THE PARALLEL ROADS OF GLEN ROT. 169 
brought into play are the nutrition of the glaciers by the 
frozen material above, and their consumption in the milder 
air below. For a period supply exceeded consumption, and 
the ice extended, tilling Glen Speau to an ever-increasing 
height, and abutting against the mountains to the north 
of that glen. But why, it may be asked, should the val- 
leys south of Glen Spean be receptacles of ice at a time 
when those north of it were receptacles of water? The 
answer is to be found in the position and the greater eleva- 
tion of the mountains south of Glen Speau. They first 
received the loads of moisture carried by the Atlantic winds, 
and not until they had been in part dried, and also warmed 
by the liberation of their latent heat, did these winds touch 
the hills north of the Glen. 
An instructive observation bearing upon this point is 
here to be noted. Had our visit been in the winter we 
should have found all the mountains covered; had it been / 
in the summer we should have found the snow all gone. 
But happily it was at a season when the aspect of the 
mountains north and south of Glen Spean exhibited their 
relative powers as snow collectors. Scanning the former 
hills from many points of view, we were hardly able to 
detect a -fleck of snow, while heavy swaths and patches /L4L, 
loaded the latter. Were the glacial epoch to return, the 
relation indicated by this observation would cause Glen 
Spean to be filled with glaciers from the south, while the 
hills and valleys on the north, visited by warmer and drier 
winds, would remain comparatively free from ice. This 
flow from the south would be reinforced from the west, and 
as long as the supply was in excess of the consumption the 
glaciers would extend, the dams which closed the glens 
increasing in height. By and by supply and consumption 
becoming approximately equal, the height of the glacier 
barriers would remain constant. Then, if milder weather 
set in, consumption would be in excess, a lowering of the 
barriers and a retreat of the ice being the consequence. 
But for a long time the conflict between supply and con- 
sumption would continue, retarding indefinitely the dis- 
appearance of the barriers, and keeping the imprisoned 
lakes in the northern glens. But however slow its retreat, 
the ice in the long run would be forced to yield. The dam 
at the mouth of Glen Eoy, which probably entered the 
glen sufficiently far to block up Glen Glaster, would gradu- 
