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

A Time in Question

There is as yet no record of what happened in Rocky Mountain National Park from the end of the Bull Lake Glaciation to the beginning of the last glaciation. However, studies in progress of plant and tree pollen obtained from sediments of this interval in other areas show changes in the local vegetation of those areas that reflect broad changes in climate. These climatic changes probably were sufficient to have affected the Park. They suggest that the time of retreat and disappearance of the late Bull Lake glaciers, possibly about 87,000 years ago and certainly before 70,000 years ago, was followed by a warm interval, somewhat like today. Then, about 70,000 years ago, the climate turned markedly colder, and glaciers may have advanced again about 45,000 years ago. If so, they probably receded about 32,000 years ago when the climate warmed again. Finally, perhaps about 27,000 years ago, the cold climate, which led to the beginning of the last widespread glaciation, set in.

The Last Glaciation

Cooling of the climate at the beginning of the last glaciation caused snowline to be lowered again. Present snowline (the average lower limit of névé in late summer) in the Park is at about 11,800 feet. This is halfway between the average altitude of the lower ends of the present glaciers (11,100 feet) and the average altitude of the crests of the cirque headwalls (12,500 feet). The average altitude of the outermost end moraines of the last glaciation is about 8,270 feet. The average altitude of the crests of cirque headwalls during the last glaciation was the same as now (12,500 feet). A point halfway between, at 10,400 feet, is a reasonable estimate of the altitude of snowline in the Park during the maximum extent of the last great glaciers, and is about 1,400 feet lower than present snow line. This does not mean that perennial snow lay everywhere above 10,400 feet, but only that it lay above this altitude on the glaciers and in places so sheltered that it did not melt in summer.

Comparison of snowline today and at peak of last glaciation.

Head of valley of Cache La Poudre River through which ice overflowed from the Colorado River. View from Trail Ridge Road. (Fig. 22) (Richard DeLong)

The glaciers formed in most of the same cirques as did the Bull Lake glaciers. From these basins, they advanced down the canyons, and may have attained their maximum length about 15,000 years ago.

pine needles and cone

This last of the major glaciations of the Ice Age is called the Pinedale Glaciation. A glance at the map will show the extent of the ice in and around Rocky Mountain National Park. On the east side of the mountains, glaciers stemmed from groups of cirques along the Continental Divide and in the Mummy Range. They flowed as tongues, mostly 8 to 10 miles long, down each major valley. The longest, about 13 miles long, was in the valley of Big Thompson River. The snouts of the glaciers rose abruptly 200 to 300 feet. In the upper parts of the canyons, the ice was 1,000 to 1,500 feet thick.

On the west side of the mountains, névé fields lay along the entire length of the Continental Divide and also on the high ridges to the west. As a result, the glaciers were nourished not only from their heads but also from their sides, and built up to such a thickness that they overflowed across the ridges in many places to form a complex network of ice streams. Some of these glaciers were over 1,500 feet thick.

Extent of glaciers in Rocky Mountain National Park and adjacent Front Range during the Pinedale Glaciation. Greater extent of preceding Bull Lake glaciers, where known, is shown by dotted lines. (click on image for an enlargement in a new window)

In the northwest part of the Park, glaciers formed in cirques along the crest of the Never Summer Mountains. They flowed eastward to the valley of the Colorado River where they merged with glaciers flowing westward from the Continental Divide to form the largest glacier in the Park. This great river of ice, from its head west of La Poudre Pass to its terminus at the chain of islands across Shadow Mountain Lake, was 20 miles long, the longest in the Park. Its headward part overflowed eastward across Milner Pass into the valley of the Cache la Poudre River.

U-shaped glaciated canyon of Fall River above Horseshoe Park. (Fig. 23) (Wayne B. Alcorn)

Much of the scenic beauty of the Park today is the work of the last great glaciers. The cirque basins were orignally roughed out by the early and intermediate glaciers. However, the steep headwalls along the Continental Divide as seen from Trail Ridge Road, or the awe-inspiring east face of Longs Peak as viewed by a climber at Chasm Lake, were steepened, deepened, and brought to their present rugged condition by the quarrying action of the last glaciers. The beautiful tarns within the cirques, such as Chasm Lake, or Frozen Lake at the head of Glacier Gorge, were scoured from bedrock by stones frozen and dragged along in the bottom of the glaciers.

Chain Lakes. Fourth, Spirit, and Verna Lakes, scoured by ice in bedrock of upper canyon of East Inlet; view from Tanina Peak. (Fig. 24) (James Larson)

Below the cirques, the great U-shaped canyons, such as the canyon of Fall River above Horseshoe Park, or Hayden Gorge, or Tyndall Gorge, so often photographed with Hallett Peak from Dream Lake, were deepened, steepened, and scoured by the last glaciers—though, like the cirques, they had been shaped by earlier glaciers. In places, the upper limit of glacial polish and other scour features high on the canyon walls show approximately where the surface of the ice lay. Above, jagged and fractured cliffs rise to the rim of the gently sloping uplands.

The ice both quarried and polished the canyon floors. Over the cliffed fronts of giant rock stairs, some more than 200 feet high, the glaciers flowed, broken at their surfaces by a maze of crevasses. At the bottom of the glaciers, ice or water penetrated fractures in the rock, loosening blocks and quarrying them from the cliff faces.

Below the cliffs the full abrasive force of stone-laden ice was brought to bear on the valley floor, scouring depressions now filled with water and linked together by streams to form a chain of lakes. Gorge Lakes, as seen from Trail Ridge, are typical, as are Emerald and Dream Lakes below Tyndall gorge and Sky Pond, Glass Lake, and others.

Elsewhere along the valleys the glaciers abraded large rock knobs, such as Glacier Knobs below Glacier Gorge, smoothing and shaping them into stream-lined forms. Smaller rounded and smoothed knobs and ledges abound along the valleys, especially near and above timberline where there is little soil or forest cover. When seen from a distance a group of these knobs resembles the backs of scattered sheep, from which their name, "roches moutonées," is derived.

Shallow glacial grooves showing the direction of local down-valley ice movement many be found on smooth slopes, rock ledges, and knobs in the upper valleys. Here also, glacial striations, the fine parallel scratches abraded on rock by stones and sand held in the base of a glacier, can be found. They too record the direction of ice movement of the last glaciers. Many rock surfaces have also been polished by sand and silt in the overriding ice. These features are less commonly found in the lower valleys because the soil and forest cover is more widespread, and because many have been removed by weathering of the bedrock. Some, easily seen, are along the one-way road up the canyon of Fall River, about 100 yards above the second switchback, where construction of the road has stripped away the soil mantle to expose the fresh, bare, grooved, striated and polished bedrock of the valley wall.

Glacially striated rock surface about 100 yards above second switchback of road up the canyon of Fall River. Arrow points at knife for scale. (Fig. 25) (Wayne B Alcorn)

The lower ends of some U-shaped canyons lie above the floors of main valleys, into which their streams descend in a series of cascades or falls. The valleys of Roaring River, Chiquita Creek, and Sundance Creek, all tributary to Fall River in Horseshoe Park, are examples. Such valleys, called hanging valleys, are believed to result from undercutting of the tributary valley by the greater eroding power of the main valley glacier. Though this is true in regions where glaciers were a few thousand feet thick, undercutting of the hanging valley of Chiquita and Sundance Creeks as much as 500 feet by a glacier at most only 800 feet thick in this part of the valley of Fall River during the last glaciation would not have been possible. Such undercutting, if of glacial origin, must have been accomplished by thicker ice during an earlier glaciation.

Glacial basin of Moraine Park bordered by large moraines of last glaciation (Pinedale). Helicopter view from southeast. (Fig. 26) (Wayne B Alcorn)

Most of the glaciers reached their outer limits in the lower valleys just inside the Park boundaries. Along these gentle slopes the solid ice fronts of the glaciers were shoved forward by the force of flowing ice behind. This thrusting from the rear caused the solid ice to break and shear up over itself, dragging up with it quantities of rock debris from beneath. The large basins of Horseshoe Park, Moraine Park, Glacier Basin, and Shadow Mountain Lake were excavated in this way.

The deposits of the last, or Pinedale Glaciation, cover the valley floors and form prominent, forested, end moraines and lateral moraines. These are large ridges, littered with numerous light-gray boulders. The end moraines loop around the lower ends of the valley basins. Lateral moraines extend from them upstream along both valley walls. The deposits are a loose mixture of sand and stones of many kinds, both large and small. The stones are mostly unbroken and unweathered. Few display glacial striations. The deposits have only a thin pale-yellow soil, or zone of weathering. Their surfaces are commonly hummocky, in places enclosing depressions and small ponds. On the whole, they look much fresher than deposits of the Bull Lake glaciers.

Bouldery Pinedale end moraines along Fall River above Aspenglen Campground. (Fig. 27) (Wayne B Alcorn)

Bouldery deposit of Pinedale lateral moraine along road to Adams Falls east of Grand Lake. (Fig. 28) (Wayne B Alcorn)

In the valley of Fall River, the Pinedale end moraine is the high bouldery ridge over which the highway passes, west of the Park entrance. From the upper end of Horseshoe Park the highway ascends the south side of the valley to Many Parks Curve through the south lateral moraine, whose contact with the deeply weathered gneiss of the valley wall is well displayed along the road. From turnouts and from Many Parks Curve you can see the north lateral moraine on the rim across the valley. Higher up, the road follows the inner edge of the south lateral moraine ridge which crosses the mouth of Hidden Valley, showing thereby that no glacier existed in that valley in Pinedale time.

There is no prominent end moraine at the lower end of Moraine Park, though boulders were deposited by the Pinedale ice as much as 220 feet above the Visitor Center on the slope of Eagle Cliff Mountain. High lateral moraines rising above both sides of the valley can be seen from the Visitor Center. As mentioned, their great size may be because they overlie Bull Lake moraines.

End moraine of Pinedale glacier in valley of Fall River. In foreground, the moraine blocks the mouth of Hidden Valley, which was not occupied by a glacier at that time. (Fig. 29) (Wayne B Alcorn)

The end moraine of the Pinedale glacier in Glacier Basin is crossed where the road ascends to the basin. From the campground, a magnificent panorama clearly shows the high lateral moraines of the last glaciation extending far up both sides of the valley. Should you climb to Bierstadt Lake, you can see that the lake is enclosed between the Pinedale lateral moraine and an older-looking lateral moraine of Bull Lake age which forms the higher ridge to the north.

In the lower part of the steep valley east of Longs Peak, the Pinedale end moraine breaches the end moraines of the Bull Lake glaciers and extends nearly to the highway.

Pinedale terminal moraine forming islands in Shadow Mountain Lake, a modern reservoir. View looking north from early Bull Lake moraine at foot of lake. (Fig. 30) (Wayne B Alcorn)

On the west side of the mountains, the end moraine of the Pinedale glacier in the valley of the Colorado River forms the peninsulars and islands that trend across the lower part of Shadow Mountain Lake. This artificial lake is dammed in the large basin excavated by the lower end of the glacier.

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Last Updated: 8-May-2007
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