Ice-dam lakes.—Whenever a continental glacier, either in advancing its front or in retiring, lies across the lines of drainage upon their downstream side, water is impounded along the ice front so as to form ice-dam lakes. Such lakes are found to-day in Greenland and in the southern Andes, and similar bodies of water of far greater size and importance came into existence in Pleistocene times each time that the continental glaciers of northern North America and Europe advanced upon or retired from suitably directed river systems. Thus above the Baltic depression, when the ice front lay to the eastward of the main watershed, each easterly sloping valley was obstructed by the ice and occupied by an ice-dam lake ([Fig. 444]), the beaches of which may all be traced to-day ([Fig. 445]).
Fig. 444.—Ice-dam lakes (in black) between the front of the late Pleistocene glacier of northern Europe and the divide near the Norwegian boundary (after G. de Geer).
One side of each ice-dam lake is formed by an ice cliff at the glacier front, and if the region is relatively flat, the remaining shores are likely to be formed by a marginal moraine which the glacier has abandoned in its retreat. In their smaller stages, therefore, ice-dam lakes on prairie country have the form of a crescent, which is the more pronounced because the waves by their attack upon the ice front flatten the curvature of its outline (see [Fig. 360], [p. 330]).
The life of an ice-dam lake is begun and ended in important changes of glacier outline, and after the draining of lakes by this process the land shores may be traced in beaches, and the ice margin by a water-laid moraine of low relief ([Fig. 359], [p. 330]).
A much smaller but in many respects similar ice-dam lake is to-day to be seen at the side of the Great Aletsch glacier, a mountain glacier of Switzerland. The traveler who makes the easy ascent of the Eggishorn may look directly down upon this crescent-shaped lake with its ice cliff on the glacier side (see [Fig. 446]).
Fig. 445.—Wave-cut terrace at an elevation of 177.5 meters above sea on the southern slope of the northern Dala valley north of Baggedalen in Sweden. To the right in the foreground is a peat bog (after Munthe).