J. F. Campbell Collection > Parallel roads of Lochaber

Lakes. — In the Lochaber system of glens are large lakes clammed by
rock barriers, or by drift. Their waste waters are we.iriuL; their dams,
r it'in,,'- drains in them, lowering the water level, and leaving beaches at
old lake shores on the hill side. In other places in the same glens lakes
have been drained by the well-known p'-ocess which is going on at Moy,
at the west end of Loch Laggan. River courses, drains, broken
dams, and old lake shores remain to show plainly where drained lakes
stood, in Glen Spean above the waterfall, and above Speau bridge : in
Glen Roy, at Bnionachan (drizzle field), and elsewhere. A higher lake
level is stronsrly marked along the shore of Loch Laggan, 819 feet, which
is beingdrained, and it corresponds to the flat top of a terrace of stratified
shingle at "Moy." Through it the river Spean has cut a drain. The
name means " a plain." magh, genitive maigh. The " plain " is at about
850 feet above the sea level, and it is opposite to a river which comes
from Loch Ghnilbin, whei-e is another " plain " of like stuS": — a dam of
detritus at about 1,200 feet above the sea level. Lakes, lake shores, and
dams associated in the same hollows with far higher beaches, natnr-
ally suggested larger and deeper lakes, higher and bigger dams, in
Lochaber. Accordingly the first observers thought of lakes. But
there was a difficulty about the needful dams at the places required to
account for lake beaches at 1,107, 1,148, 1.067, and 855 feet. To
account for the Glen Roy lake a dam is needed about Roy bridge, at
least as high as the Scotch water parting between the Spean and the
Spey at one source of the Roy That is at 1,151 feet at the " Col,"
and corresponds nearly to the highest beach mark in Glen Roy, 1,148.
The dam must have been as wide as the distance between the opposite
contour lines of 1 ,200 feet, which are nearly three miles apart above
Roy bridge, and it must have been about 900 feet deep, where the
rock foundation now is 300 feet above the sea. Because of existing
dams of detritus at about 850 and at 1,200 feet at the end of Loch
Laggan, Loch Ghuilbin, and Loch Treig, another such dam might have
existed as high as 1,200 feet at Roy bridge, on the same drainage sys-
tem, but no remnant of such a dam of detritus is there, A rock dam
is above the waterfall in Glen Spean with glacial marks upon it, and
with a river drain cut through it, but there is no rock dam at Roy
bridge. Measured by the high level beach, a theoretical Glen Roy lake
was about 10 miles long, and overflowed eastward into the Spey Glen
instead of westward into Glen Spean, but there is no remnant of a dam
to account for that lake shore about Roy bridge Other dams are
needed to hold the lake at the high level. It seemed to me that high
terraces are faintly visible in Glen Spean above the bridge, far outside
of Glen Roy on the face of " Meall nan luatha." The needful dam in
that case must have been out about the river Lochy where there is
no such dam nearer than the American coast.
Ice Dams. — Failing dams of rock or of drift, like those which hold
Loch Lagrnan. Loch Ghuilbin, and Loch Treig and those which
formerly held lakes in Glen Spean and Glen Roy, and remain broken
rniiis: ice dams were suggested, like the dam which holds a lake in
Switzerland. They were suggested by Agassiz, who was a Swiss.
It is supposed by the supporters of this explanation for high level