Fig. 446.—View of the Márjelen Lake at the side of the Great Aletsch glacier, seen looking directly down from the summit of the Eggishorn (after a photograph by I. D. Scott).
Glacier lobe lakes.—Upon the sites of the former lobes of the Pleistocene glacier of North America are found the basins of the Laurentian River system, the largest freshwater lakes in the world. There has been much controversy concerning the manner of formation of these lakes, but the view which has seemed to have the largest following is that they were excavated by the eroding action of the continental glacier over the drainage basins of former rivers. It is but one phase of the long controversy between opposing schools, which have advocated on the one hand the efficiency of glacier ice as an eroding agent, and upon the other its supposed protection from the weathering processes. The positions and the outlines of the several lakes of the series sufficiently proclaim their connection with the former glacial lobes, and the name which we have adopted leaves the exact manner of their formation a still open question. The recognition of the importance of the glacial anticyclone, in giving shape to the glacier surface and in effecting a transfer of snow from the central to the marginal portions, has had the effect of emphasizing the relative importance of erosion under the marginal and lobate portions. Thus the importance of ice lobes has been greatly accentuated, though this applies only to the shaping of the basins and not in any important way to the impounding of the present waters. The present Laurentian Lakes owe their existence to the elevation by successive uplifts of the country to the northward and eastward, since the glacier retired from the lake region. When the ice front lay to the northward of the Ottawa River, the discharge of the upper lakes was by a channel through Nipissing River and Lake and thence down the Ottawa River to a gulf in the lower St. Lawrence. The uplift of the land has had the effect of raising a barrier where the former outlet existed, and diverting the waters to a roundabout channel by way of Detroit and Lake Erie (see [Fig. 365], [p. 335]).
Fig. 447.—Diagrams to illustrate the arrangement and the characters of rock-basin lakes, together with a map of such lakes from the Bighorn Mountains in Wyoming.
Rock-basin lakes.—The reversed grades which develop in a valley deepened by mountain glaciers—the back-tilted treads of the cascade stairway (see [p. 376])—furnish a series of basins hollowed in rock which are strung along the course of the valley like pearls upon a thread, or, far better, like the larger beads in a rosary ([Fig. 447]). This characteristic arrangement accounts for the name “Paternoster Lakes” which has sometimes been applied to them in Europe. Their positions in series within U-shaped mountain valleys, and their rock shores with characteristically smoothed and striated surfaces, make them easy of determination. In the higher portions of the valley, where the treads of the cascade stairway are relatively narrow, such lakes are often approximately circular in outline, but in the lower levels and upon wider treads they may be ribbon-like, though lakes of this type are to a large extent replaced in the lower levels by the valley moraine type or a combination of the two.
Fig. 448.—Convict Lake, a lake behind a moraine dam within a glaciated valley of the Sierra Nevadas, California (after a photograph by Fairbanks).
Valley moraine lakes.—The recessional moraines which mark the halting stations of mountain glaciers, while retiring up their valleys, form dams in the later river and so produce a type of lake which is in contrast with the morainal lakes which result from continental glaciation. They may, therefore, be distinguished by the name valley moraine lakes. Their positions on the bed of a U-shaped mountain valley, and the glacial materials which compose the dams, are sufficient for their identification ([Fig. 448]). Moraine Lake and Lake Louise in the Canadian Rockies are typical examples. Rock basin and valley moraine lakes may occur in alternation or combined in mountain valleys.