720 
PROFESSOR PRESTWICH ON THE ORIGIN 
It would appear therefore from the want of agreement between the levels of the 
“ roads ” and of the cols, and from the want of horizontality in the lines of the “ roads ” 
that they cannot be referred to the definite zones of an established and uniform water- 
level, regulated by the height of the cols, and combined with long continued shore action. 
The facts admit however of ready explanation on the principle that the momentum of 
the sliding mass of detritus, varying necessarily according to the angle of slope, would, 
in those cases where the slopes were greatest—other conditions being alike—carry 
that body beloiv the surface of the water; while in other cases where the slopes were 
less, it proved insufficient to effect the descent quite to that level. Or the difference in 
the slide might have been due to the lesser rapidity with which the level of the lake 
fell. Thus the height of “ road ” No. 4, which is always above the level of the escape- 
col at Makoul, may have arisen from the slopes of the hill at that level being, on the 
whole, somewhat less steep than at the higher “roadsor the main cause may have 
been the slower discharge of the water arising from the vastly greater size of the lake, 
while the pass was no wider, if so wide. 
The diminished momentum on lesser slopes may also be one cause why the “ roads,” 
in some places where the gradients are much smaller, almost disappear, which would 
not be the case to the same extent if the “roads ” had been formed by long continued 
shore action. 
Not only therefore is the variable inclination and w T ant of horizontality of the 
“roads” compatible with the hypothesis of detritus sliding down a slope, but this 
seems to me the only hypothesis on which the differences of level between the cols and 
the “ roads,” together with the curves in the lines of the latter, can be explained. 
§16. Further considerations in connexion with the Great Ice-sheet, suggested by the 
Local Phenomena of Lochaber. 
The Till of Scotland has been described as the product of the moraine profonde of 
the old glaciers. In the sense attaching to this term in existing glaciers this use of it is 
likely to lead to misconception. The original definition was intended to denote one 
form of moraine in contradistinction to the three forms of lateral, medial, and terminal 
moraines ; while in the sense in which it has sometimes been used by geologists it 
would include the product of these several moraines and perhaps more. 
The moraine profonde (couche de boue) of Agassiz, generally, is that layer of rock 
fragments, pebbles, and rock powder which is held in the ice at the bottom of a glacier 
and spread in places over its bed. It is rarely more than a few inches or a fev T feet thick, 
and the rocky bed is more often bare. Dolfuss-Ausset describes it to be “ materials 
buried under the glacier,” including however in that definition those which have fallen 
in front in the course of ablation.'"' Hogard says “the layer of mud, sand, pebbles, 
and blocks beneath the glacier constitutes the moraine profonde which forms almost 
* 1 Materiaux pour l’Etude des Glaciers,’ vol. v., p. 415. 
