CORNISH POST-TERTIARY GEOLOGY. 49 



case. Yet, to explain the thoroughness of Pleistocene denudation by- 

 agencies suflficiently powerful to sweep away the debris resulting 

 from their erosion, is in a measure to hint at the introduction of ice 

 in some form. There is no reason to think that Cornwall was lower 

 during the period preceding the formation of the old beaches than 

 its present altitude ; but, on the contrary, it is possible that it may 

 have been much higher. Supposing the erosion of the English 

 Channel, as Mr. Godwin-Austen suggests, to have been begun in 

 the early part of the Pleistocene epoch by a river flowing westward, 

 the transport of Cretaceous materials obtained from the eastern 

 districts would have furnished a plentiful supply of flints for in- 

 corporation in the subsequent beaches, and their presence both in 

 the raised and modern beaches would be easily accounted for. As 

 the influence of glacial conditions became felt, the highlands would 

 become covered by snow, liberating great quantities of surface- 

 water with torrential power during intermittent warmer periods, 

 and during the succeeding amelioration ; by these causes great 

 surface waste would take place, the valleys would be deepened, 

 and the existing deposits in exposed situations swept down with 

 recently derived materials to lower levels now beneath the sea. 

 The alternative version postulates more than this, namely, the 

 descent of glaciers from the snow-covered highlands. In both 

 cases the observation I have applied to the old boulder gravels holds 

 good, namely, the fact that the present area of Cornwall (at least 

 in its more western parts) would rej)resent but the watershed, or, in 

 the case of ice, the generative sources, and tracks of its nascent flow, 

 and all moraine material, etc., would be swept down to districts 

 now beneath the sea, whilst roches moutonnees, etc., would be very 

 unlikely to survive the subsequent extensive weathering of exposed 

 rock surfaces which took place during the accumulation of the Head. 

 Reasons have been assigned for believing that coast-ice helped 

 to transport some of the materials found in the raised beaches of 

 Brittany. 1 The large granite boulder pointed out by Mr. Williams 

 rests on the old beach platform in Barnstaple Bay, and is surrounded 

 by the raised beach materials which also rest on it. It is evidently 

 an erratic. Some months after my paper on the Pleistocene Deposits 



1 Annales de la Soc. Geol. du Xord, T. 4, p. 186, April 18th, 1877. 



4 



