1 86 



PHYSICAL GEOLOGY 



formed in several 

 ways. The rock may 

 have been scooped 

 out by glaciers, form- 

 ing rock basins (p. 

 145). Many such 

 exist in mountainous 

 regions affected by 

 glaciation. River 

 valleys may have 



been 

 drift 

 large 

 Lake 

 Lake 



dammed by 

 so as to form 

 lakes, such as 



Geneva and 



Constance 



in 



Fig. 174. — Lake Marjelen, formed by the damming of 



a valley by the Aletsch Glacier. 



Switzerland. 1 he 



many lakes which add so much to the attractiveness of the Adiron- 

 dacks are the result of the repeated damming of old river courses (Fig. 

 173). The uneven surface of the drift is often dotted with lakes and 

 ponds which rest in the inequalities formed by the irregularly de- 

 posited material. Basins may be pro- 

 duced by a combination of the above 

 methods. The finger lakes of central 

 and western New York (Cayuga Lake 

 and Seneca Lake), Lake Chelan, Wash- 

 ington, and Lake Como, Italy, are the 

 result of the deepening of old river 

 valleys which lay in the direction of 

 the movement of the ice and of the 

 damming of their outlets. The bodies 

 of water thus formed are long and 

 remarkably deep. Many temporary 

 lakes were formed between the ice front 

 and moraines and also when glaciers 

 moved up a slope, thus preventing 

 the waters from taking their natural 

 course. Glacial Lake Agassiz is such 

 an example (p. 656). Valley' glaciers 

 sometimes dam their tributary valleys, 

 thus forming lakes in them. Lake 



Fig. 175. — Map showing the 

 probable preglacial course of the 

 Genesee River (shaded) and the pres- 

 ent drainage. 



