GLACIAL DAMS, LAKES, AND WATERFALLS. 365 



terraces of an old ice-barrier lake, the uppermost standing at 

 the height of the pass into the next glen, but the cause of the 

 others is not so apparent ; * the glacier which served as a bar- 

 rier has long since disappeared, with all its Scotch com- 

 panions. 



The Mattmark See, representing the second subspecies, is in 

 the Saas valley, between Monte Rosa and the Rhone, where 

 the Allalin Glacier advances across the main valley bottom, f 

 It differs from the preceding only in the relative position of 

 lake and barrier, and in the lake's level always being deter- 

 mined by flow over the ice or its moraine. The Lac du 

 Combal is in the same way held back by the Glacier de Miage 

 at the southern base of Mont Blanc. Several temporary Swiss 

 lakes of this construction have caused great damage by burst- 

 ing through their barriers. A famous case is that of the 

 Gietroz Glacier in the valley of Bagnes, south of Martigny, in 

 1818. The lake grew to be a mile long, seven hundred feet 

 wide, and two hundred deep. An attempt was made to drain 

 it by cutting 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. J 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 an hour, and was felt three hundred miles 

 down-stream, destroying all the villages on the lower plain, 

 and strewing the fields with stones, sand, and mud. || 



* Lyell, " Antiquity of Man," p. 300. A good bibliography of this old lake 

 is given by W. Jolly, in " Nature," May 20, 1880, p. 68. 



f Heliotypes of this and the Merjelen See are given in " Glaciers," by Sbaler 

 and Davis, Boston, 1881. 



% Lyell, "Principles," vol. i, p. 348. " Bibliotheque Universelle de Geneve," 

 vol. xxi, 1827, p. 227 ; vol. xxii, p. 58 ; vol. xxv, p. 24, etc. 



* Sonklar, "Die Oetzthaler Gebirgsgruppe," Gotha, 1860, p. 154. Stotter, 

 "Die Gletscher des Vernagtthales in Tirol und ihre Geschichte," Innsbruck, 

 1840, p. 15. 



I H. Strachey, " Royal Geographical Society Journal," vol. xxiii, 1853, p. 55. 

 Compare Drew, Jummoo, and Kashmir. 



