354 Mr Milne on the Parallel Eoads of Lochaber, 



Bhelf in Glen Roy; in which case it would be only 10 or 11 feet 

 above that in Glen Gluoy, a difference quite within the limits of 

 error. 



I have observed several places along the Caledonian Canal, where 

 there are traces of one or more horizontal terraces, at a height of 

 from 650 to 690 feet above the sea. From these considerations, I 

 infer the possibility of there having been a blockage which applied 

 not merely to Glen Gluoy, but to other glens opening into the great 

 Caledonian valley ; and it would, therefore, be most important, that 

 future observers should turn their attention to the adjoining dis- 

 tricts. 



My explanation of the Lochaber shelves depends entirely on the 

 accuracy of the supposition that the valleys were, in the lower parts 

 of them, filled up with detrital matter, capable of being gradually 

 worn down and washed away. This supposition is not only not 

 improbable on general principles, but is verified to a great extent 

 by the remains of such detrital matter at and above the heights re- 

 quired for it. Thus, in Glen Collarig, there are to be seen, near 

 the east end, and within about half a mile of the place where the 

 blockage must have existed, enormous heaps of boulder-clay, gravel, 

 and sand. These detrital deposits must have existed in Glen Col- 

 larig before the shelves were formed, because shelves 2 and 3 are 

 seen distinctly indented upon these deposits ; and I was particularly 

 struck with the fact, that these deposits reach to a height of more 

 than 100 feet above shelf 2. Here is proof, that in Glen Collarig, 

 before the formation of the lake which filled it, there was detrital 

 matter of sufiicient depth and consistency to have retained water at 

 the required height. At the place where shelf 2 terminates in this 

 glen, the valley, even at present, is only about 236 feet deep, and 

 300 yards wide, so that the depth of detrital matter does not ex- 

 ceed the limits of probability — nay, is exemplified by the occur- 

 rence of much larger accumulations of detritus in all parts of the 

 Highlands. 



It is here proper to explain, that there are in these valleys, as 

 elsewhere in Scotland, two distinct sorts of superficial deposits, — 

 the one consisting of the well-known boulder-clay, and the other of 

 ordinary gravel and sand. This bloulder-clay exhibits the same 

 general characters, which it commonly possesses elsewhere ; it is un- 

 stratified, exceedingly obdurate, of a dark-bluish colour, and filled 

 with water-worn boulders. This boulder-clay I found at the fol- 

 lowing places ; — Spean Bridge, where it is covered by sand ; Bo- 

 huntine Hill, where it is covered with laminated clay, sloping to the 

 centre of the valley, and about 250 feet below shelf 4; Bohina, on 

 the south side of Glen Boy; Inverlair Bridge, near Loch Laggan ; 

 Glen Glaster (on the west side of the valley), from 50 to 80 feet 

 above shelf 3 ; Glen Collarig (near the Gap), where it rises above 

 shelf 2 ; Glen Gluoy, as seen at the water-shed between it and 

 Glen Roy, and on a level with shelf 1 . The deposit occurs also at 



