Fig. 449.—Lake basins produced by successive slides from the steep walls of a glaciated mountain valley (after Russell).
Landslide lakes.—The sheer-walled valleys which are carved by mountain glaciers are too steep to long retain their perpendicularity when the support of the glacier has been removed. Aided by the ever present joint planes, which admit water to the rock, they succumb to frost action, and further give way in avalanches whenever the rock is of sufficiently porous material to become saturated with water. Landslides sometimes occur successively until the original valley wall has been replaced by a terraced slope. The treads of the steps in this terrace have generally a backward-sloping grade, so that basins are formed to be filled by relatively long and narrow lakes or by successions of small pools ([Fig. 449] and [plate 23 B]).
Fig. 450.—Lake Garda, a border lake upon the site of a piedmont apron at the margin of the Alpine highland (after Penck and Brückner).
When the avalanched material is so disposed as to dam the valley, much larger lakes of this type come into existence. During an earthquake which occurred on January 25, 1348, there was a landslide within the valley of the Gail, Carinthia, which destroyed seventeen villages and produced a lake which even to-day is represented by a great marsh.
Border lakes.—Whenever mountain glaciers push out their fronts beyond the borders of the mountain range by which they are nourished, they spread upon the foreland in broad aprons about which morainic accumulations are particularly heavy. This elevation of morainal walls about the margins of the aprons yields natural basins that are occupied by lakes so soon as the glacier retires its front within the valley. Because such lakes are found at the borders of upland districts they have been called border lakes. The beautiful Lakes Constance, Lucerne, Maggiore, Lugano, Como, and Garda ([Fig. 450]), on the borders of the Alpine highland, are all of this type.
Plate 23.
A. View of the American Fall at Niagara, showing the accumulation of rocks beneath (after Grabau).