n through all the valleys on the
mountain-slopes to the north and south of it, so that this valley had
become, as it were, the great drainage-bed for the masses of ice thus
poured into it laterally, and moving down the valley from east to west
as one immense glacier. It is natural to suppose, that, at the
breaking-up of the great sheet of ice which, if my view of the case is
correct, must have covered the whole country at this time, the ice would
yield more readily in a valley like that of Glen Roy, lying open to the
south and receiving the full force of the sun, than in those on the
opposite side of Glen Spean, opening to the north. At all events, it is
evident that at some time posterior to this universal glacial period,
when the ice began to retreat, Glen Roy became the basin of a glacial
lake such as we now find in the Alps of Switzerland, where occasionally
a closed valley becomes a trough, as it were, into which the water from
the surrounding hills is drained. In such a lake no animals are found,
such as exist in any other sheet of fresh water, and this would account
for the absence of any organic remains on the terraces of Glen Roy. But
at first sight it seemed that this theory was open in one respect to the
same objection as the other. What prevented this sheet of water from
spreading east and west in Glen Spean? If it not only filled Glen Roy,
but extended to the southern side of Glen Spean immediately opposite
the opening of Glen Roy, what prevented it from filling the whole of
that valley also? In endeavoring to answer this question, I found the
solution of the mystery.

The bed of Glen Spean, through its whole extent from east to west, is
marked, as I have said, by glacial action, in rectilinear scratches and
furrows. This westward track of the main glacier is crossed transversely
near the centre of the valley by two other glacier-tracks cutting it at
right angles. Upon tracing these cross-tracks carefully, I became
satisfied, that, after the surrounding ice had begun to yield, after the
masses of ice which descended from the northern and southern slopes of
the mountains into Glen Spean had begun to retreat, and to form local
limited glaciers, two of those lateral glaciers, one coming down from
Ben Nevis on the southwest, the other from Loch Treig on the southeast,
extended farther than the others and stretched across Glen Spean.[C]
These two glaciers for a long time formed barriers across the western
and ea