OF THE PARALLEL ROADS OF LOCHABER. 
681 
Although the data thus furnished by the position of the glaciated surfaces and 
erratic blocks are not numerous, still they are sufficient to lead to the conclusion that, 
on the Ben Nevis range the ice-sheet may have extended to a height of from 3,000 
to 3,500 feet ; and on the hills to the north of the Spean to a height of not less, and 
possibly more, than 2,000 to 2,500 feet. The facts may seem somewhat conflicting, 
but considering the complicated movements that must have attended the growth of 
the ice-sheet, they can hardly be otherwise until we have more precise data respecting 
the direction of the striae and the origin of the boulders (see Map, Plate 46). 
For this great mantle of ice necessarily commenced with local glaciers descending 
from every mountain range, and, as those of one group became confluent in the glens 
of that group, they flowed into the wider intervening valleys, where they met with 
the glaciers of other ranges. Here they would consequently become subject to much 
mutual interference, causing deviation and deflections from their normal course into 
lines altogether aberrant and dependant upon the relative mass and force of the con¬ 
tributory glaciers. In the Alps at the present day the glaciers do not go beyond the 
first local stage, being confined to the separate slopes of each mountain group, and 
ending on reaching the intervening valleys, so that in no case do the glaciers of one 
mountain group come into collision with those of another range. But amongst the 
mountains of Scotland, where the number of local centres was great, the gradients 
variable, and the interferences frequent, the results produced have nothing analogous 
in the existing glaciers of Europe. Geologists have sought for such conditions in the 
ice-sheet of Greenland, and of this we know little except at its borders. 
Owing to the conflicting forces thus brought into play, the old ice-sheet, as is well 
known, did not always follow the natural channels of drainage, but was constantly 
forced, not only up valleys, but over intervening ranges of hills. At the points of 
junction of opposing glaciers, blockages must also have often ensued that tended to 
heap up and accumulate, not only large bodies of ice, but likewise large quaritities 
of moraine detritus. With a general movement of the whole mass seaward, it was 
subject to innumerable deflections at the confluence of contributing glacier-systems of 
variable force, and to formidable checks and stoppages at those points where the 
ice-masses were in more direct antagonism in valleys open to many tributaries, or 
where the main valley was blocked previous to the descent of the glaciers from the 
tributaries. The ice would then necessarily take the lines of least resistance, and no 
longer be guided by the lines of natural drainage. 
At the present day, the effects of conflicting ice-masses are confined to the com- 
pression by confluent glaciers in the narrow glens or in higher and more open mountain 
basins. Under certain circumstances this may give rise to the horseback ridges of 
some glaciers,”' and to the convexity noticed in some great central ice-areas. Sir 
* I cannot conceive that the medial ridges of some of the Alpine glaciers, for example, can be entirely 
due to the more rapid melting of the ice on the sides of the glacier. Observations on this point are 
however wanting. 
4 s 2 
