Fig. 426.—Schematic diagram to show the manner of formation of glacier cornices.
One further ice feature shaped by differential melting around rock particles remains to be mentioned. Wherever the seasonal snowfalls of the névé are exposed in crevasses, they are generally found to be separated by layers of dirt, and lines of pebbles similarly separate those ice layers which are revealed at the foot of the glacier. In either case, if the sun’s rays can reach these layers in an opened crevasse, the half-buried rock fragments are warmed by the sun upon their exposed surfaces and slowly melt their way down the ice surface, thus removing from it a thin layer of snow or ice and causing that part above the pebble layer to project like a cornice. This process will go on until the overhanging cornice protects the pebbles from any further warming by the sun, but each lower pebble layer that is reached by the sun will produce an additional cornice, so that the original surface may at the bottom have been retired by the process a number of inches. These features are described as glacier cornices ([Fig. 426]).
Fig. 427.—Superglacial stream upon the Great Aletsch glacier.
Glacier drainage.—Already in the early morning of every warm summer day, active melting has begun upon the surface of the Swiss glaciers. Rills of icy water soon make their way along depressions upon the surface, and are joined to one another so that they sometimes form brooks of considerable size ([Fig. 427]). Such streams continue their serpentine courses until these are intersected by a crevasse down which the waters plunge in a whirling vortex which soon develops a vertical shaft of circular section within the ice. Such shafts with their descending columns of whirling water are the well-known moulins, or “mills”, which may be detected from a distance by their gurgling sounds. The first plunge of the water may not reach to the bottom of the glacier, in which case the stream finds a passageway below the surface but above the floor until another crevasse is encountered and a new plunge made, here perhaps to the bottom. Once upon the valley floor the stream is joined by others, and pursues its course within an ice tunnel of its own making ([Fig. 421], [p. 394]) until it issues at the glacier front.
The coarser of the rock débris which was gathered up by the stream upon the glacier surface is deposited within the tunnel in imperfect assortment (gravel and sand), while all finer material and that lifted from the floor (rock flour) is retained in suspension and gives to the escaping stream its opaque white appearance. This glacier milk may generally be traced far down the valleys or out upon the foreland, and is often the traveler’s first indication that a range which he is approaching supports glaciers.
Fig. 428.—Ideal form of the surface left on the site of the apron of a piedmont glacier. M, moraine; T, outwash; C, basin usually occupied by a lake; D, drumlins (after Penck).
Deposits within the vacated valley.—For every excavation of the higher portions of the upland through glacial sculpture, there is a corresponding deposit of the excavated materials in lower levels. So far as these materials are deposited directly by the ice, they form the lateral, medial, ground, and terminal moraines already described. A considerable proportion of them are, however, deposited by the water outside the terminal moraine; but as with the shrinking glacier the ice front retires in halting movements over the area earlier ice-covered, the terminal moraines are ranged along the vacated valley as recessional moraines, each with a valley train of outwash below. About the apron of the piedmont glacier, such deposits are particularly heavy ([Fig. 428]). During the “ice age” the Swiss glaciers extended down the valleys below the existing ice remnants and spread upon the Swiss foreland as great piedmont glaciers such as may now be seen in Alaska. To-day we find there moraines and glacial outwash, a lake in the middle of the apron site, and sometimes a group of radiating drumlins like those found within the ice lobes of the continental glacier in southern Wisconsin ([Fig. 429], and [Fig. 344], [p. 317]).