1 8 ALBRECHT PENCK 



mountains had been protected during the Great Ice Age by a cover- 

 ing of snow, and that the destruction has since begun. But the fact 

 that the surfaces of the old glaciers served as a base-level of destruc- 

 tion, as the surfaces of actual glaciers do, leaves no doubt that also 

 in the glacial period the destructive process went on. Its youth 

 must, therefore, result from other causes, as has been believed until 

 now. There are many reasons for assuming that the Alps were 

 elevated during the Great Ice Age, and that this elevation was a true 

 vertical upheaval. The youth of the high mountain crests might 

 be caused by such a recent upheaval, still going on. 



That the surfaces of the valley glaciers serve as a base-level of 

 destruction for the hanging glaciers is a fact which corresponds to 

 the same relation between the surfaces of side and main rivers. The 

 other fact, that also the snow-line is such a level, helps us to under- 

 stand the relation between the rates of glacial and fluviatile erosion. 

 The snow-line becomes visible in the forms of those mountains which 

 rise above the snow-line of the glacial period in such a way as to show 

 that above it the destruction of the mountains went on more quickly 

 than farther down. The sudden increase of the destruction of 

 mountains cannot be directly caused by a sudden climatic change 

 between the elevations below and those above that line, causing a 

 difference of weathering. We know there is such a gradual transition 

 of the climate in the neighborhood of the snowline, that it is very 

 difficult to recognize the climatic features determining its position. 

 The increase of destruction above the glacial snow-line is not due to 

 an increase of weathering above it but is caused by the development, 

 of a new agency, degrading land at a faster rate than the running 

 water. This agency is the glacier ice. 



Ice does not protect its bottom, as commonly believed; it attacks 

 vigorously. These attacks become most visible where glaciated 

 surfaces extend into the neighborhood of non-glaciated areas. There 

 we have a continual sapping of the latter. They cannot be as easily 

 recognized on surfaces which were totally covered with ice, for these 

 suffer by a general degradation. Therefore mountains which have 

 been totally covered with ice will not exhibit the same features as 

 those which had- only local glaciers. They will have no corries, and 

 they will conserve, under their covering of ice their rounded surface; 



