will certainly continue into the future, and the present landscape will inevitably change. Every year sees some little modification here and there. These small changes are not linked in our thinking with the vast sweep of geologic time, probably because our own lives are so very short. With some reflection we seem to catch glimpses of eternity as we examine the ancient gray cliffs and mighty peaks of Rocky Mountain National Park.