612 D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 
the lake fell to Shelf 3, the waters in Glen Collarig reached to C in fig. 10, and 
thereafter, on the removal of the blockage between Craig Dhu and Bohuntine, 
the waters of the lowest shelf came up Glen Collarig to the place (viz., D), 
where the detrital blockage above referred to existed. 
When the lake sank to the lowest shelf* (No. 4) in the glens, by the removal 
of the blockage between Bohina and Bohuntine, the waters of the lake occupied 
the whole valley of the Spean. Shelf 4 is traceable as far as the east end of 
Loch Laggan ; and towards the west to Teindrish on the one side and Corry 
Cholzie and Corry N’Eoin on the other side. 
And what was the blockage at this point? The intervening space between 
the two shelves at this their Western termination is no less than five miles in 
length. Could there have been a detrital barrier here also? There are good 
grounds for believing that there was. The whole of this district, as Dr 
CHAMBERS explains, consists of “a@ mass of gravel 11 miles long by perhaps 2 
broad, and reaching an elevation of 612 feet above the sea.” TI have traversed the 
district in many directions, and can attest that it presents an enormous accumu- 
lation of drift deposits,—not gravel only, but also of sand and clay ;—at one 
spot only does rock come to the surface. Streams cut through this extensive 
drift platean from the Aonachmore hills, situated to the south. There are no 
less than five mountain torrents in the course of two miles in this part 
of the district. These streams present deep gashes through the detritus, 
and when they reach the base of the hilly range, unite into considerable 
rivers, which run, some north to join the Spean, others west towards Fort- 
William. The scouring out of detritus along the base of these hills has been 
very great. The result has been a valley deep and wide in an east and 
west direction. In company with the Rev. Mr Cameron, minister of the 
parish, I walked along this valley towards Fort-William. Part of it consists 
of an elongated marsh, formerly a lake, whose margin had been about 20 feet 
above the marsh, the surrounding cliffs being detritus. Farther west I came 
upon a small lake, the banks of which, composed of detritus, are about 170 feet 
high, showing on their sides two or three terraces, proofs that either the lake 
had subsided from one level to another, or that the river had eroded first on one 
side and then on another at the above levels before reaching its present channel. 
The river has now reached rock, so that further subsidence is arrested. 
In different parts of this lower district, knolls and banks of detritus stand 
up above the general level. It would therefore require no great amount of 
restoration to supply a detrital blockage sufficient for damming the great lake 
indicated by the lowest Glen Roy shelf, which is 854 feet above the sea. 
* T have already explained, that the subsidence of the lake from Shelf 3 to 4 did not take place 
all at once. It sank at first only about 78 feet, and formed an intermediate road visible in Glen 
Collarig, at its north end, on both sides. ‘ 

