8 ALBRECHT PENCE 



approach one another until they finally coalesce. While, however, 

 the surfaces must be arranged in a descending order, the same is 

 not the case with the bottom. It is stated from observation that 

 glaciers can also move on reversed slopes as long as they have a suffi- 

 cient surface slope ; that is, as long as the surface slope is considerably 

 greater than the reversed bottom slope. To keep up glacial move- 

 ment, it is necessary that the curves of successive cross-sections be 

 arranged in a descending order. If we therefore, have, at the lower 

 end of a glacier, a series of cross-sections of diminishing size, their 

 bottoms may rise, if their surfaces slope so steeply that their centers 

 of gravity form a continually descending line. Therefore we find 

 in the bottoms of glaciated valleys reversed slopes, and we must 

 expect to find them chiefly near the ends of the old glaciers. Here, 

 indeed, most of the larger lakes of the Alps are found. 



The general arrangement of the Alpine glaciation during the Great 

 Ice Age was the following: The interior valleys of the mountain 

 chains were filled up with enormous, flat cakes of ice, of 2,000- 

 2,500 m elevation, interrupted by the higher ridges, from which deep 

 affluents poured into the mer de glace. Its surface sloped down in 

 the center very gently, and with increasing steepness toward its rim. 

 Under this steep marginal slope lie the existing and former lake 

 basins of the Alps. The location of their reversed slopes indicates 

 the region where the glacier's erosive action gradually ceased. It 

 has long been recognized that the depth of these lakes is far greater 

 in the south than in the north, but no adequate explanation has been 

 given. This phenomenon is consistent with the fact that the marginal 

 slope of the Alpine glaciation at the south side of the mountains was 

 twice or thrice as steep as on the north side ; here a far greater reversed 

 slope could be overcome by the glaciers. It must be borne in mind, 

 however, that thick morainic deposits occur at the lower ends of the 

 troughs. The lake basins of the Alps, therefore, are not alone formed 

 by glacial erosion; they are partially dammed up by the thick gravel 

 deposits which the glaciers accumulated at their ends, and this 

 accumulation assumes, in the south side of the Alps, a far greater 

 thickness than on the north side, because it is concentrated over a 

 less extended area. This damming up raises the levels of the Italian 

 lakes far higher than those of the south German lakes, and the differ- 



