SLUISKIN RIDGE AND COLUMBIA CREST
From a photograph copyright by A.H. Barnes
Beginning then in its cirque, as a river often begins in its lake, the glacier flows downward, river-like, along a course of least resistance. Here it pours over a precipice in broken falls to flatten out in perfect texture in the even stretch below. Here it plunges down rapids, breaking into crevasses as the river in corresponding phase breaks into ripples. Here it rises smoothly over rocks upon its bottom. Here it strikes against a wall of rock and turns sharply. The parallel between the glacier and the river is striking and consistent, notwithstanding that the geologist for technical reasons will quarrel with you if you picturesquely call your glacier a river of ice. Any elevated viewpoint will disclose several or many of these mighty streams flowing in snakelike curves down the mountainside, the greater streams swollen here and there by tributaries as rivers are swollen by entering creeks. And all eventually reach a point, determined by temperature and therefore not constant, where the river of ice becomes the river of water.
Beginning white and pure, the glacier gradually clothes itself in rock and dirt. Gathering as it moves narrow edges of matter filched from the shores, later on it heaps these up upon its lower banks. They are lateral moraines. Two merging glaciers unite the material carried on their joined edges and form a medial moraine, a ribbon broadening and thickening as it descends; a glacier made up of several tributaries carries as many medial moraines. It also carries much unorganized matter fallen from the cliffs or scraped from the bottom. Approaching the snout, all these accumulations merge into one moraine; and so soiled has the ice now become that it is difficult to tell which is ice and which is rock. At its snout is an ice-cave far inside of which the resultant river originates.
But the glacier has one very important function which the river does not share. Far up at its beginnings it freezes to the back wall of its cirque, and, moving forward, pulls out, or plucks out, as the geologists have it, masses of rock which it carries away in its current. The resulting cavities in the back of the cirque fill with ice, which in its turn freezes fast and plucks out more rock. And presently the back wall of the cirque, undermined, falls on the ice and also is carried away. There is left a precipice, often sheerly perpendicular; and, as the process repeats itself, this precipice moves backward. At the beginning of this process, it must be understood, the glacier lies upon a tilted surface far more elevated than now when you see it in its old age, sunk deep in its self-dug trench; and, while it is plucking backward and breaking off an ever-increasing precipice above it, it is plucking downward, too. If the rock is even in structure, this downward cutting may be very nearly perpendicular, but if the rock lies in strata of varying hardness, shelves form where the harder strata are encountered because it takes longer to cut them through; in this way are formed the long series of steps which we often see in empty glacial cirques.
By this process of backward and downward plucking, the Carbon Glacier bit its way into the north side of the great volcano until it invaded the very foundations of the summit and created the Willis Wall which drops avalanches thirty-six hundred feet to the glacier below. Willis Wall is nearly perpendicular because the lava rock at this point was homogeneous. But in the alternating shale and limestone strata of Glacier National Park, on the other hand, the glaciers of old dug cirques of many shelves. The monster ice-streams which dug Glacier's mighty valleys have vanished, but often tiny remainders are still seen upon the cirques' topmost shelves.