1882 .] 
351 
[Davis. 
by Agassiz at the Parallel Roads of Glen Roy, near Ben Nevis, 
in Scotland ; 1 these are simply the shore-line 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 appar- 
ent 2 ; the glacier which served as a barrier has long since disap- 
peared with all its Scotch companions. 
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 3 : it differs 
from the preceding only in the relative position of lake and bar- 
rier, and in the lake's level always being determined 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 Mt. Blanc. 
Several temporary Swiss lakes of this construction have caused 
great damage by bursting 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 hun- 
dred 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. 4 In the Tyrol, the Vernagt glacier has 
many times caused disastrous floods by its inability to hold up 
the lake formed behind it. 5 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 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. 6 
1 Agassiz, Geol. Soc. Proc. in, 1842, 331; here described as a lateral glacier closing 
the main valley. 
2 Lyell, Antiquity of Man, 300. A good bibliography of this old lake is given by 
W. Jolly in Nature, May 20, 1880, p. 68. 
3 Heliotypes of this and the Merjelen See are given in “ Glaciers,” by Shaler and 
Davis, Boston, 1881. 
4 Lyell, Principles, i, 348. Biblioth. Univ. de Geneve, xxi, 1827, 227 ; xxii, 58 ; 
xxv, 24, etc. 
5 Sonklar, Die Oetzthaler Gebirgsgruppe, Gotha, 1860, 154. Stotter, Die Gletscher 
des Vernagtthales in Tirol und ihre Geschichte, Innsbruck, 1840, 15. 
6 H. Strachey, Royal Geogr. Soc. Journ., xxm, 1853, 55. Compare Drew, Jummoo 
and Kashmir 
