Part 4

Since we know that the streams did not begin to cut these gorges until the large Pleistocene glaciers had disappeared from those sites, approximately 10,000 years have been required for their formation. Thus the average maximum rate of down-cutting has been of the magnitude of 0.002 to 0.003 inch per year. With these figures as a foundation it is not so difficult to comprehend that the much larger valleys of the park could not have been eroded in less than several millions of years.

Another common, though seldom noticed, post-glacial feature of the park is the alluvial fan. These are fan-shaped accumulations of gravel deposited by swift, tributary streams where they enter a main valley. Some of them have grown so large as to dam the stream in the major valley and cause a lake (Figure 2). St. Mary, Lower St. Mary, Lower Two Medicine, and Waterton Lakes are held in by such dams. The alluvial fan of Divide Creek which holds in St. Mary Lake can easily be distinguished from Going-to-the-Sun Highway on the north side of the lake near its outlet. The St. Mary Entrance Station is located on this fan. The lower lake is dammed by a large fan built into the St. Mary Valley by Swiftcurrent Creek. The straight section of highway between the town of Babb and the St. Mary River bridge lies on the lower part of this fan. Inasmuch as the Pleistocene glaciers undoubtedly removed any such fans made previously, those which are present today must have been constructed since disappearance of the ice, and are then not more than 12,000 years old. Most of them are somewhat older, possibly by as much as two or three thousand years, than the gorges mentioned above, because the latter are located nearer the source of the glaciers, and their sites were thus still covered by ice after the fans had already begun to form. After the Pleistocene glaciers began their final retreat several thousand years elapsed before they disappeared from the mountains.

One of the most conspicuous of all post-glacial features is the talus cone, an accumulation of angular rock fragments which fall from cliffs. It is only at the base of a crevice or chimney that this material takes the apparent form of a distinct cone. Elsewhere it is referred to as a talus slope or simply as talus, or, in the parlance of some mountaineers, as scree. Although several thousand years have been required for their formation most talus accumulations in the park are still actively growing, especially in spring and early summer when rocks are pried loose by the alternate freezing and thawing of moisture within fractures. The artillery-like crack made when a falling rock crashes to the base of a high cliff is a familiar sound to anyone who has spent much time in the mountains.

The Future

We know that the processes of erosion and weathering will continue, that alluvial fans and talus cones will grow larger, and gorges will be eroded deeper, and as a result the mountains will be cut down to lower elevations. But, as we have seen, this event will require much time. If the present climate continues for a few more years our remaining glaciers will disappear, but there is nothing in geologic history which says they won't return again, possibly even to the size of their heyday in the Pleistocene. And if history repeats itself, and all past geologic history has been a repetition, then the mountains will eventually be worn down to an uneventful plain and the sea will invade the land again.

But certain breeds of man are the only despoilers of mountains that we need fear, so if the good citizens of our land keep the human invader and his dams and earth-moving equipment out of our national parks these grand mountains will endure for many thousands, yes, even millions of years.

Footnotes

Dr. Dyson worked as a ranger-naturalist in Glacier National Park for eight summers starting in 1935.

Argillite is the term used by geologists for a rock, originally a shale, which has been recrystallized or made harder by greater pressure. In external appearance it looks like shale.

A dike is like a sill in all respects except that it cuts across adjacent layers instead of paralleling them.

For a complete discussion of glaciers and their effects see Special Bulletin No. 2 (Glaciers and Glaciation in Glacier National Park) of the Glacier Natural History Association.

PRINTED IN U. S. A. BY GLACIER NATURAL HISTORY ASSOCIATION IN COOPERATION WITH NATIONAL PARK SERVICE DEPARTMENT OF INTERIOR 1953 O'NEIL PRINTERS--KALISPELL, MONTANA

Principal Aims of the GLACIER NATURAL HISTORY ASSOCIATION, Inc. Glacier National Park West Glacier, Montana

Organized for the purpose of cooperating with the National Park Service by assisting the Naturalist Department of Glacier National Park in the development of a broad public understanding of the geology, plant and animal life, history, Indians and related subjects bearing on the park region. It aids in the development of the Glacier National Park museum library, museums and wayside exhibits; offers books on natural history pertaining to this area for sale to the public; assists in the acquisition of non-federally owned lands within the park in behalf of the United States government; and cooperates with government projects in the completion and development of Glacier National Park as needed.

Revenue derived from the activities of the Glacier Natural History Association is devoted entirely to the purposes outlined. Any person interested in the furtherance of these purposes may become a member upon payment of the annual fee of one dollar. Gifts and donations are accepted for land acquisition or general use.

Bulletin No. 1--Motorists Guide to the Going-to-the-Sun Highway, 1947--Price 25 Cents. Bulletin No. 2--Glaciers and Glaciation in Glacier National Park, 1948--Price 25 Cents. Bulletin No. 3--Geologic Story of Glacier National Park, 1949--Price 25 Cents. Bulletin No. 4--Trees and Forests of Glacier National Park, 1950--Price 50 Cents. Bulletin No. 5--101 Wildflowers of Glacier National Park, 1952--Price 50 Cents.

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