1892-93.] Dr Munro on a BemarhaUe Glacier- Lake. 
53 
On a Remarkable Glacier-Lake, formed by a Branch of 
the Hard anger- Jokul, near Eidfiord, Norway. By 
Robert Munro, M.D., M.A. (With a Plate.) 
(Read March 20, 1893.) 
Lakes whose causal conditions are due to the direct interposition 
of glaciers, may he formed in. one or other of the two following 
ways. First, when a glacier descending a lateral valley protrudes 
so far as to dam up a river in the main valley, the obstructing 
element being either the actual ice or the morainic debris deposited 
from it. Or, secondly, when a glacier occupying a main valley 
blocks up the outlet of some tributary stream. In both instances 
the water accumulates behind the obstruction and so forms a lake, 
varying in size according to the height of the barrier and the 
surface configuration of the district immediately above it. When 
the obstruction consists partially or wholly of ice, the stream 
issuing from the lake becomes liable to sudden inundations, as the 
pent up waters sometimes burst a passage through or underneath 
the ice, and so cause havoc among the industrial products of man 
along its whole downward course. A typical example of this kind 
of lake may now be witnessed in the Mattmark See, situated in the 
upper part of the Saas Valley, in Switzerland, where the Saaser 
Yisp has become dammed up by a mound of debris carried into the 
valley by the Allalin glacier. This glacier has, of late years, been 
decreasing in size, and the ice itself no longer forms part of the 
dam, which is thus exclusively composed of the accumulated debris 
of the terminal moraine, over which the river is now gradually 
deepening its channel. At an earlier period, however, certainly not 
many centuries ago, the glacier extended right across the valley, a 
fact clearly shown by the fresh-looking morainic deposits containing 
fragments of “gabbro,” which are to be seen high up on the 
opposite slope. That during this period the Mattmark See was 
subject to violent outbursts, may be inferred from the traditional 
dread of inundations which is still prevalent among the inhabitants. 
