1863.] JAMIESON PARALLEL ROADS OF GLEN ROY. 247 



glaciers I have indicated forming a large proportion of the main 

 stream. This then would close up the mouths of Glen Gluoy and 

 Glen Spean ; and so long as the ice exceeded sufficiently the height 

 of the watersheds at the top of the glens, these cols would determine 

 the level of the water in the lakes. 



But in Glen Roy there are three lines, and this barrier across the 

 mouth of Glen Spean, although it might serve for the lowest, leaves 

 the two higher ones unaccounted for. In order that a lake could 

 exist in Glen Roy at the height of the upper lines, something must 

 have prevented the water escaping by Makoul, and also by the Glen 

 Glaster Col. 



In order to explain this we must go to Loch Treig. Let us sup- 

 pose a glacier issuing from the mouth of Glen Treig, and let it pro- 

 trude across Glen 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 Glaster and Makoul ; and, so long as the icy barriers main- 

 tained a sufficient height, the water rilling Glen Eoy would have to 

 escape by the col at the top of that Glen into the head of Strath- 

 spey. This col, therefore, would determine the level of the lake, 

 and keep it at the upper line as long as this state of things lasted. 



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. That 

 watershed would now determine the height of the lake, and there- 

 fore keep its surface at the middle line so long as this second state 

 of matters lasted. 



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 Makoul, 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 suffi- 

 cient height. When this latter finally gave way, Glen Roy would at 

 length be emptied. 



Grant then these two ice-streams, one in the Great Caledonian 

 Valley and the other at 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. 



Now there is good reason to believe that both of these glaciers 

 existed. Indeed the evidence for that of Glen Treig is probably 

 more complete than for any other glacier in the kingdom. I have 

 in a former paper detailed part of that evidence* ; I have shown how 

 the rocks at the entrance are intensely worn for many hundred feet 

 up the hills on either side, presenting in their rounded outlines, 

 scored surfaces, and perched boulders, all the marks of glacier-action. 

 The profound pool of Loch Treig, lying in a deep rock-basin, is, 

 adopting the views of Professor Ramsay, a still further proof. But 

 it was not until my last visit that I was aware of some remarkably 

 fine moraines that demonstrate still more forcibly the former pre- 

 sence of the ice, and which I shall now describe. I am the more 

 * Quart. Journ. Greol. Soc. vol. xviii. p. 170. 



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