D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER, 601 
856 feet. This time the lake sank so low, that it could not enter the lateral 
valley of Glen Collarig through the gap. But what is very important, the lake 
entered Glen Collarig at its south or lower end, and its beach mark is distinct 
on both sides of that Glen up to near the middle of it, as is shown on the map. 
Some blockage of a permanent character must therefore have existed about 
this part of Glen Collarig, to prevent the two highest shelves, Nos. 2 and 8, 
going farther south, and also to prevent the lowest shelf, No. 4, going further 
north than the points where these shelves respectively stop. To this important 
discovery I will afterwards advert more particularly. 
This lower shelf, No. 4, in Glen Spean runs a certain distance west, 
towards the glen occupied by the Caledonian Canal. It also runs up the 
whole length of Glen Spean to the east end of Loch Laggan at Mukkoul, 
where the lake, when at that level, discharged eastward by the Makkoul valley 
(now dry) into Strath Spey. 
The blockage which caused the waters of the lake to stand at this level, and 
forced them to flow out at its east end, must have extended across the country, 
between a spur of Ben Nevis and Teandrish, a distance, as the crow flies, of 
about 4 or 5 miles. This district, called Unichan, lying between Fort-William 
and Spean Bridge, consists now of tolerably flat ground composed mostly on its 
surface, of gravel, clay and sand. 
The Author's Theory. 
2. The view which I support is, that the lakes at all these different levels 
were kept in by an accumulation of detritus, which being from time to time 
‘lowered in level, caused the subsidence of the lakes, and being scooped away, 
allowed of the extension of the lakes, beyond the original blockage, to lower 
parts of the glens. 
Mr Darwin and others represent the impossibility of having barriers 
several hundred feet in height, and more than a mile long, composed of such 
loose materials as gravel or clay. 
_The objection would be well founded, if it was necessary to suppose that 
the lakes were kept in by barriers resembling Dam-dykes, which is the 
representation given of the blockage by some writers. This, however, is not a 
correct representation of the nature of the blockage suggested. 
It is manifest that all this district of the Highlands was formerly covered by 
detritus, up to the height of at least 2000 feet above the present level of the 
sea; and that this detritus filled the valleys, including even the Great Glen which 
stretches from Fort-William to Inverness. I assume that the detritus had been 
deposited and spread over the country, when this part of Europe was sub- 
merged beneath the sea. 
