THE PARALLEL ROADS OF GLEN ROT. 171 
terraces, no doubt, are due in part to the descending drift 
arrested by the water, and in part to the fretting of the 
wavelets, and the rearrangement of the stirred detritus, 
along the belts of contact of lake and hill. The descent of 
matter must have been frequent when the drift was un- 
bound by the rootlets which hold it together now. In 
some cases, it may be remarked, the visibility of the roads 
is materially augmented by differences of vegetation. The 
grass upon the terraces is not always of the same character 
as that above and below them, while on heather-covered 
hills the absence of the dark shrub from the roads greatly 
enhances their conspicuousness. 
The annexed sketch of a model (p. 173) will enable the ^ 
reader to grasp the essential features of the problem and J 
its solution. Glen Gluoy and Glen Roy are lateral valleys 
which open into Glen Spean. Let us suppose Glen Spean 
tilled from v to w with ice of a uniform elevation of 1,500 
feet above the sea, the ice not filling the upper part of that 
glen. The ice would thrust itself for some distance up the 
lateral valleys, closing all their mouths. The streams 
from the mountains right and left of Glen Gluoy would 
pour their waters into that glen, forming a lake, the level 
of which would be determined by the height of the col, at 
A, 1.170 feet above the sea. Over this col the water would 
flow into Glen Roy. But in Glen Roy it could not rise 
higher than 1,150 feet, the height of the col at B, over 
which it would flow into Glen Spey. 
The water halting at these 'svels for a sufficient time, 
would form the single road in Glen Gluoy and the highest 
road in Glen Roy. This state of things would continue 
as long as the ice dam was sufficiently high to dominate 
the cols at A and B; but when through change of climate 
the gradually sinking darn reached, in succession, the levels 
of these cols, the water would then begin to flow over the 
dam instead of over the cols. Let us suppose the wasting 
of the ice to continue until a connection was established 
between Glen Roy and Glen Glaster, a common lake would 
then fill both these glens, the level of which would be 
determined by that of the col C, over which the water 
would pour for an indefinite period into Glen Spean. 
During this period the second Glen Roy road and the 
highest road of Glen Glaster would be formed. The ice 
subsiding still further, a connection would eventually be 
