ain
comparatively free from ice. This flow from the south would be
reinforced from the west, and as long as the supply was in excess of
the consumption the glaciers would extend, the dams which closed the
glens increasing in height. By-and-by supply and consumption becoming
approximately equal, the height of the glacier barriers would remain
constant. Then, as milder weather set in, consumption would be in
excess, a lowering of the barriers and a retreat of the ice being the
consequence. But for a long time the conflict between supply and
consumption would continue, retarding indefinitely the disappearance
of the barriers, and keeping the imprisoned lakes in the northern
glens. But however slow its retreat, the ice in the long run would be
forced to yield. The dam at the mouth of Glen Roy, which probably
entered the glen sufficiently far to block up Glen Glaster, would
gradually retreat. Glen Glaster and its col being opened, the
subsidence of the lake eighty feet, from the level of the highest to
that of the second parallel road, would follow as a consequence. I
think this the most probable course of things, but it is also possible
that Glen Glaster may have been blocked by a glacier from Glen Trieg.
The ice dam continuing to retreat, at length permitted Glen Roy to
connect itself with upper Glen Spean. A continuous lake then filled
both glens, the level of which, as already explained, was determined
by the col at Makul, above the head of Loch Laggan. The last to yield
was the portion of the glacier which derived nutrition from Ben Nevis,
and probably also from the mountains north and south of Loch Arkaig.
But it at length yielded, and the waters in the glens resumed the
courses which they pursue to-day.

For the removal of the ice barriers no cataclysm is to be invoked; the
gradual melting of the dam would produce the entire series of
phenomena. In sinking from col to col the water would flow over a
gradually melting barrier, the surface of the imprisoned lake not
remaining sufficiently long at any particular level to produce a shelf
comparable to the parallel roads. By temporary halts in the process
of melting due to atmospheric conditions or to the character of the
dam itself, or through local softness in the drift, small
pseudo-terraces would be formed, which, to the perplexity of some
observers, are seen upon the flanks of the glens to-day.

In presence then of the fact that the barriers which stoppe