684 
PROFESSOR PRESTWICH ON THE ORIGIN 
the joint action of this stream and of the great stream coming’ from Ben Nevis and the 
Lochy.This direction of the ice-flow is confirmed by an observation of Mr. Campbell, 
who found, at a point 1 mile N.W. of Spean Bridge, ice-striae pointing W.N.W., 
indicating that the westernly ice-stream of the Spean Valley was there deflected 
northward towards the Great Glen. The local variations in this area may be due to 
variations in the flow of the ice at different periods of the growth of the ice-sheet. 
With respect to the ice-flow higher up Strath Spean, I do not see how it was 
possible for the Treig glacier, after pushing across that valley, to have sent off two 
lateral branches, one up and the other down the valley, while the central portion con¬ 
tinued to advance up the hill to the pass of Glen Glaster. Although the ice-striae 
opposite Glen Treig are transverse to the valley they are not directed towards Glen 
Glaster, but point more westward, while lower down (the valley here runs east and 
west) the influence of the mass of the Ben Nevis range of glaciers shows a deflection 
of the ice-stream over Craig Dhu, in the direction of Glen Boy. Higher up the valley, 
the striae near the Bough Burn point, not up and down the main valley, but in the 
direction of the hills north of Strath Spean (which rise to the height of 3,000 to 
3,400 feet), as though a glacier had descended from them along Glens Eeitheil and 
Chaoruinn into the valley of the Spean ; while nearer Moy, where the valley turns 
a few degrees more northward, the striae again conform nearly to the direction of the 
main valley. The following are the corrected bearings I took of these striae :— 
1. A little east of Tulloch . . . E.S.E. and W.N.W. 
2. Nearly opposite Inverlaire . . E. and W. 
3. Nearly opposite Glen Treig . . S. 22° E. and N. 22° W. 
4. Near the Bough Burn .... N. 7° W. and S. 7° E. 
5. About l-§- mile west of Moy . . W. 32° S. and E. 32° N. 
While therefore I agree in considering that the striae Nos. 1, 2, and 3 are due to 
the Treig glacier, I think that its whole mass turned and flowed westward down the 
Spean Valley, and that the striae Nos. 4 and 5 are due to other glaciers descending 
southward from the opposite hills into the Bough Burn and Strath Spean, where they 
met and were stopped by the Treig glacier, when, driven to take an easterly course up 
Strath Spean, they travelled in confluence with the glaciers of Glen Gulban, or Ghuil- 
binn, and of other tributary glens, over the pass of Makoul into Strath Spey. 
Whether or not another portion of the Spean Wiley ice passed up the valley of 
Glen Ptoy and over its col into Glen Spey, I think far from decided. The glaciers 
from the Ben Nevis range may have so dammed up the entrance to Glen Boy that 
no glacier belonging to that valley could pass out into Glen Spean. Nevertheless, 
during the first extension of the ice-sheet, the glaciers from the mountains at the head 
* I am unable, however, to account for the rocks at the first of these places being most bared on their 
west side, unless it be due to the pressure of the great Lochy and Nevis ice-stream. 
