408 MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 
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 pre- 
sent 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 existence 
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 MacCuntoce# 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 position of the 
blockage applicable to Glen Gluoy, I should like to examine 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 shelf 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 Caledonial 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 districts. 
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 
required for it. Thus, in Glen Collarig, there are to be seen, near the east end, 
* It is to be regretted that Mr Darwin, when he visited Lochaber, was not provided with a 
spirit-level. His statement as to the horizontality of this shelf at Kilfinnin, depends entirely on 
ocular inspection and barometric measurements. 
