376 



GLACIEE LAKE— THE MAEJELEN SEE. 



[Ch. XVI. 



CH 



rock having been planed and nibbed off, in the manner 



some 



under the terminal arch of the great glacier of the Viesch, 

 a tributary of the Upper Ehone. It was in autumn (Sept. 



durin 



summer 



many 



Under the arch on one side was a floor 



of white granite streaked, not only with straight furrows 

 freshly made, but also with many parallel black lines which 

 had been ruled by fragments of soft, dark blue slate, fixed in 

 the moving ice. According as the impinging stones had been 

 harder or softer than the floor over which they grated, they 

 had cut rectilinear furrows in the rock which would last, or 



left superficial black 



mar 



which the glacier torrent 



of the ensuing winter would speedily wash away. 



Glacier lake — Marjelen See. — There are several instances 

 in the Himalaya where glaciers descending from lateral val- 

 leys cross the main valley and convert a portion of it into a 



the river. The converse of this may 



dammin 



sometim 



valley causes a lake by blocking up the lower end of a tribu- 



tary valley. 



example of this occurs in the Swiss Alp 



few miles above Brieg, in the Canton of Yalais, where the 

 great glacier of Aletsch gives rise in this way to a small lake 

 called the Marjelen See, which, after lasting for periods varying 

 from three to five years, is periodically drained by changes 

 which take place in the internal structure of the glacier. Rents 

 or ' crevasses 5 in the ice open and give passage to the waters, 

 which escape in a few hours, producing destructive inunda- 

 tions in the country below. Nothing is then left but a small 



earn 



interval of about a year, is again filled, the water rising to 

 its old level, and so continuing for several years. This old 

 level is determined, not by the height of the glacier-dam, but 

 by that of a watershed, or col, which separates the Marjelen 

 See from the adjoining valley of the Yiesch glacier, which 



lies to the eastward. (See fig. 23.) 



Marj elen 



miles in circumference 



normal 



month 



of 





