GLACIAL FEATURES OF THE ALPS n 



glacier passed over the Pass of Porlezza and reached the eastern 

 deep part of Lake Lugano, which drains into Lake Maggiore. If 

 the morainic deposits to the south of the eastern part of Lake Lugano 

 were higher, that lake would discharge into Lake Como, and the rela- 

 tion of the eastern part of Lake Lugano to Lake Como would 

 become very clear. It is also a finger-lake, lying around the palm- 

 lake of Lake Como, but separated from it by a low pass. 



The finger-lakes and finger-valleys, with their reversed drainage, 

 are a very conspicuous feature in the regions of glacial diffluence. 

 Many phenomena indicate that they were not originally there, 

 and that their formation was connected with the diffluence of the 

 glaciers. Lake Orta is a typical finger-lake in the region of diffluence 

 of the old glacier of the Ticino, and has a reversed drainage. It occu- 

 pies, however, a valley whose catchment basin has a peculiar arrange- 

 ment, as if it would be drained toward the Po plain. It can be shown 

 that the Lecco branch of Lake Como came very lately into use as 

 an outlet for a branch of the Adda glacier. It can be further deter- 

 mined that the different fingers in the large fans of the glaciers on 

 the north side of the Alps are younger than the gravels of the first 

 glacial epoch. How glacial diffluence controls the formation of these 

 features can be shown by the study of those forms which are origi- 

 nated by it. 



When a glacier fills a valley above the height of the notches in its 

 watershed, it flows over these notches, if it is not hindered by ice 

 masses on the other side of the pass. By overflowing, it exercises 

 on the pass a conspicuous erosive action, by which the pass is lowered 

 and widened. This can be seen in all passes which have been over- 

 flowed by ice. On the St. Gotthard and on the Grimsel, ice-marks are 

 very clear, roches montonnees with their scour and pluck sides spread 

 over the culminating surface, and little mountain tarns indicate that 

 the glacier eroded flat basins. The longer the ice-action goes on, 

 the more the pass is lowered; the height of the watershed is leveled 

 down to the floor of the valley, into which the glacier pours, and 

 a vast flat is formed which begins at the shoulders of the valleys 

 from which the ice branched. Many Alpine passes belong to this, 

 type. They can be reached from the glacier valley only by a sharp 

 ascent on its trough-side, while the descent into the neighboring 



