364 THE ICE A GE IN NORTH AMERICA. 



there. The following accurate and interesting account of 

 them is from the pen of Professor William M. Davis : * 



Glacial lakes are now of little importance ; a few occur in 

 the higher mountain-chains, but they are trifling in size, and 

 rank with many other species only as curiosities unless they 

 become of disastrous importance in the valleys below, from the 

 floods that follow a giving way of their barriers. Three sub- 

 species are easily distinguished : First, when the advance of a 

 glacier down a main valley closes the mouth of a lateral ravine ; 

 second, when a glacier from a side valley obstructs the main 

 stream ; third, when the great ice-sheet of early Quaternary 

 times melted away so as to disclose the upper part of a valley 

 sloping gently against it. 



The Merjelen See in Switzerland serves as the type of the 

 first subspecies ; it is held back by the Great Aletsch Glacier 

 in a little valley behind the Eggischhorn, a favorite point of 

 view, from which the lake below and the whole stretch of the 

 ice-stream in the main valley can be seen. Sometimes the 

 waters find an outlet through the ice-barrier ; then the slow 

 accumulation of months rushes out in a torrent, flooding the 

 valley of the Massa below, and leaving miniature bergs broken 

 from the glacier stranded on the rocky bottom ; subsequently, 

 another motion of the glacier closes the outlet, and the basin 

 slowly fills again. The highest level of such a lake will be 

 determined either by free escape across the ice, when it will 

 have a variable maximum, or by flow over a pass at the head 

 of its lateral valley. The latter is the case with the Merjelen 

 See, and the level of the pass is marked by a faint terrace and 

 by a change of color on the rocks around the shore, f 



Although rare at present, these lakes have had a consider- 

 able importance in the- past. An extinct example was early 

 recognized by Agassiz at the Parallel Koads of Glen Roy, near 

 Ben Nevis, in Scotland ; \ these are simply the shore-line 



* " Proceedings of the Boston Society of Natural History," vol. xxi, pp. 

 350, 351. 



f L. Agassiz, "Etudes sur les Glaciers" (1840), pp. 218, 220. Lyell, 

 "Principles of Geology," vol. i, p. 372 ; " Antiquity of Man," p. 309. 



\ Acrassiz, " Geological Society Proceedings," vol. iii, 1842, p. 331 ; here 

 described as a lateral glacier closing the main valley. 



