OF THE PARALLEL ROADS OF LOCHABER, 
665 
the Great Glen/’ He meets the difficulty as to the efficiency of a dam entirely detrital, by 
supposing that the lakes might have been formed while the sea still stood at the height 
of 500 to 600 feet higher than at present, that the rivers ran in higher channels, and 
that the valleys had been deepened since that period. 
In opposition to these views, it is contended that the erratic boulders have been 
transported by land ice, and supposing even the possibility of a detrital accumulation 
by sea action sufficient to block the several lake-glens, how, it is asked, could it have 
escaped filling up Loch Lochy and Loch Linnhe; while any denuding force that could 
possibly have again excavated these deep depressions, could hardly have failed to 
remove the intervening barriers which separate the several lochs in the Great Glen. 
Nor, supposing that barriers such as Mr. Milne Home describes had existed, do I 
imagine they could have formed impermeable dams to any large bodies of water. It 
might be possible for tenacious moraine clay or Till to form impervious barriers, but 
stratified beds of loose “gravel, sand, and clay” formed by marine currents could not 
possibly be water-tight. Had even the sea outside tended to stand as high as the 
lake, though the level of those inner waters might have been maintained some few 
feet higher, still as the land rose, and the sea-level gradually fell, so would the surface 
of the lakes subside by percolation, and their height would be regulated by that of 
the sea-level, and not, as they evidently have been, by the height of the several cols. 
Mr. Milne Home suggests that the lakes need not have been very deep—that the 
glens themselves were so obstructed with detritus as to reduce their depth to 100 feet. 
If so, it is not easy to see how the upper end of Glen Hoy could be so comparatively 
free from detritus, and yet so large an accumulation of moraine matter and gravel 
remain in its lower end, below its junction with Glen Glaster, where the scouring 
action of the outflowing water must have been greater. 
The argument that the valleys may have been subsecpiently deepened is disproved 
by the fact that, with the exception of the narrow river-cut channel, the valley floors 
remain as when left after the previous general glaciation, glaciated rocks showing 
everywhere in the bed of the valley of Glen Spean, and in places in Glen Hoy. 
The “ marine theory which regards the “ roads” as old sea beaches, was briefly 
alluded to by Macculloch. It was afterwards brought forward with his usual power 
of illustration, but subsequently abandoned, by Darwin. It had an able supporter in 
the late Dr. H. Chambers, and has more recently found strenuous advocates in Professor 
Nicoi/' and Mr. J. F. Campbell.! The manifest objection to this theory is the difficulty 
to conceive beaches to have been formed in glens such as those of the Hoy and the Spean, 
without similar beaches having been formed in the adjacent valleys ; and that on this 
hypothesis the terrace in Glen Gluoy should be on the level of the upper terrace of 
Glen Hoy, whereas there is a difference of 18 feet between them. The whole of the 
“roads” ought at least to have been common to the three glens, where the conditions 
* Quart. Journ. Geol. Soc., vol. xxv., p. 283, 1869. 
t ‘ The Parallel Roads of Lochaber,’ privately printed, November, 1877. 
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