686 
PROFESSOR PRESTWICH ON THE ORIGIN 
intervening between the rock and the glacier to the smallest possible amount. It is 
only when the glacier issues from these contracted ravines into valleys where the 
ground is comparatively flat, and where the ice is free to spread out laterally, that it 
loses its extreme power of abrasion, and may pass over detrital matter without causing 
disturbance, except by the direct action of pressure. Charpentier* * * § mentions that on 
one occasion the Glacier du Tour, in the valley of Chamouni, descended into that valley, 
and advanced a distance of 80 feet over a bed of gravel. When at the end of five 
years the glacier retreated, the gravel was found to be undisturbed, and even the tufts 
of Alpine plants on its surface were found in their placed 
Mr. Doughty j notices a similar state of things in some of the Norwegian glaciers. 
Alluding to the ’ploughing-out power of glaciers over loose materials, he says : “From 
what I saw in the Norwegian glaciers, I should believe this action has been rather 
exaggerated : the snout of the glacier ploughs out a little, and rises over the rest 
From the plate accompanying his paper I imagine that this happened where glaciers 
debouched into a transverse valley with small gradients. 
Colonel God win-Austen, || although he describes the disturbing pow 7 er of glaciers 
in ploughing up the soil and turf, also speaks of some of the glacier moraines of the 
Himalayas overlying loose sand in the bed of valleys. 
So when the glaciers of the Lochaber and other mountain ranges emerged from the 
contracted glens down which they first descended, into the broader and more open 
river valleys, they probably spread out and over some portion of the moraine debris, 
previously swept down in their front, before they coalesced and formed another stream. 
It is to be presumed, also, that in the case of glaciers meeting in opposition, their 
driving force would be more or less neutralised or stayed, according to the angle at 
which they met. In the Alps, where each glacier system is limited to its own centre, 
and there is no clashing of opposing glaciers in intervening valleys, we have no means 
of judging what would be the results of such conditions, but in Scotland, during 
the growth of the great ice-sheet, such clashing was inevitable It is not as 
where glaciers meet at a small angle, and, becoming confluent, continue uninter¬ 
ruptedly a downward course ; but where, issuing from glens belonging to different 
mountain chains into intervening valleys, they not only meet with altered gradients, 
but come also into direct collision with the glaciers of other systems. In these cases 
* ‘ Essai sur les Glaciers,’ pp. 42, 97, 1841. 
t Dolfcss-Ausset mentions that the lower glacier of the Aar, where the gradient is small, passes over 
and buries the materials composing the terminal moraine; and he instances a large block which was 
10 metres in front of the glacier in 1848, 2 metres in 1849 when it was nearly hidden by other debris, 
and in 1850 was covered by the ice, and has not since (1864) been seen.—Op. cit., vol. v., p. 415. 
J ‘ On the Tdstedal-braa Glaciers,’ London, 1866. 
§ The italics are mine.—J. P. 
|| Op. cit., pp. 24, 47. The author informs me that sand and gravel may frequently be seen under the 
front of these glaciers, bat his impression is that they do not pass far under. There was, however, no 
opportunity of determining the actual case. 
