Mr Milne on the Parallel Itoads of Lochaber, 353 



high), are specimens indicating the prodigious accumulations once 

 existing in the great glen. 



To this point I shall revert. But, in the mean time, taking for 

 granted that such detritus did fill the lower parts of the valleys, it 

 is easy to understand how it should have dammed up the waters 

 into lakes, and how, by a gradual and long-continued process of 

 wearing down, this detrital blockage should have been lowered to 

 the requisite extent. 



I have endeavoured to explain the damming back and the de- 

 pressing of the lakes to their successive levels, without imagining 

 that the level of the sea was then different from what it is at pre- 

 sent. If the sea stood at a higher level, then the difficulties of the 

 explanation become less ; because the valleys must then have been 

 previously less excavated than they now are, by the operation of 

 rivers. There are good reasons for believing, that since the period 

 of the deposit of the boulder- clay in Scotland, the sea has stood at 

 least 1000 feet higher on the land than at present. Of course, it 

 must have been after the land rose out of the sea to some extent, 

 that the Lochaber shelves could have been formed by lakes ; but the 

 lowest of these might have existed when the sea stood 900 feet 

 above its present level, in which case the depth of detrital matter 

 required to dam up the valleys would be comparatively small. 



I have attempted to explain how the valleys of Glen Roy, Glen 

 Collarig, and Glen Spean were blocked up. There still remains 

 Glen Gluoy, which, as before mentioned, contains two shelves, one 

 of which is about 29 feet above the highest of Glen Roy. Glen 

 Gluoy being unconnected with the other valleys, requires a separate 

 blockage. There would be no great difficulty in imagining the ex- 

 istence of detrital blockage in this glen, at the place where its 

 shelves terminate towards the west, as it is generally, throughout 

 its whole course, exceedingly narrow ; and being unconnected with 

 Glen Roy (though MacCulloch states the reverse), its blockage may 

 have been worn down at periods, and in a way independently of 

 Glen Roy and Glen Collarig. 



Before, however, forming a very decided opinion as to the posi- 

 tion of the blockage applicable to Glen Gluoy, I should like to ex- 

 amine more particularly than I was able to do, some of the other 

 Glens which open into the Caledonian valley on both sides, with the 

 view of ascertaining whether they contain traces of horizontal shelves 

 about the same height. Mr Darwin takes notice of one in the valley 

 of Kilfinnin,'"* about 10 miles to the eastward, and which he says 

 is (by his barometric observations) about 40 feet above the highest 



* It is to be regretted that Mr Darwin, when he visited Lochaber, was not 

 provided with a spirit-level. His statcnicnt as to the horizontality of this 

 shelf at Kilfinuin, depends entirely on ocular inspection and barometric uitu- 

 sureracnts. 



