
MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 409 
and withinabout half a mile of the place where the blockage must have existed. 
enormous heaps of boulder-clay, gravel, and sand. These detrital deposits must 
have existed in Glen Collarig before the shelves were formed, because shelves 2 
and 3 are seen distinctly indented upon these deposits; and I was particularly 
struck with the fact, that these deposits reach to a height of more than 100 feet 
above shelf 2. Here is proof, that in Glen Collarig, before the formation of the 
lake which filled it, there was detrital matter of sufficient depth and consistency 
to have retained water at the required height. At the place where shelf 2 termi- 
nates in this glen, the valley, even at present, is only about 236 feet deep, and 
300 yards wide, so that the depth of detrital matter does not exceed the limits of 
probability—nay, is exemplified by the occurrence of much larger accumulations 
of detritus in all parts of the Highlands. 
It is here proper to explain, that there are in these valleys, as elsewhere in 
Scotland, two distinct sorts of superficial deposits,—the one consisting of the well- 
known boulder-clay, and the other of ordinary gravel and sand. This boulder-clay 
exhibits the same general characters, which it commonly possesses elsewhere ; it 
is unstratified, exceedingly obdurate, of a dark-bluish colour, and filled with water- 
worn boulders. This boulder-clay I found at the following places ;—Spean Bridge, 
where it is covered by sand; Bohuntine Hill, where it is covered with laminated 
clay, sloping to the centre of the vailey, and about’250 feet below shelf 4; Bohina, 
on the south side of Glen Roy; Inverlair Bridge, near Loch Laggan; Glen Glaster 
(on the west side of the valley), from 50 to 80 feet above shelf3; Glen Collarig 
(near the gap), where it rises above shelf 2; Glen Gluoy, as seen at the water- 
shed between it and Glen Roy, and on a level with shelf 1. The deposit occurs 
also at Clenichan, at the river Roy, where the mica-slate rocks, through which 
the river now runs, are covered immediately by boulder-clay,—the boulder-clay 
being here covered by deposits of irregularly stratified beds of gravel and sand. 
from 150 to 200 feet thick. At this place, I observed among the boulders in the 
hill, granites (with red and grey varieties), old conglomerate, and red porphyry.— 
rocks, all of which must have come from a distance. 
From the fact that this boulder-clay occupies alike the highest and lowest 
parts of the glens; and, more especially, that in several places it is seen distinctly 
covered over by laminated clay as well as by stratified gravel and sand, it may be 
inferred that the boulder-clay, with its imbedded blocks, was deposited, certainly 
not after the drainage of the lakes, but either before the valleys were occupied 
with water, or during that period. 
In regard to gravel and sand, I do not remember having, in Glen Roy or its con- 
tiguous valleys, observed any considerable beds of it, so high up as the boulder-clay. 
But at lower levels, there are everywhere enormous cliffs of it to be seen, several of 
which I measured, and found to exceed 180 feet in height. These cliffs are formed 
out of the ancient bottom of the lake or estuary which filled the valleys, and are 
