626 D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 
although it might serve for the lowest, leaves the two higher ones unaccounted for. In order 
that a lake in Glen Roy could exist at the height of the upper lines, something must have 
prevented the waters escaping by Mukkoul, and also by the Glen Glaster col. 
“Tn order to explain this, we must go to Loch Treig. Let us suppose a glacier issuing from 
the mouth of Glen Treig, and let it protrude across Loch Spean until it rests on the hills upon 
the north side of that valley.* This would cut off all outlet to the eastward, both by Glen 
Glastert and Mukkoul, and so long as the icy barriers maintained a sufficient height, the water 
filling Glen Roy would have to escape by the col at the top of that glen into the head of 
Strathspey. 
“ Now let the Glen Treig glacier shrink a little. This would open the Glen Glaster col, and 
let out all the water above its level.t 
“Then let the Glen Treig glacier shrink again, until it withdrew out of Glen Spean. That 
valley being now clear, the water would escape by the outlet at Mukkoul, which would then 
determine the level of the lake and keep it at the lowest line, so long as the ice stream across 
the mouth of Glen Spean maintained itself of sufficient height. 
‘Grant then these two ice streams, one in the Great Caledonian Valley, and the other in 
Glen Treig, and the problem of the Parallel Roads can be solved, provided we allow that glaciers 
have the power to dam such deep bodies of water as must have occupied Glen Gluoy and Glen 
Roy.” 
Mr JAMIESON explains how he supposes the different ice streams moved. 
Thus, with regard to the Arkaig glacier, he says that he found on the rocky 
“Ridge between Loch Lochy and the Spean, such markings as might be made by ice 
mounting over it from Loch Lochy. The rocks were bared, where the presence of a glacier 
wheeling round from the Great Caledonian Valley into the mouth of Glen Spean, would have 
operated most strongly” (p. 246). 
Mr Jamieson, at the conclusion of his paper, makes the following candid 
admissions :— ; 
“The greatest difficulty that I find in supposing the parallel roads to have been formed by 
glacier dammed lakes, arises from a consideration of the depth of water the ice had to retain; for 
it is evident that the moraines were too insignificant to have done much of the duty. One 
might think that by the hydrostatic pressure of a column of water some hundreds of feet high 
(the water) would have found an escape beneath the ice. If, however, the height or thickness 
of the glacier were sufficiently in excess of the depth of the water, I imagine there would be 
pressure enough to keep it in” (p. 257). 
Mr Jotiy adopts Mr JAmigson’s views regarding the existence of a glacier 
in each of the valleys of Arkaig, Corry M‘Eoin, and Treig. He, however, sug- 
gests an additional glacier from the valley of the Laire (adjoining Loch Treig 
on the west) which filling Spean valley might flow down towards the west. 
“ Tf (says he) a glacier issued from Corry M‘Koin, and added its tributary ice to the Spean 
* If Loch Spean existed at this time, of which there can be no doubt, it is difficult to understand 
how a glacier could protrude across it, in the very middle of the Loch. 
+ To do this, the glacier must have filled Glen Glaster, and crossed the valley of the Roy. 
+ The level here referred to is that of Shelf 3. If the ice shrunk back from Glen Glaster, there would 
be no ice barrier across Glen Roy for Shelf 3, which extends down that Glen, beyond the mouth of 
Glen Glaster. 

