Vol. 56.] OF GLACIAL EROSION AT LOCH LOCHY. 201 



common practice among some geologists of late years to attach 

 very small importance to the erosive power of glaciers. 1 This 

 has been partly due to the fact that some lake-basins, formerly 

 attributed to glacial erosion alone, have been shown to have owed 

 their formation, in part at least, to other causes, and that doubts 

 have always existed whether large deep lakes such as Loch Ness 

 could have been excavated by glacial action. It is therefore well to 

 point out that the origin of lake-basins is not the same question as 

 the erosion of the sides or even the bottom of a valley by glacial 

 action. I have not the slightest wish to enter into the question of 

 the origin of lake-basins — a subject on which the last word has not 

 yet been said — but I think it is evident tbat the slopes nearLaggan 

 have been cut away, and to a very great extent by glacial action, 

 because no such sloping surface as is there exposed can have been 

 produced by ordinary freshwater erosion. 



There can, I think, be no reasonable question that, as has been 

 shown by Sir Archibald Geikie, 2 the Great Glen of Scotland, at the 

 commencement of the Glacial Epoch, was an ordinary river-valley. 

 In this case the lateral glens, of which the higher portions remain 

 almost as they were in pre-Glacial times, must have contimied down 

 to the valley, with ridges between them. The lower portions of 

 these ridges, up to a height of about 1000 feet, have been completely 

 swept away. It is impossible with any degree of precision to 

 estimate the magnitude of these ridges ; but, taking into consideration 

 the facts that some of the higher glens, despite the Glacial denuda- 

 tion which has manifestly worn down the crests that separate them, 

 are still upwards of 500 feet deep, and that the flat bottom of the 

 main valley at Laggan is 3 furlongs or 2000 feet broad, it is 

 probable that a thickness of at least 250 or 300 feet of rock has been 

 removed throughout a considerable portion of the valley. The fact 

 that the stream-beds from the higher glens are cut off by the slope 

 and have formed shallow ravines in it is evidence that the Glacial 

 erosion cut into the hillsides to a greater depth than the streams 

 formerly did, and that not only the intervening spurs were abraded, 

 but even the hillside from which they projected was planed away. 



(ii) The second point is of at least equal interest. Most geologists 

 will heartily concur with Sir Archibald Geikie's remarks in his recent 

 address to Section C of the British Association at Dover on the 

 importance of obtaining trustworthy estimates of geological time. 

 Now the latest subdivision of geological time, that which has 

 elapsed since the Glacial Epoch, is precisely that with which we 

 ought to be best acquainted. But it is notorious that the different 

 attempts which have been hitherto made to estimate the number of 

 years that have passed since glaciers disappeared from the valleys 



1 Quite recently (in Geol. Mag. 1899, p. 486) Mr. A. Harker has made precisely 

 similar remarks on the efficacy of glacial erosion, when describing its effects in the 

 island of Skye, which is only about 40 miles west of Loch Lochy. 



2 ' Scenery of Scotland ' 1st ed. (1865) p. 180, 



Q.J.G. S. No. 221. p 



