OF THE PARALLEL ROADS OF LOCHABER. 
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present very changing outlines and dimensions. The following is an ideal sketch, by 
Colonel God win-Austen, of one of these lakes 
Fig. 7. 
a. Lakes. b. Moraine debris. c. Glacier ice. d. Ice cliffs. 
We can therefore imagine that, under the effects of the continued thaw, the old ice- 
sheet of Scotland became covered with pools and lakes, which would go on filling until 
the water reached the lip of the basin, when the surplus waters would escape along 
the natural lines of drainage, either on or beneath the ice, to some lower levels; but 
when, owing to exceptional circumstances such as those which prevailed in the Spean 
and Boy Valleys, the ice had been heaped up in larger masses so as to raise high the 
water-level, then these lakes, dammed back pending the removal of the barriers, 
made their temporary outflow over any cols lying at a lower level than the barriers 
and leading into adjacent valleys free from such blockages. 
So while every mountain side was contributing its rills and rivulets to innumerable 
temporary pools and tarns on the melting ice, their waters were, in such instances as 
those presented by the Lochaber Valleys, retained, and formed lakes ultimately extend¬ 
ing the length and depth of the glens. If there were no passes at levels lower than the 
barrier at the entrance, then the lake would go on unceasingly filling until the water 
overflowed the barrier itself, or burst a passage through it, in which case, as no per¬ 
manent fixed water-level had been established—for after the first overflow the barrier 
would be speedily destroyed—the waters, draining off at once, would have left little 
or no mark of their presence on the hill sides. 
In Lochaber, however, the main barriers were of such height and permanence, and 
the cols were so placed, that the waters found earlier outlets over the latter than was 
practicable through the former, and they thus obtained, at successive intervals, fixed 
and definite levels. I believe that to these conditions the levels or lines of the 
parallel roads are owing; but I further believe that it was not, as commonly held, by 
length of time and long continuance of the same water level that the “roads” were 
actually shaped, but by the sudden discharge of the water on successively breaking 
through minor barriers at each col. 
There are, no doubt, shore beaches formed by the other classes of lakes, but as they 
have the ordinary origin I do not think it necessary to discuss them here. The 
more or less permanent lakes, formed in lateral valleys by main glaciers, have in many 
cases given rise to such shore lines. The structure of these shingle ridges is, however, 
very different to that of the Lochaber “ roads.” They are formed by the soil and 
rubble, carried down the hill sides by the rain, rivulets, or frost, and spread out by the 
water, as in A, fig. 8, or aided by the erosion of yielding slopes, as in B, and form in 
mdccclxxix. 4 u 
