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

Glaciers Reborn

About 3,800 years ago the climate cooled again. Snow accumulated in the cirques, and small glaciers formed and began to move outward from the shelter of the headwalls into the cirque basins. These little glaciers were never more than half a mile long. Their end moraines are mostly in the cirque basins or just below them. The moraines are fresh and very bouldery. Stones in them show but little glacial wear because they were not carried far by the ice. Three and locally four such moraines are present in the most protected cirques, those facing north or northeast. The outermost two of the four have a thin soil cover that supports a grassy tundra vegetation. The third has almost no soil, but a few sparse pioneer plants grow on them, especially on their down-valley slopes. The innermost are fresh, with no soil or plant cover. Lichens are more abundant on the surface of boulders on the outer moraines than on the intermediate, and are very sparse or lacking on the inner. The size of individual lichen on the boulders and Carbon-14 dates of organic debris in the deposits tell us that these moraines were formed by small advances of the glaciers at different times. One of the two older moraines, and possibly both, formed between 3,800 and 2,550 years ago; the third formed between 1,850 and 950 years ago (100 A.D.-1000 A.D.); and the youngest formed during the last 300 years, chiefly about 100 years ago (1870 A.D.). This period, from 3,800 years ago to the present, is called the Little Ice Age or Neoglaciation.

Very bouldery moraine of late readvance of last glaciation below Alberta Falls. View from Bear Lake parking area. (Fig. 43) (Wayne B Alcorn)

The moraines can be seen only by climbing into the cirques or looking down into them from the Continental Divide. Among the most accessible are those below Andrews Glacier, Taylor Glacier, Chiefshead Glacier, and Mills Glacier. In some places, the little glaciers were so heavily laden with debris from the cliffs at their heads that, when the ice melted, a lobate glacier-shaped mass of rock debris was left behind. Such deposits are called rock glaciers. The deposits at the foot of Tyndall Glacier and Taylor Glacier are rock glaciers.

The Glaciers Today

The glaciers in the Park today are remnant from the last expansion of the small glaciers which began to develop in empty cirques about 3,800 years ago. They are not relicts of the glaciers of the Ice Age and are as different from them as they are from the large modern glaciers in Alaska. Some, in fact, may be only 300 years old, for they may have disappeared briefly during the dry years of the 11th and 12th centuries and re-developed during the 18th century. Today, these glaciers are barely alive. All are in east- or north-facing cirques where snow accumulation and shelter from the sun are greatest. Few show any significant forward motion, though growth lines can be seen on most. A few are merely masses of ice that is no longer moving. Snowline, or the lower level of névé on the glaciers in summer, is within a few yards of the outer edge of the ice. This means that the formation of new ice each year is barely able to replace the annual loss by melting and evaporation. Though some glaciers still extend to their innermost end moraines, none are actively growing against them. Many glaciers have receded from their end moraines, leaving a basin occupied by a small pond. Though a few of these ponds are milky, showing that glacial silt is still being carried into them by glacial meltwater, most are sufficiently clear to suggest that the glacier is inactive. The most obvious sign of decay is the extent to which the glaciers have wasted away: Their lower ends are thin, their middle parts concave, and their upper parts shrunken. A broad band of whitish fresh rock, from which the ice has downwasted, stares from the base of cliffs around the glaciers. Many glaciers are less than 250 feet thick, meaning that the ice is no longer thick enough to flow. They are literally starving for nourishment.

Two moraines of the older glaciers of the Little Ice Age, probably formed between 3,800 and 2,550 years ago, at head of canyon of Spruce Creek. (Fig. 44) (Gerald M. Richmond)

Modern active ice-cored rock glacier below Tyndall glacier (1960). (Fig. 45) (Chester O. Harris)

Modern glaciers
a. Tyndall glacier (1963) (Fig. 46) (National Park Service)
b. Rowe glacier (1952) (Fig. 47) (Thomas Hornbeim)
c. Sprague glacier (1950) (Fig. 48) (Dwight L. Hamilton)
d. Moomaw glacier (1963), a small mass of stagnant ice and névé. (Fig. 49) (Jack Dreibus)

Andrews Glacier is one of the few still active, due apparently to unusually strong drifting of snow across the Continental Divide at its head. Its profile is convex, it is more than 250 feet thick, and it shows some evidence of movement. But even Andrews Glacier is in a bare subsistence condition.

The most recent severe decay of the glaciers took place during the droughts of the 1930's. The upper parts of some glaciers are known to have thickened between 1947 and 1957, but no advance of the glacier snouts has been reported. It is surprising that they are able to exist at all, for they are among the most southerly in the Rocky Mountains and therefore most sensitive to changes in climate. Arapaho Glacier, at the head of the North Fork of Boulder Creek, just south of the Park, is the most southerly existing glacier. About 1,000 years ago, small glaciers probably existed in mountains as far south as Taos, New Mexico; about 3,000 years ago they existed in mountains as far south as Santa Fe.

Andrews Glacier through the years (a, top - e, bottom).
a. 1916 (Fig. 50) (National Park Service)
b. 1929 (Fig. 51) (National Park Service)
c. 1939 (Fig. 52) (National Park Service)
d. 1949 (Fig. 53) (George Hockman)
e. 1969 (Fig. 54) (Benjamin Slater)

And Tomorrow—?

The present interglacial period, that is, the time since the retreat of the last large glaciers in the mountains, has lasted about 12,000 years. Its warmest part was before the development of new small glaciers about 3,800 years ago. Scientists tell us that many of the interglacial periods which separated the major glacial advances of the ice age lasted from 10,000 to 15,000 years. Is another long cold period to be expected? Theoretically, this is possible, but positive signs are difficult to identify. Periods of a few years when winters have been harder than normal are well documented and remembered by older inhabitants; short periods when the climate has been warmer than normal, as during the 1930's, are remembered by many. Such short-term fluctuations in climate are normal and should not unduly concern us. Another long cold period may begin in a few millenia—or in a time about equivalent to that which has elapsed since the beginning of the Bronze Age. But until the small glaciers of today extend beyond their outermost moraines of 3,800 years ago in the high cirque basins, there would seem to be no cause for alarm. Of more significance to man would be a period of increased warmth and complete disappearance of the ice. A long-term drought could severely affect the semi-arid west. The small glaciers of the Park are very sensitive to climatic change. At present, they are barely holding their own.

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Last Updated: 8-May-2007
Copyrighted by Rocky Mountain Nature Association