THE PARALLEL ROADS OF GLEN ROY 237 



doubtedly have been able to maintain themselves had 

 they ever been there; in presence of the fact that great 

 glaciers once most certainly filled these valleys that the 

 whole region, as proved by Mr. Jamieson, is filled with 

 the traces of their action; the theory which ascribes 

 the parallel roads to lakes dammed by barriers of ice 

 has, in my opinion, a degree of probability on its 

 side which amounts to a practical demonstration of its 

 truth. 



Into the details of the terrace formation I do not enter. 

 Mr. Darwin and Mr. Jamieson on the one side, and Sir 

 John Lubbock on the other, deal with true causes. The 

 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 

 unbound 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 char- 

 acter 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. 238) will enable the 

 reader to grasp the essential features of the problem and 

 its solution. Glen Gluoy and Glen Roy are lateral val- 

 leys which open into Glen Spean. Let us suppose Glen 

 Spean filled from v to w with ice of a uniform elevation 

 of 1,600 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. 



