Transition from the ice cap to the mountain glacier.—A study of existing glaciers leads inevitably to the conclusion that although subject to short period advances and retreats, yet, broadly speaking, glaciers are now gradually wasting away, surrounded by wide areas upon which are the evidences of their recent occupation. We are thus living in a receding hemicycle of glaciation.
Fig. 408.—Schematic diagram to show the relationships of glacier types formed in succession during a receding hemicycle of glaciation.
Many mountain districts which now support small glaciers only, or none at all, were once nearly or quite submerged beneath snow and ice. If once covered by an ice carapace or cap, our present interest in them begins at that stage of the receding hemicycle when the rock surface has made its reappearance above the surface of the snow-ice mass. At this stage intensive frostwork, the characteristic high level weathering, begins, and cirques develop above the scars of those earlier amphitheaters formed in the advancing hemicycle.
The piedmont glacier.—In this early stage of transition from the ice cap to the mountain glacier, the ice flows outward to the mountain front in ill-defined streams divided by the projecting ridges, and upon reaching the mountain front these streams deploy upon it so as to coalesce in a great stagnant ice apron whose upper surface slopes gently forward at an angle of a few degrees at the most ([Fig. 408], stage I). This is the piedmont glacier, a type found to-day in the high latitudes of Alaska and in the southern Andes ([Fig. 409] and [pl. 18 B]).
Fig. 409.—Map of the Malaspina glacier of Alaska, the best known of existing piedmont glaciers (after Russell).
During this stage the cirques may be but poorly defined, and ice flows in both directions from rock divides so that the streams transect the range, and later, after the glaciers have disappeared, may expose a pass smoothed and polished upon its floor and with striæ directed in opposite directions from the highest point. The pass of the Grimsel in Switzerland furnishes an excellent illustration of such earlier transection of the range.
