
MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 407 
river Roy of detritus from the valley? It is manifest, from many appearances 
along its sides, that the river Roy has cut down at least 200 feet below what was 
the original bottom (whether of lake or estuary), formed when the waters stood 
at shelf 4 ; so that the height of the supposed barrier to retain the waters at shelf 
2 would not exceed 600 feet above the bottom of the valley, and might be much 
less, if the valley were more filled up. Mr Darwin considers it probable (p. 83), 
that the buttresses existing on the sides of Glen Roy indicate, that the valley, up- 
wards from Bridge of Roy, had been filled with detrital matter to the very level of 
shelf 4; in which case the blockage or barrier requisite to form a lake at the level 
of shelf 2, would have been only about 300 feet above the bottom of the valley. 
My belief, however, is, that the whole not only of the lower part of Glen Roy, 
but also of the district about Unachan, High Bridge, and Fort-William, was blocked 
up with detrital matter, which, in the course of time was washed away by rivers ; 
and that, when the blockage of Glen Roy was removed, the depressed waters stand- 
ing at shelf 4 were dammed back by detrital accumulations near Unachan, so as to 
force a discharge by the Pass of Mukkul. This 4th, or lowest shelf, seems to me 
to stretch much farther to the north, on both sides of the Spean, than former ob- 
servers have noticed. On the hills flanking the east side, this shelf can be traced 
to within nearly a mile of Spean Bridge. On the opposite side of the valley, it can 
be traced to within 6 or 7 miles of Fort-William. The width of the valley where 
this shelf on both sides ceases to be visible is about 4 miles. Across the mouth 
of this valley, a little beyond a line joining the extreme visible points of shelf 4, 
lies the high and elongated hill of Tomnempearaichin, the top of which I found, 
by the level, to be only 50 or 60 feet below shelf 4; and there is no great 
difficulty in imagining that the whole of this district, as far as Fort-William, 
where the enclosing hills are greatly higher, was filled by detritus. There are, 
even now, detrital remnants of enormous size, of which the well-known Hill of 
Tomnahurich at Inverness (about 180 feet high and half a mile long), and a hill 
to the west of it (240 feet high), are specimens indicating the prodigious accumu- 
lations once existing in the great glen. 
To this point I shall revert. But, in the mean time, taking for granted that 
such detritus did fill the lower parts of the valleys, it is easy to understand how 
it should have dammed up the waters into lakes, and how, by a gradual and long- 
continued process of wearing down, this detrital blockage should have been lowered 
to the requisite extent. 
I have endeavoured to explain the damming back and the depressing of the 
lakes to their successive levels, without imagining that the level of the sea was 
then different from what it is at present. Ifthe sea stood at a higher level, then 
the difficulties of the explanation become less ; because the valleys must then have 
been previously less excavated than they now are, by the operation of rivers. 
There are good reasons for believing, that since the period of the deposit of the 
VOL. XVI. PART III. 5 
