Geological Survey 18th Annual Report (Part II) Glaciers of Mount Rainier Rocks of Mount Rainier

Mount Rainier is a typical example of a lofty volcanic cone built largely of projectiles, but containing also many lava streams. It belongs with the class of volcanic mountains known as composite cones.

At one time the mountain was more lofty than it now is, its reduction in height being due to an explosive eruption which blew away the upper 2,000 feet of the original cone, leaving a great crater in the truncated remnant. After the loss of its summit the mountain was not symmetrical; the rim of its great summit crater was highest on the west, and lowest and probably breached on the eastward side.

At a more recent date two smaller craters were formed by mild explosive eruptions within the great crater and nearly filled it. The building of these secondary craters partially restored the symmetrical outline of the top of the mountain, but gave to it a dome-shaped instead of a conical summit. Whether glaciers were formed on the mountain previous to its truncation, or appeared only after that event, is unknown.

There is no evidence to show that the higher portion of the mountain was exposed to stream erosion previous to the gathering of perennial snows and the formation of glaciers. Broad névé fields were formed about the sides of the mountain, and extended from the summit far down its sides. The lower limit of the snow fields fluctuated with climatic changes, which also caused many variations in the size and extent of the glaciers flowing from them. The descending névé, on spreading as the area to be covered increased with decrease of elevation, became divided and gave origin to primary glaciers which carved deep canyons in the middle slope of the mountain. The backward cutting of the heads of the canyons led to the excavation of more or less well-defined amphitheaters.

Below the extremities of the primary glaciers the rivers formed by the melting ice carved deep canyons, not only in the lower slopes of the mountain, but in the platform or plateau on which it is situated.

Secondary glaciers, originating on the interspaces between the primary glaciers, excavated depressions, the rims of which now stand as ridges and peaks about the middle slopes of the main mountain mass.

The modifications in the general history due to a great extension of the glaciers during the Glacial epoch remain to be studied. All of the glaciers about Mount Rainier are receding and shrinking within the valley walls that confine them. Evidence of a marked climatic change unfavorable to the existence of perennial ice is thus indicated, which is in harmony with similar evidence furnished by nearly all the known glaciers of the northern hemisphere.