Lakes whose causal conditions are due to the direct interposition of glaciers, may be 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 débris 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.