DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 211 



Ice-Dams. 



The foregoing changes in lines of drainage due to the 

 Glacial period were produced by deposits of earthy mate- 

 rial in preglacial channels. Another class of temporary 

 but equally interesting changes were produced by the ice 

 itself acting directly as a barrier. 



Many such lakes on a small scale are still in existence 

 in various parts of the world. The Merjelen See in Switz- 

 erland is a well-known instance. This is a small body of 

 water held back by the great Aletsch Glacier, in a little 

 valley leading to that of the Fiesch Glacier, behind the 

 Eggischorn. At irregular intervals the ice-barrier gives 

 way, and allows the water to rush out in a torrent and 

 flood the valley below. Afterwards the ice closes up again, 

 and the water reaccumulates in preparation for another 

 flood. 



Other instances in the Alps are found in the Mattmark 

 See, which fills the portion of the Saas Valley between 

 Monte Rosa and the Rhone. This body of water is held 

 in place by the Allalin Glacier, which here crosses the 

 main valley. The Lac du Combal is held back by the 

 Glacier de Miage at the southern base of Mont Blanc. 

 " A more famous case is that of the Gietroz Glacier in the 

 valley of Bagnes, south of Martigny. In 1818 this lake 

 had grown to be a mile long, and was 700 feet wide and 

 200 feet deep. An attempt was made to drain it by cut- 

 ting through the ice, and about half the water was slowly 

 drawn off in this way; but then the barrier broke, and 

 the rest of the lake was emptied in half an hour, causing 

 a dreadful flood in the valley below. In the Tyrol, the 

 Vernagt Glacier has many times caused disastrous floods 

 by its inability to hold up the lake formed behind it. In 

 the northwestern Himalaya, the upper branches of the 

 Indus are sometimes held back in this way. A noted 

 flood occurred in 1835 ; it advanced twenty-five miles in 



