OF THE PARALLEL ROADS OF LOCHABER. 
687 
their further course then becomes a matter of the resolution of forces, dependent upon 
the mass and weight of the several glaciers and on the lines of least resistance, and 
these may be in the same direction as the natural lines of drainage, or may be in 
a direction contrary to or across those lines. Each case must be examined by itself, 
and its various conditions ascertained before it can be determined whether or not the 
conclusion is in harmony with all the postulates. 
The moraine detritus, in the sense I take it, is not therefore the product only of the 
moraine profonde of Agassiz, but is the sum of all the debris carried down by a glacier 
and projected in its front, or subsequently ploughed up by the advancing glacier, and 
overlaid by the rolling stream of ice.'" It is this which forms that unstratified mass 
of decomposed and ground-up rock, rock debris, angular fragments and pebbles, and 
subangular striated fragments—grey and argillaceous in this district, but varying, as 
is well known, in texture and colour in others, according to the nature of the local 
rocks—which so often lies upon the glaciated rock surface. It is to this sub-glacial 
product that I should restrict the term “ Till,” or else designate it as land-till. 
Under the various circumstances here noted, the terminal moraines of separate 
glaciers may have remained isolated or have become confluent; may have been passed 
over by the ice, or pushed indefinitely forward ; or they may have been piled or heaped 
up between conflicting glaciers. These several contingencies will, I think, account, 
more readily than would subsequent partial denudation, for the many otherwise inex¬ 
plicable positions in which the moraine detritus is met with—in the valleys, on the 
sides of hills, and on the top of passes ; wherever, in fact, the advancing glaciers have 
filled valleys, have been forced up on to higher ground, or have anywhere met and 
contended. The result of this has been, that generally in the narrower and in the 
steeper valleys, where the ice-flow was little impeded, the moraine debris has been 
swept forward and removed; that in the broader and flatter valleys it has been left in 
isolated patches and ridges at various levels, while in the larger plains it occurs in more 
continuous and extended sheets.! The same causes would, in some places, produce 
contortions in the Till, and in others would press it out into great lenticular masses. 
In the Spean Valley, moraine detritus forms mounds at intervals all down the 
valley, but them character is much masked by subsequent water action, owing to 
which they are covered with gravel and often levelled on the top so as to form terraces. 
The large accumulation of this debris in front of Glen Treig, and at the entrance of 
the Rough Burn Glen, I attribute to the arrested glacier action resulting from the 
meeting of the glacier from Glen Treig with the one descending from the glens on the 
opposite side of Strath Spean; while the terraces fronting these and the Gulban mound 
* It is in this wider sense apparently that the term moraine profonde is used by ILogard and 
Dolfuss-Ausset ; but as that term is generally understood in Agassiz’s more contracted sense, its use 
geologically seems to me objectionable. 
t This would explain the difficulty, alluded to by various writers, that arises from the absence of glacial 
moraine detritus in many highly glaciated tracts of the Highlands. 
MDCCCLXXIX. 4 T 
