J. F. Campbell Collection > On the parallel roads of Lochaber

600 D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER.
iu what part of the glens the shelves cease, for there, or near that part, the
l)lockage whatever it was, most prol)al)ly existed.
In Glen Roy, as will be seeit from the map annexed (Plate XLI.), the highest
shelf, No. 2, exists in the upper part of the glen, on both sides, but does not come
further down the glen than a certain point on each bank. These two points, it
will be observed, are nearly ojjposite to each other. Here, therefore, the lake
is generally assumed to have terminated, when it stood at its highest level,
and here a blockage of some kind must be sought for. Let it be also
noticed, that this shelf, No. 2, enters the side valley, called Glen Collarig,
through a hollow or depression called the " Gap," but only for a certain
distance; — and there, another blockage of some kind must have existed, to
account for the stoppage of the shelf.
When the Lake sank to its next, the 1068 feet level (Shelf 3), it formed
a beach line not only in the upper part of Glen Roy, but in a lower part,
i.e., in a part about one quarter of a mile loAver down the Glen than the spot
where No. 2 shelf stopped. The blockage, or a jiart of it, to allow this, must
have been lowered 81 feet," and must have occuj^ied a situation further south.
This lower shelf, No. 3, is ti-aceable into Glen Glaster, and approaches the col
whei'e there is the old river course, by which the lake, when at that level,
overflowed towards what is now called the Rough Burn into Glen Si^ean.
This lower shelf, continues in Glen Roy on both sides, and, like Shelf 2,
stops at certain points, as may be seen on the map, nearly ojiposite to one
another.
This lower .shelf also goes through the gap into Glen Collarig ; — and it
goes a little lieyond the place where shelf No. 2 stopped. Here, therefore,
as in Glen Roy, something occurred to allow the lake in Glen Collarig to
reach a little farther south, and to be kept up there at that lower level. To
allow of this extension of the lake, both in Glen Roy and in Glen Collarig,
there must have been a scooping away of the blockage. This is an important
fact, because the blockage must have been of such a nature as to be capable
of being lowered vertically, and of being scooped away horizontally.
We now come to the next subsidence of the lake in Glen Roy, as indicated
by Shelf 4, which stands at a height of 856 feet above the sea.
This shelf goes up Glen Roy only for a certain distance, of course stopping
at or neai- the part where the l)ottom of the valley rises to a level higher tlian
* It may be proper to explain that the lake did not siibsido at once 81 fact. Two iatermodiatc
shelves are visible between Shelves 2 and 3, in Glen Gla.ster (east side), in Glen Collarig, iu Glen Koy
on the soutli side above Achavaddj'-, and on Ben Erin — i.e., the hill above the Gap. One of these is
about 14 feet below Shelf 2, the other about 32 feet above Shelf 3. So also, when the lake subsided
from shelf 3 to shelf 4, there was a halt long enough to allow an intcrniediato shelf to be formed at a
lieight of 990 feet, which is very conspicuous in C!len Collarig. None of these intermediate shelves
ate marked on the Ordnance Maps ; they were, however, pointed out by me to the Surveyor.?.