714 
PROFESSOR PRESTWICK OK THE ORIGIN 
Fig. 20. 
Glen Roy from the detrital bank above Acliavady, looking north. 
a. Gravel of reconstructed moraine detritus.30 feet ? 
to. Roughly bedded moraine detritus (its full depth is not shown) 100 ,, ? 
The moraine detritus probably extends in patches higher up Glen Hoy, but it is so 
covered by coarse gravel and rubble, which in some sections is from 15 to 20 feet thick, 
that it is rarely exposed. The section of the “roacl,” fig. 13, shows, however, a mass of 
the moraine matter remaining on a ledge on the slope of the hills, and fig. 15 indicates 
also its existence at their base on the same east side of the valley. 
The vast size of the glacier needed to block the entrance of the Spean Valley has 
always been felt to be a difficult problem. We have to imagine a barrier 4 miles long 
and having a height of not less than 900 feet above the sea-level; but it must be 
borne in mind that the bed of the Spean is here 200 feet above the sea, and that, at 
the distance of half a mile on either side, the ground is 500 feet high, and thence rises 
gradually to the height of 1,000 feet or more. The difficulty is much lessened if we 
consider the barrier to have been formed not only by a remnant of the old ice-sheet, 
but also by the detrital mass which forms Unachan Hill (rising to the height of 613 
feet), and which is spread over some of the adjacent hills to the height of 800 feet or 
more, and extends also irregularly on the one side to the Great Glen and on the other 
to the flanks of Ben Nevis. The large quantity of this moraine detritus at the 
entrance of the Spean Valley, and on the flanks of the hills on either side, together 
with the extensive overlying beds of rolled gravel and sand formed of its wrecked 
portions, and spreading thence to the Lochy and Loch Linn he,* renders it also 
* Chambers speaks of ifc as a mass of gravel 11 miles long by perhaps 2 broad. 
