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 hike 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 

 its solution. Glen Gluoy and Glen Roy are lateral valleys 

 which open into Glen Spean. Let us suppose Glen Spean 

 filled 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 ] 3vels 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 dam 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 CJlen Roy road and the 

 highest road of Glen Glaster would be formed. The ice 

 subsiding still further, a connection would eventually be 



