Correspondence — W. M. Davis. SSl 



CORRESPONDENCE. 



GLACIAL EEOSION OF SNOWDON. 



Sir, — Professor Gregory's article on " The Pre-Glacial Valleys of 

 Arran and Snowdon " in your April number does not give, as far as 

 Snowdon is concerned, sufficient evidence for the form that he 

 assumes for the Preglacial valleys. The question is, as he says, 

 "' merely how far the glaciers enlarged and deepened these pre- 

 existing valleys." He c|uotes Ramsay as to the local glaciers 

 " escaping from the high bounding-walls of the upper parts of their 

 valleys ", and infers from this that Ramsay thought " the valleys 

 [that is the valleys in their existing form] were pre-Glacial". That 

 was very likely Ramsay's opinion, but as he thought that the 

 Snowdon glaciers merely scoured the pre-existing cwms and valleys, 

 and as he gave practically no consideration to their possible excava- 

 tion by glacial erosion, his views on the mooted question count for 

 little. ^ 



Professor Gregory goes on to say : " The uplift of Snowdonia led 

 to the erosion of canyons along the floors of the old valleys, the width 

 of which indicates that they had been worn down to base-level and 

 their walls cut backward before the Glacial period." It is here 

 implied that the width of the Preglacial " canyons " is known ; but 

 as a matter of fact, their width as well as their depth is in question. 

 The question cannot be satisfactorily solved simply by assuming, as 

 Professor Gregory seems to do, that the canyons had some such 

 width as the valleys possess to-day ; for in that case the branch 

 valleys would have established accordant junctions with them, 

 and the present hanging relation of the branch valleys would remain 

 unexplained. This difficulty Professor Gregory proposes to remove 

 by assuming that '' if the major valleys be regarded as a reticular 

 system formed along fractures which gaped open at the uplift of 

 the country . . . the [greater J depths of the main valleys as compared 

 with some of their tributaries is a natural consequence of the more 

 rapid denudation along the tectonic ruptures". But no evidence 

 whatever is given to prox^e that the main valleys of Preglacial time 

 were formed in this highly hypothetical manner, nor is any reticular 

 valley system of such origin known elsewhere, for the newly deepened 

 valleys of other broadly uplifted regions lie, in the vast majority of 

 cases, along essentially the same courses that they followed before 

 uplift. Indeed, until a reticular system of valleys occupying gaping 

 fractures is shown to exist in some recently elevated region, the 

 gratuitous supposition that the main valleys around Snowdon are 

 of that extraordinary origin has no value. 



A profile from Snowdon across the valley of Lake Cwellyn to 

 Mynydd Mawr is introduced to illustrate the contrast between the 

 theories of strong and of slight glacial erosion. " The areas in solid 

 black represent the amount of solid rock removed on the alternative 

 hypothesis that the valley had been excavated approximately to its 



