Reports and Proceedings — Geological Society of London. 285 



S.— March 24, 1909.— Professor W. J. Sollas, LL.D., Sc.D., F.R.S., 

 President, in the Chair. 



The following communication was read : — 



"Glacial Erosion in ]^orth Wales." By Professor William Morris 

 Davis, For. Corr. G. S. 



An excursion around Snowdon in September, 1907, followed by 

 a further visit in 1908, led the author to the conclusion that a large- 

 featured, round-shouldered, full-bodied mountain of pre-Glacial time, 

 had been converted by erosion during the Glacial Period — and chiefly 

 by glacial erosion — into the sharp-featured, hollow-chested, narrow- 

 spurred mountain of to-day. The peculiar indifference of topographic 

 form to the trend of formation boundaries and the insequent stream 

 arrangement, are what might be expected as the result of prolonged 

 erosion upon a mass of complicated and resistant structure. The 

 author discusses Ramsay's theory of a plain of marine denudation, and 

 is of opinion that the upland seems to deserve classification rather 

 with peneplains ; he suggests for it a Tertiary date, which would 

 not be inconsistent with the erosion of open valleys in the uplifted 

 peneplain after its elevation, and argues that Snowdon and its high 

 neighbours had a relief of some 2,000 feet above the plain. As the 

 result of a comparison with the non-glaciated regions of Devon, it is 

 considered that the dissection of J^orth Wales must have been some- 

 what less developed in pre-Glacial times than in Devon to-day. On 

 this assumption it is possible to make a tentative restoration of the 

 pre-Glacial form of Snowdon : subdued mountains, with dome-like 

 summits and rounded spurs, drained by prevailing graded streams of 

 accordant levels at their junctions. In fact, the characters were those 

 of ' moels ' such as Moel Tryfaen or like the well-worn Appalachian 

 Mountains of jN^orth Carolina or the Cevennes. 



The chief abnormal featui'es of Snowdon are the following : — 

 Alongside the graded summit and slopes of a ' moel ' stand the 

 head-cliffs of a rock-walled cwm, in the floor of which talus is now 

 accumulating. The cwm-floors are generally stepped, sometimes 

 more than once, and the streams cascade down into the valleys. The 

 cross-profile of the valleys is often a fine catenary curve, down the 

 sides of which streams fall from hanging valleys. The slope of the 

 main valleys occasionally decreases even to the point of reversal, as 

 where lakes occur ; and in the immediate neighbourhood of smoothly 

 graded, waste-covered slopes knobby or craggy ledges and bars of 

 rock often appear. After pointing out that such features are generally 

 associated with glaciation, the author proceeds to discuss two out of 

 four possible hypotheses put forward : "that glaciers are essentially 

 protective agencies," or that they "are active destructive agencies". 

 The consequences deduced from the hypotheses are confronted with 

 the actual facts, and it is found that these, and especially those 

 relating to rock-steps, cannot be explained on the protection-theory, 

 while the theory of a destructive agency seems to explain most of 

 the facts. 



These facts are dealt with under the following heads : — Valley- 

 head curves; valley-floors, lakes; valley-floors, rock steps ; valley- 



