98 MR DAVID MILNE HOME: MEMOIR ON THE 
is owing to the same cause that Shelves 3 and 4, especially the former, are 
wider here than elsewhere, as the Ordnance Map (on the 6-inch scale) shows. 
This point being of extreme importance towards the settlement of the 
question, I have given a map (see Plate XIII.) indicating the position of the 
different blockages. It will be observed that the blockages in Glen Glaster and 
Glen Roy (EF and GH) form one line. The most’probable supposition is, that 
the blockages at both places were due to the same mass of detritus prevailing 
over the whole of this district. 
A considerable stream, as the Ordnance map shows, now crosses the place 
where that blockage existed, so that it would be exposed to the risk of being 
cut through, and its materials removed by the operation of running water. 
The succession of changes on the blockages, to allow of the subsidence and 
extension of the lakes, would be as follows :-— 
(1.) When the lake of Shelf 2 was flowing over the col at the head of Glen 
Roy, a lowering of the Glen Glaster blockage (EF) took place, first to the 
extent of 14 feet, next to the extent of 36 feet, and lastly 32 feet more, when 
the surface of the lake would reach the rocky col between Glen Glaster and 
flow out towards the Rough Burn.* 
If the blockage there consisted of detritus, no long time would elapse 
between the successive erosions ; and, accordingly, the “ Roads” formed at these 
intermediate lines are only discernible at a few places. 
(2.) The next great subsidence was from Shelf 3 to Shelf 4; the vertical 
distance between the two being 211 feet. 
This was effected by a lowering of the Boheenie blockage (HG). But it was not 
all accomplished at once. <A shelf intermediate between 3 and 4 was discovered 
by Mr Jotty and me in Glen Collarig, at a distance below Shelf 3 of 78 feet. 
This lower line was pointed out by us to the Ordnance Surveyors. 
Moreover, above Shelf 4, between the mouth of Loch Treig and the Laire 
Burn, there is a Shelf about 30 feet above Shelf 4, indicating another inter- 
mediate subsidence. 
The Boheenie blockage (GH), was therefore lowered from time to time, 
till it was totally removed, and thereafter the waters of the Glen Spean lake 
passed up into Glen Roy and Glen Glaster. 
(3.) The only other blockage requiring notice is, that which kept in the 
lake of Shelf 4, viz., extending across the Unachan Moor, between Teandrish 
and Corry N’Eoin. It is marked by the thick line KL on the map (Plate XIII). 
I agree with Sir THomas Dick LAvDER, in the position which he assigned 
to this blockage. At my last visit, I think I discovered a remnant of it where 
it had joined the steep bank to the north of the Corry. 
* These intermediate shelves are described by Dr Cuampers and myself in our respective previous 
Memoirs. See also my last Memoir, p. 600. 

