l62 



PHYSICAL GEOLOGY 



long occupied by glaciers, has been accomplished to any great extent 



by subglacial stream erosion. 



If the streams which issue from glaciers are ponded by terminal 



moraines, lakes are formed in which they deposit their loads. If 



they have a free 

 course, however, they 

 will carry their loads 

 of rock flour and peb- 

 bles farther down the 

 valleys. The coarse 

 gravel will soon be 

 dropped, but the finer 

 material may be car- 

 ried some distance. 

 When, however, a 

 stream thus loaded 

 reaches a more gentle 

 grade, it may lose so 

 much velocity that 

 it becomes overloaded 

 and compelled to take 



Fig. 146. — The union of the Rhone and Arve rivers 

 near Geneva, Switzerland. The water of the Rhone, hav- 

 ing been filtered by Lake Geneva, is clear and blue, while 

 that of the Arve is grey with the rock flour carried into 

 it by glacial streams. To the right is seen the cement 

 works for recovering the Arve sediments. (Hobbs, Earth 

 Features.) 



a braided course (p. 86). The stratified deposits laid down in 

 valleys by glacial streams are called valley trains (p. 178). 



As has been stated 

 (p. 1 59), streams flow- 

 ing from glaciers are 

 milky with rock flour, 

 while those which 

 gather their water 

 from the land sur- 

 face may be yellow 

 with the clay of the 

 weathered rock which 

 they bear along, but 

 streams filtered by 

 lakes are clear. At 

 the confluence of the 

 Rhone and the Arve (Fig. 146) a striking contrast is seen between 

 the clear water flowing from Lake Geneva and the turbid water of 

 the Arve which has its source in the glacier of that name. For a 



y\ 



Fig. 147. — Block diagram showing a valley blocked 

 by a moraine; the stream having been diverted from its 

 old course has cut a steep-sided, postglacial gorge. 



