OF THE PARALLEL ROADS OF LOCHABER. 
715 
tolerably certain that, before the rupture of the barrier, the quantity of moraine 
detritus here was much greater than now. The accumulation of this detritus, like 
that of the ice, resulted from the block of the many large glaciers at this point. 
The existing shape of Unachan Hill does not represent that of the original mass of 
moraine detritus. This was more transverse to the valley, and the present form is 
due to the scour of the water as it escaped seaward in the direction of Loch Linnhe 
on the bursting of the barrier, and to subsequent denudation. I saw the moraine 
debris only near its base, but I am confirmed in my opinion by the observations of 
Mr. Campbell, who remarks that he found the hill to be “ made chiefly of deep 
moraine stuff with many large polished boulders in it, covered with rolled gravel 
and peat where I could get at it.”'" (See Map, Plate 46.) 
I therefore think it probable that this barrier, like the one at the entrance of Glen 
Roy, although formed chiefly of ice, was largely supplemented and strengthened by 
unstratified argillaceous moraine detritus. I also think that this detritus extended 
within the barrier and there reduced the depth of the valley; for the several terraces, 
such as those of Inverroy and the opposite ones of Inch, have a foundation of moraine 
debris with a capping of drift gravel, resulting apparently from the levelling caused 
by the escaping waters on the rupture of the barrier. As the waters fell lower they, 
in combination with subsequent river action, cut out deeper channels through the 
moraine detritus, leaving its escarped flanks rising on both sides of the valley. 
The following section gives generally the position of these detrital terraces between 
Roy Bridge and Spean Bridge. This and the following two sections (figs. 22, 23) 
have the same horizontal and vertical scale of 1 inch to the mile. 
Fig. 21. 
Section across the Spean at Inverroy near Roy Bridge. a. River gravel. 
b. Moraine detritus and gravel (represented somewhat too thick). * Ice-worn surface of rock. 
This section also shows the depth the lake—level of “ road ” No. 4—would have had 
1 mile inside the barrier; while figs. 22 and 23 give sections of the valley at the line 
where the barrier is placed by the Ordnance Survey, and again 1 mile beyond, where 
the detrital mass of Unachan Hill intervenes. 
In these sections the height of the ground is represented in relation to the present 
sea-level, but as we have shown reason to conclude that the lakes were formed during 
* Op. cit ., p. 20. 
