FRONT OF LEWIS RANGE, NORTH SIDE OF SWIFTCURRENT VALLEY. THE LEWIS OVERTHRUST LIES AT THE BASE OF THE CLIFF. THREE LARGE TALUS CONES ARE VISIBLE BELOW MT. ALTYN ON THE LEFT. (DYSON PHOTO)
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
[1]Dr. Dyson worked as a ranger-naturalist in Glacier National Park for eight summers starting in 1935.
[2]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.
[3]A dike is like a sill in all respects except that it cuts across adjacent layers instead of paralleling them.