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valleys on the north, visited by milder 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 closing the glens increasing in height. By-and-by supply 
and consumption becoming approximately equal, the height of 
the glacier barriers would remain constant. Then, as milder 
weather set in, consumption would be in excess, and a retreat of 
the ice would be the consequence. But for a long time the 
conflict between supply and consumption would continue, 
retarding indefinitely the disappearance of the barriers, and 
keeping the imprisoned lakes in the northern glens. But how- 
ever slow its retreat, the ice in the long run would be forced to 
yield. The dam at the mouth of Glen Roy, which probably 
entered the glen sufficiently far to block up Glen Glaster, would 
gradually retreat. Glen Glaster and its col being opened, the 
subsidence of the lake 80 feet, from the level of the highest to 
that of the second parallel road, would follow as a consequence. 
I think this the most probable course of things ; but it is also 
possible that Glen Glaster may have been blocked by a glacier 
from Glen Trieg. The ice dam continuing to retreat, at length 
permitted Glen Roy to connect itself with upper Glen Spean. 
A continuous lake then filled both glens, the level of which, as 
already explained, was determined by the col at Makul, above 
the head of Loch Laggan. The last to yield was the portion of 
the glacier which derived nutrition from Ben Nevis, and prob- 
ably also from the mountains north and south of Loch Arkaig. 
But it at length yielded, and the waters in the glens resumed 
the courses which they pursue to-day. 
For the removal of the ice barriers no cataclysm is to be 
invoked ; the gradual melting of the dam would produce the 
entire series of phenomena. In sinking from col to col the 
water would flow over a melting barrier, the surface of the im- 
prisoned lakes not remaining sufficiently long at any particular 
level to produce a shelf comparable to the parallel roads. By 
temporary halts in the process of melting due to atmospheric 
conditions or to the character of the dam itself, or through local 
softness in the drift, small pseudo-terraces would be formed 
which, to the perplexity of some observers, are seen upon the 
flanks of the glens to-day. 
In presence, then, of the fact that the barriers which stopped 
these glens to a height, it may be, of 1,500 feet above the 
bottom of Glen Spean, have dissolved and left not a wreck 
behind ; in presence of the fact, insisted on by Professor Geikie, 
that barriers of detritus would undoubtedly have been able to 
maintain themselves had they ever been there ; in presence of 
the fact that great glaciers once most certainly filled these 
