670 
PROFESSOR PRESTWlCH ON THE ORIGIN 
lake formed by the smaller lateral glacier crossing the main valley. With the further 
progress of the glacier, the fissure is carried lower down the valley and again the lake 
basin is fronted by a solid and compact ice, and the lake refills. 
With the great length of time required by Mr. Jamieson, and with the continued 
travel of the glaciers, it seems impossible but that the ice-barriers of the Lochaber 
lakes should have presented from time to time fissured portions by which their waters 
would have escaped, and the escape of such large bodies of water could hardly have 
been effected without producing a more damaging and permanent effect on the barriers 
than is caused by such small lakes as the Merjelen See, or even their total removal. 
It is true, that if, as in the case of such glacier lakes, the escape were only temporary, 
their waters might, apart from these occasional accidents, be considered as virtually 
permanent, for they could continue to fill to the one level, as often as they were 
emptied. But besides the doubt whether a breach made by the much larger bodies 
forming the Lochaber lakes could be readily repaired, is the doubt whether, looking at 
the great size of those lakes, they would have had time to refill up to the level of the 
cols before the movement of the glacier brought other fissures into play. It must be 
remembered, also, that in the Merjelen See and other such lakes the broadside pre¬ 
sented by the glacier is but a few hundred feet in length, whereas the Lochaber 
barriers were to be measured by several thousands of feet, and consequently the chances 
of fissures occurring would be proportionally increased. If, on the other hand, owing 
to the change of climate, the glaciers were no longer advancing, is it possible that ice- 
barriers, so durable as to have lasted during the many ages required on this theory 
for the formation of the “ roads,” could have existed for that length of time ? 
The Cols .—On the hypothesis that the “ parallel roads ” were formed by the long 
continued action of freshwater lakes in barred glens, it follows that rivers as large at 
least as the present Boy and Spean, must have flowed, during long successive periods, 
over the cols at the head of Glen Boy, of Glen Glaster, and of Makoul. The late 
Professor Nicol, in 1869, was the first to draw attention to the consequences of this 
assumption, and remarked * that in none of the cols “is there the slightest trace of an 
ancient river,” such as there must have been, had “ each of these passes been the exit 
of a river of considerable size and flowing in a narrow valley for a long period.” This 
important objection is confirmed by facts incidentally mentioned by other observers. 
At the same time, Professor Nicol sees in all the cols evidence of water action 
compatible with what he considers might have been incidental to a narrow sea-strait. 
In the Glen Gluoy Pass he found a line of stones on either side, but no indication of any 
stream of water having ever run along it. Over the pass from Glen Boy to Glen Spey, 
a considerable river, in all probability larger than the Boy at present, would have 
flowed and left its impress.! But no notch is cut there, or on the sloping declivity down 
to Loch Spey, nor has any delta been formed. On the contrary, the bottom of the 
* Quart. Journ. Geol. Soc., vol. xxv., p. 284. 
t Even the tiny stream from the Merjelen See has cut a channel through moraine matter (Ltell). 
