10 



Prof. J. Prestwich. On the Origin [^ a y 1> 



The observations of MacLaren, Chambers, Milne-Home, Jamieson, 

 James Geikie and others, sufficiently prove the great thickness of the 

 ice covering in this part of Scotland during the first period of intense 

 glaciation. On the flanks of the Ben Nevis range, glacial striae extend 

 to a height of more than 2,000 feet, while on the hills facing this 

 range, and on the opposite side of the Spean Valley, such striae have 

 been traced to nearly the same height, as for example to nearly the 

 summit of Craig Dhu near the entrance of Glen Roy, and on the hills 

 above the Rough Burn by Mr. Milne-Home to a height of 1,600 feet. 

 Everywhere, the rocks in the lower parts of Glen Spean are intensely 

 glaciated, as are also those at the head of Glen Roy and around the 

 Cols of Glen Glaster and Makoul. At the same time, the erratic 

 blocks with those beds of sand and gravel of foreign origin which have 

 been found widely distributed over the hills around Glen Roy to 

 heights of from 800 to 2,100 feet, afford confirmatory testimony of 

 the depth of the land ice which then covered the country. There can 

 be little doubt therefore of the vast extent and general spread of the 

 ice-sheet at the first period of glaciation. 



With the incoming of this glacial period, local glaciers must have 

 descended from every mountain range, and so long as the glacier of 

 one steep glen became confluent with another of the same chain flow- 

 ing in the same general direction, so long would their course be 

 uninterrupted, and the propelling and abrading force maintained, as 

 in the Alps at the present day ; but when, emerging from these glens 

 into valleys of small gradients dividing the several mountain chains, 

 they met with glaciers descending from these other ranges, their pro- 

 gress was not only subject to be checked, and their forces neutralised, 

 but their course diverted, for if the lines of natural drainage 

 were barred, the ice took those of least resistance, although such 

 might be up-hill and against the lines of drainage. This, however, 

 could not be effected without excessive pressure and heaping up of 

 the ice at the points of junction. 



These interferences must have been especially frequent in the valley 

 of the Spean. On the one side, the glaciers descending the steep 

 ravines of Larig Leachach, the Cour and others on the northern 

 flank of the Ben Nevis range, issued into Glen Spean and projected 

 across it to the Glen Roy hills opposite. Below to the west, the great 

 Nevis Glen glacier emerged into the valley of the Lochy, while 

 above to the east the great glacier from Glen Treig turned down 

 Glen Spean, but, meeting with the aforesaid group of glaciers from 

 |3en Nevis, was partly diverted over the flanks of Craig Dhu, and 

 upon the entrance to Glen Roy. 



While the glaciers from this system of mountains were becoming 

 confluent in and idling Glen Spean, those from the opposite range 

 of hills were descending Glen Roy, Glen Feitheil, the Rough 



