near Port Clais, St. Davids. 463 



by Dr. Strahan on the geology of South "Wales 1 in the Jubilee volume 

 of the Geologists' Association. 



" 1. The highest member, or head, is a talus of angular fragments of 

 limestone, obviously derived from the neighbouring cliffs, and con- 

 taining no other rocks. 



"2. Under this and mingling with it is a foxy-red sand, very 

 variable in thickness, and containing an occasional snail-shell. This 

 is evidently a blown sand. 



" 3. The lowest member is a layer of well-rounded pebbles of lime- 

 stone, crowded with shells of recent species. . . ." 



Dr. Strahan gives a list of these shells, and states that the shingle 

 rests on a water- worn platform about 10 to 15 feet above the 

 present level of the shore (or about 25 feet above Ordnance Datum), 

 and that the head and the shingle are always firmly cemented by 

 carbonate of lime. The proof of the pre-Glacial age of this series 

 of deposits depends on Mr. Tiddeman's observations at several points 

 on the Gower coast, where glacial deposits were found frequently 

 to rest upon but never to underlie the raised beach deposits. 



In South Pembrokeshire I have observed the 'platform' and 

 its associated deposits at several points, but although the shingle, 

 blown sand, and head always occupy the same relative positions as 

 in Gower, the three members are seldom well displayed in any one 

 section, and in no case has a glacial deposit yet been observed clearly 

 resting upon the raised beach series. Mr. E. L. Dixon, who has mapped 

 the raised beach in this district, remarks that at Bullum's Bay (Caldey) 

 "the glacial deposit appears to overlie the raised beach, though the 

 exposure is obscure, and the evidence of superposition is not so 

 conclusive as in Gower". 2 Again, in the Milford Haven district the 

 platform has nowhere been found in clear association with glacial 

 deposits. 3 



Near Porth Clais in North-West Pembrokeshire, the stratigraphical 

 relation between the raised beach and the glacial deposit is perfectly 

 clear and conclusive. It is of interest to note that the evidence is 

 to be found in the immediate vicinity of a section of the raised 

 beach and head, figured by Prestwich in his great paper on "The 

 liaised Beaches and Bubble Drift or 'Head'". 4 The sketch for 

 the figure must have been taken from a point very close to the 

 place where the glaciated surfaces are now exposed. 



Prestwich mentions two raised beach sections, one " a short distance 

 west of Porth Claus [Clais] Harbour", the other "half a mile W. 

 of Porth Claus [Clais] Harbour". The first is illustrated by a figure 

 which shows merely head resting on " Beach consisting of subangular 

 fragments of Cambrian and pre-Cambrian Bocks, granite, veinstones, 

 porphyry, white quartz pebbles, and a few flint pebbles". The 

 second exposure, which was pointed out to Prestwich by Dr. Hicks, 

 is described simply as " Beach 8 feet thick and covered, by 15 feet 

 of Head". One of these exposures, probably the second, must be 



1 See Geology in the Field, 1910, pp. 851-2. 



2 Summary of Progress for 1905 (Mem. Geol. Suit.), p. 70. 



3 E. L. Dixon, Summary of Progress for 1906 (Mem. Geol. Surv.), p. 63. 



4 Quart. Journ. Geol. Soc, vol. xlviii, 1902, p. 293. 



