OF THE PARALLEL ROADS OF LOCHABER. 
685 
of Glen Roy must have descended along that valley, receiving tributary ice-streams 
from Glen Turret and Glen Glaster. Traces of the presence of this glacier are to be 
met with only in a few places, as the bed of the valley of Glen Roy is so filled with 
detrital matter that the bare rock is rarely seen. I found, however, a well-marked 
roche moutonnee close under the bridge crossing the Turret above Dalriach, and 
another a short distance lower down the Roy, while on the slopes of Tombrahn the 
rocks are glaciated both below and above the two higher “roads.” I also look 
upon the traces of moraine detritus on the slope of the hills above Achavady (see 
figs. 15 and 19), and the large bed of it between Achavady and Bohuntine, as equally 
due to the action of this glacier. 
This Roy glacier moved down Glen Roy until it met the ice from the Ben Nevis 
range and the Spean Valley, when a block ensued. Whether or not its course would 
then be reversed by the larger and stronger mass, or whether it would only be rendered 
stationary, while the upper part of the great opposing united ice-streams passed on and 
over-rode the smaller glacier, is a question for future investigation. I should presume 
that the latter would be the more probable case. This would be in accordance with 
the opinion of Mr. Jamieson’s, that an ice-current flowed outward over the col from 
Glen Roy; at the same time, I would suggest that the glaciation at that spot, which 
I had not the opportunity of visiting, if not due to the Roy Glacier may be due to a 
glacier issuing from Glen Agie and the other adjacent glens, and diverted eastward by 
the block in Glen Roy. A reversal of the entire current seems to me to be a physical 
impossibility, owing to the enormous inertia of the mass of ice, and the vast friction 
to be overcome.'" At the same time, I can conceive the possibility of the Ben Nevis 
ice, from its much greater height and mass, over-riding the Glen Roy glacier. 
§ 8. Distribution of the Moraine Detritus. 
The many checks and blocks that must have occurred during the formation and 
growth of the great ice-sheet, may serve to explain much that is peculiar in the distri¬ 
bution of the moraine detritus, not only in Lochaber, but in other parts of the country. 
In the glaciers of the Alps, where the ice-flow is uninterrupted and the-gradients 
are considerable, the glacier detritus is carried down and deposited in front of each 
glacier as a terminal moraine—the materials being derived chiefly from the lateral 
and medial moraines, while the subglacial detritus or moraine profonde usually con¬ 
tributes but little ; for so long as the glaciers remain confined within the walls of 
narrow and steep ravines, so long do they press forward on their beds with such force 
as either to leave the rock bare, or else to reduce the quantity of silt and stones 
* Unless, possibly, the work were very gradual and prolonged, the upper layer of the underlying glacier 
being first moved back by the advancing Ben Nevis ice, and so on successively as layer after layer came 
under its action. 
