600 D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 
in what part of the glens the shelves cease, for there, or near that part, the 
blockage whatever it was, most probably existed. 
In Glen Roy, as will be seen 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 opposite 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 a 
a beach line not only in the upper part of Glen Roy, but in a lower part, 
i.é., in a part about one quarter of a mile lower down the Glen than the spot 
where No. 2 shelf stopped. The blockage, or a part of-it, to allow this, must 
have been lowered 81 feet,* and must have occupied a situation further south. 
This lower shelf, No. 3, is traceable into Glen Glaster, and approaches the col 
where 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 Spean. 
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 opposite to one 
another. 
This lower shelf also goes through the gap into Glen Collarig ;—and it 
goes a little beyond 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 near the part where the bottom of the valley rises to a level higher than 
* It may be proper to explain that the lake did not subside at once 81 feet. Two intermediate 
shelves are visible between Shelves 2 and 3, in Glen Glaster (east side), in Glen Collarig, in Glen Roy 
on the south side above Achavaddy, and on Ben Erin—.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 intermediate shelf to be formed at a 
height of 990 feet, which is very conspicuous in Glen Collarig. None of these intermediate shelves 
are marked on the Ordnance Maps; they were, however, pointed out by me to the Surveyors. 


