In many ways the most interesting of all the ice streams on Mount Rainier is the Carbon Glacier, the great ice river on the north side, which flows between those two charming natural gardens, Moraine Park and Spray Park. (Fig. 19.) The third glacier in point of length, it heads, curiously, not on the summit, but in a profound, walled-in amphitheater, inset low into the mountain's flank. (Fig. 15.) This amphitheater is what is technically known as a glacial cirque, a horseshoe-shaped basin elaborated by the ice from a deep gash that existed originally in the volcano's side. It has the distinction of being the largest of all the ice-sculptured cirques on Mount Rainier, and one of the grandest in the world. It measures more than a mile and a half in diameter, while its head wall towers a sheer 3,600 feet. So well proportioned is the great hollow, however, and so simple are its outlines that the eye finds difficulty in correctly estimating the dimensions. Not until an avalanche breaks from the 300-foot névé cliff above and hurls itself over the precipice with crashing thunder, does one begin to realize the depth of the colossal recess. The falling snow mass is several seconds in descending, and though weighing hundreds of tons, seemingly floats down with the leisureliness of a feather.
These avalanches were once believed to be the authors of the cirque. They were thought to have worn back the head wall little by little, even as a waterfall causes the cliff under it to recede. But the real manner in which glacial cirques evolve is better understood to-day. It is now known that cirques are produced primarily by the eroding action of the ice masses embedded in them. Slowly creeping forward, these ice masses, shod as they are with débris derived from the encircling cliffs, scour and scoop out their hollow sites, and enlarge and deepen them by degrees. Seconding this work is the rock-splitting action of water freezing in the interstices of the rock walls. This process is particularly effective in the great cleft at the glacier's head, between ice and cliff. This abyss is periodically filled with fresh snows, which freeze to the rock; then, as the glacier moves away, it tears or plucks out the frost-split fragments from the wall. Thus the latter is continually being undercut. The overhanging portions fall down, as decomposition lessens their cohesion, and so the entire cliff recedes.
A glacier, accordingly, may be said, literally, to gnaw headward into the mountain. But, as it does so, it also attacks the cliffs that flank it, and as a consequence, the depression in which it lies tends to widen and to become semicircular in plan. In its greatest perfection a glacial cirque is horseshoe shaped in outline. The Carbon Glacier's amphitheater, it will be noticed, consists really of two twin cirques, separated by an angular buttress. (Fig. 15.) But this projection, which is the remnant of a formerly long spur dividing the original cavity, is fast being eliminated by the undermining process, so that in time the head wall will describe a smooth, uninterrupted horseshoe curve.
In its headward growth the Carbon Glacier, as one may readily observe on the map, has encroached considerably upon the summit platform of the mountain, the massive northwest portion of the crater rim of which Liberty Cap is the highest point. In so doing it has made great inroads upon the névé fields that send down the avalanches, and has reduced this source of supply. On the other hand, by deploying laterally, the glacier has succeeded in capturing part of the névés formerly tributary to the ice fields to the west, and has made good some of the losses due to its headward cutting. But, after all, these are events of relatively slight importance in the glacier's career; for like the lower ice fields of the Nisqually, and like most glaciers on the lower slopes of the mountain, the Carbon Glacier is not wholly dependent upon the summit-névés for its supply of ice. The avalanches, imposing though they are, contribute but a minor portion of its total bulk. Most of its mass is derived directly from the low hanging snow clouds, or is blown into the cirque by eddying winds. How abundantly capable these agents are to create large ice bodies at low altitudes is convincingly demonstrated by the extensive névé fields immediately west of the Carbon Glacier, to which the name Russell Glacier has been applied. It is to be noted, however, that these ice fields lie spread out on shelves fairly exposed to sun and wind. How much better adapted for the accumulation of snow is the Carbon Glacier's amphitheater. Not only does it constitute an admirably designed catchment basin for wind-blown snow, but an effective conserver of the névés collecting in it. Opening to the north only, its encircling cliffs thoroughly shield the contained ice mass from the sun. By its very form, moreover, it tends to prolong the glacier's life, for the latter lies compactly in the hollow with a relatively small surface exposed to melting. The cirque, therefore, is at once the product of the glacier and its generator and conserver.
Of the lower course of the Carbon Glacier, little need here be said, as it does not differ materially from the lower courses of the glaciers already described. It may be mentioned, however, that toward its terminus the glacier makes a steep descent and develops a series of parallel medial moraines (fig. 16), and that it reaches down to an elevation of 3,365 feet, almost 600 feet lower than any other ice stream on Mount Rainier. A beautiful cave usually forms at the point of exit of the Carbon River. (Fig. 17.)
Last Updated: 07-May-2007