near Port Clais, St. Davids. 465 



can be closely inspected across a fault-chasm a few yards wide, 

 and the point of greatest interest (indicated by * in the diagram) 

 where the glacial deposit rests on striated boulders is easily accessible. 

 Here (see Figure) several boulders of the raised beach remain firmly 

 cemented to the platform, but most of the shingle has been swept 

 away, and the boulders are covered by yellowish clayey drift. The 

 boulder (a) is a ' beach ' boulder of greenish igneous rock (probably 

 local), and its upper surface is smoothed, polished, and covered with 

 well-marked striations, of which a few run irregularly, but by far 

 the greater number run uniformly slightly west of a N.W. by S.E. 

 line. Before my visit this striated surface was hidden by soil, 1 

 and its remarkable freshness is due to this protective covering. The 

 boulder (b), an almost spherical mass of Cambrian purple sandstone, 

 lies next to (a) and is also a raised beach boulder, but it has been 

 long exposed to the weather and the glacial striations have almost 

 disappeared. The block Qc) is not a beach boulder, but a piece of 

 the igneous dyke which here forms the platform of the raised beach. 

 It has one smooth water-worn surface covered by striations, now 

 partly obliterated by weathering. These boulders lie at the margin 

 of the preserved portion of the raised beach. To the west, under the 

 head and glacial drift, boulders and shingle extend continuously for 

 more than 50 yards, but to the east there is nothing but Boulder-clay. 



The glacial character of the deposit superposed on the raised beach 

 is well-marked. It is an unstratified gravelly Boulder-clay packed 

 with blocks and pebbles of Cambrian purple sandstone in great 

 abundance, boulders and pebbles of local igneous rocks such as 

 the pre-Cambrian granophyre (Dimetian), and the later norites and 

 diabases of the St. Davids Head intrusive rocks, with flint pebbles 

 and other rocks probably not of local origin. It contains well-striated 

 pebbles, and one huge diabase boulder (weighing some tons) which 

 projects conspicuously upon the seaward face of the drift is at 

 least a mile from the nearest exposure of any similar intrusive 

 rock. Since this deposit rests in part on a striated surface and 

 contains striated stones and erratic boulders its glacial origin is clear. 



The succession may now be compared with that on the Gower 

 coast 2 — 



I have recently found in this district a representative of the post- 

 Glacial head, which will be described on another occasion. The 

 agreement in general level of the raised beach in the two cases is 

 very close, and differences such as the absence of shells and of 



1 The soil has been replaced upon the glaciated surface and covered by 

 a large slab of rock. The point * is 20 yards west of the large angular boulder 

 indicated in the diagram. 



2 See Mr. B. H. Tiddeman's figure reproduced in Geology in the Field, p. 852. 

 DECADE V. — VOL. VHI. — NO. X. 30 



