466 A. L. Leach — Glacial Drift and Raised Beach, Port Clais. 



blown sand, the presence of abundant igneous pebbles, and the looser 

 character of the shingle and head at Porth Clais as compared with 

 Grower are due to local conditions. In Grower the Raised Beach Series 

 accumulated on a limestone platform, bordered by a sandy shore, 

 which subsequently yielded the blown sand, and overhung by 

 limestone cliffs whence the angular debris or head was derived. 

 Carbonate of lime was therefore abundantly present to supply the 

 calcareous cement by which the shingle and head are firmly concreted. 

 Near Porth Clais, on the other hand, where the rock-platform was 

 cut in non-calcareous shales (penetrated by igneous intrusions), and 

 shelved rapidly into fairly deep water, neither the shingle nor the 

 head contained any appreciable amount of calcareous material, and 

 these members are but loosely compacted. They were, therefore, 

 readily broken up when the Boulder-clay passed over them, and, in 

 fact, the head is only clearly preserved under the lee of the steep 

 cliff on the west ; elsewhere so much glacial material has become 

 mixed with it that the two are not separable. From the evidence 

 given above the raised beach near Porth Clais, in respect to its 

 elevation, constituent members, and relation to the glacial beds 

 appears to correspond with that of Gower, and is similarly pre-Glacial 

 in the sense that it underlies the lowest visible glacial deposit. 

 But the mode of transport of the large igneous boulders, which 

 occur as erratics in the raised beach both here and in South Pembroke- 

 shire, is a problem requiring further consideration, and if the 

 solution be found in the hypothesis that they were transported by 

 drifting boulder-bearing ice, then the formation of the raised beach 

 must be associated with a climate sufficiently cold to permit large 

 masses of shore-ice to form around the beach boulders, and to drift 

 several miles southward along the coast. The glaciated boulders 

 described in this paper were not striated by shore-ice, which 

 admittedly can in Arctic regions l striate stones and the rock-platform 

 beneath. The definite and uniform direction of the striations is 

 against this view, and shows that they were caused by land-ice 

 moving in from the W.N.W. (approximately), and moreover no 

 striations can be observed on beach boulders which are not directly 

 overlapped by the Boulder-clay. 



Summary. 



1. The raised beach near Porth Clais consists of shingle and 

 large wave-worn boulders resting on a rock-platform, and overlain 

 by angular head or rubble of local rocks. 



2. Glacial drift (Boulder-clay) overlies the head at the western 

 end of the section, and towards the east has ploughed into the 

 raised beach and rests on the solid rock. Some of the boulders 

 of the raised beach and a small piece of the rock-platform are 

 definitely striated under the Glacial drift. 



3. The raised beach is pre-Glacial in the sense that it underlies 

 the Glacial deposit. 



4. The raised beach contains non-local boulders which may have 

 been transported by drifting shore-ice. 



1 Colonel H. W. Fielden, Quart. Journ. Geol. Soc, vol. xxxiv, p. 556, 1878. 



