A Guide to the Geology of Rocky Mountain National Park, Colorado
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WE HAVE now brought our history of the park down to a series of events which took place in comparatively modern times, geologically speaking; that is, within the last million years. This epoch is called the Ice Age, or the Pleistocene.

About one million years ago the climate of the earth grew cold—just why is not known. Probably it was owing to a combination of causes one of which may have been a variation in the amount of the sun's heat due to sun spots. It was cold enough so that in regions of heavy snowfall, like that around Hudson Bay, the snows of one winter did not entirely melt in the following summer. Snow fields began to accumulate, and as they grew in depth the snow was turned to ice by compaction.


The great fields resembled the present ice caps of Greenland and the Antarctic. They were thousands of feet thick, and the ice at the base, unable to support the weight of the ice above, was squeezed out in all directions, gradually spreading until it covered much of the northern part of the continent. These great ice sheets were known as continental glaciers. The force of their flow was determined by the thickness of the ice in their areas of accumulation; they could not move into the higher altitudes and so never reached Colorado.

There were five separate cold periods and five different advances of the ice sheets, separated by extended warm periods in which the glaciers melted back or entirely disappeared. These interglacial periods had durations of from 25,000 to 325,000 years.

The last or Wisconsin Ice Sheet as it is called, melted away only 25,000 years ago, and the world today may be experiencing an interglacial period. In this case another ice advance is to be expected sometime in the distant future.


At the time that the great ice sheets were spreading over the northeastern part of North America, valley glaciers accumulated in the mountain gorges of the Rockies and moved down these stream-cut valleys as rivers of ice. They were in no way connected with the continental ice sheets, except that they accumulated during the same cold periods and presumably can be correlated with them. To these ancient ice rivers is due much of the magnificent scenery of the present mountains, particularly the perpendicular cliffs, towering peaks, and the beautiful lakes which lie between.

Only two of the possible five different advances of the valley glaciers can be definitely recognized in the park by differences in the weathering and decay of the gravel and boulders which the ice carried frozen in its mass and which were deposited when the ice melted. This is true because the last glaciation destroyed all evidences of the earlier advances, except in places where the earlier glaciers extended beyond the limits of the last advance of the ice. But we are getting ahead of our story; we should begin by describing the accumulation and flow of valley glaciers.

Snow will accumulate to the greatest depths where the snowfall is greatest and where the temperature is sufficiently cold to prevent melting. Such conditions are found near the heads of mountain valleys. Here, even in the comparatively mild climate of the present, the snow is drifted by the strong west winds of winter into the heads of valleys east of the divide where it feeds such modern glaciers as Tyndall and Andrews. If you have time by all means visit these glaciers which lie about 5 miles from Bear Lake. You may need horses, but the trip is well worth while.

Let us assume that you have climbed up the trail to the Flattop peneplain covered with its angular boulders, and stand on the edge of the cirque of either Tyndall or Andrews Glacier. The cirque is the great bowl-shaped depression cut by the ice at the head of the valley. In it the snow accumulates and by its own pressure is hardened into ice. As the snow and ice increase in depth, their weight proves too much for the strength of the ice at the base and this is squeezed outward and begins its flow down the steep valley floor. Bear in mind that the valley was first cut by streams and that the glacier is merely taking advantage of the channel thus prepared for it.

On the valley floor underneath the ice are loose rocks and boulders. The great weight of the ice above presses the basal ice around these until they are enveloped by it and carried down the valley as the ice advances. As they are scraped along over the bedrock they grind and polish it, leaving long scratches which indicate the direction of the ice flow. The grinding rocks are themselves ground smooth on the side which is down, and occasionally, striking some obstruction, may be turned over in the moving ice and ground on another side. Thus, instead of being rounded as in a stream, they are ground with angular sides or facets and are termed "faceted boulders." The grinding of the rocks upon each other produces a white powder or "rock flour" which gives to the waters that flow from the melting ice a peculiar white chalky appearance. Upon reaching the first lake below the glacier the coarser particles settle out and the remaining suspended particles cause the water to appear blue in color.

As the glacier grinds down the valley, sweeping away projecting points of rock and straightening its course, the cliffs are undercut and rock slides pour masses of earth and rock fragments upon the surface of the ice. As the ice continues its flow it comes into lower and warmer altitudes until finally the point is reached at which the ice melts as rapidly as it advances. Here the load of boulders, gravel, and sand which it carries is piled in a ridge along its melting front. Such a ridge is called a terminal moraine. The material which is plowed up and dumped along the sides of the glacier is called the lateral moraine (fig. 7).

If the general climate grows warmer after a terminal moraine is formed, the ice front melts back until it stands at a higher altitude. At this point where melting and rate of advance just balance, another terminal moraine is built. Such a moraine is called a recessional moraine. Most of these moraines form natural dams across the valleys, producing the lakes for which this mountain country is famous (fig. 16). Where several recessional moraines are present a series of lakes may be formed, such as the series in the valley below Tyndall Glacier (fig. 9). At the foot of Andrews Glacier is the lake of figure 8. The recessional terminal moraine forming the dam of sand, gravel, and boulders which impounds this lake can be clearly seen from the cliff above the glacier.

Figure 8.—Andrews Glacier from the terminal moraine below the lake. This small glacier extends but little beyond the cirque or rock basin in which its snow and ice accumulate. That the movement of the ice is greater in the center than on the sides is shown by the curved lines formed by dust upon the surface. Photograph by Carroll H. Wegemann.

The peculiar shape of the valleys below Tyndall and Andrews Glaciers may be noted. Although originally stream cut, they no longer have the typical V-shape of stream-cut canyons, but are U-shaped in cross section. This shape is typical of glaciated valleys and is due to the gouging action of the ice, which lowers the floor of the valley at the same time that it grinds out its sides (fig. 9).

Figure 9.—Dream Lake with Hallett Peak on the left and Flattop Mountain on the right. In the center is the valley below Tyndall Glacier. This was originally stream-cut but has been reshaped by the glaciers of the Ice Age. Note that the valley is U-shaped in cross section, that it has been smoothed and straightened by the grinding away by the ice of projecting points of rock, and that its floor is covered with glacial boulders. Photograph by George A. Grant

Let us now turn our attention to the surface of the ice itself. Dust and dirt have accumulated on it and, as the ice moves, these have been concentrated in streaks which bend downward near the middle of the glacier showing that the flow is more rapid in the middle than on the sides because of friction against the rock walls (fig. 8).

The valley floor beneath the glacier is apparently uneven, and the ice therefore moves forward over a series of levels and steep slopes like a river cascading over rapids with smooth reaches of water between. This peculiarity of glaciers is much more apparent on long glaciers than on such small ones as we are examining, and it is responsible for the formation of cracks or crevasses which occur where the ice breaks as it flows over the crests of the steep slopes. These crevasses, where covered and concealed by drifting snow, are extremely dangerous to one traversing the surface of a large glacier. They lead downward to depths of 50 feet or more and in some places open into ice caves through which sweep streams of water formed from the melting ice.

Irregularities in the rock floor over which the glacier moves may be caused by differences in hardness of the bedrock or by the presence of zones of fractures in the ancient rocks of the mountains. Such zones weaken the rocks and make them less able to withstand erosion. Water from the melting ice on warm days seeps into the fractures and on freezing tends to loosen fragments of the rock. These fragments are plucked out by the pressure of the moving ice, thus lowering the valley floor in the fracture zones more than in the unfractured rock and causing the steep inclines. Where two glaciers join, the increased amount of ice below the junction increases the erosive power of the glacier and produces the same effect.

Mention has been made of the cirque or basin-like depression at the head of a glacier. Apparently this is formed by the plucking action of the ice, just described. It is the point at which the glacier begins its work, and it is the last retreat of the shrinking mass of ice as it melts away when the climate changes. In brief, it is the point at which glacial action is longest continued. As the ice plucks out more and more of the loosened rock fragments which its freezing waters pry apart, the glacier eats its way back into the mountain at the head of the valley and in doing so produces a cliff by undermining. After the ice entirely melts away the peculiar shape of the basin or cirque thus formed is apparent.

One of the finest examples of such a cirque is that above Chasm Lake on the northeast side of Longs Peak (fig. 2). The vertical cliff which forms the wall of this cirque is more than a quarter of a mile in height. During the Ice Age, Longs Peak was surrounded by several glaciers, the cirques of which cut back into the mountain on all sides. The spectacular scenery of the peak's lofty cliffs is due entirely to this cutting by ice. Another fine example is the cirque on Sundance Mountain to be seen from Trail Ridge Road just above Rainbow Curve (fig. 11). These are cited merely as examples, for cirques are to be seen almost everywhere in the higher parts of the park.

Figure 11.—Sundance Mountain from Trail Ridge Road above Rainbow Curve. The basin cut into the mountainside is the cirque or gathering basin of a former glacier. In its formation water freezing in crevices of the rock pried off fragments by its expansion and these, enveloped by the glacial ice, were carried away as the ice moved. The cirque was thus constantly enlarged and the great cliff at its back maintained by rock falls due to undercutting. Most of the cliffs in the park are the work of former glaciers. Photograph by Carroll H. Wegemann

Whether the glaciers of today date back to the Ice Age is a difficult question to answer. They may be the last remnants of those ancient rivers of ice, slowly dying in the warmth of the present climate; or, due to some change in temperature or wind direction, they may be newly arrived in the valleys left vacant by their predecessors. One likes to believe that the first interpretation is the true one.

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Last Updated: 11-Dec-2006