292 



PROF. J. PRESTWICH 0]!^ THE RAISED 



The lower angular rubble, which looks like an ordinarj- talus, 

 maintains for some distance a thickness of from 8 to 12 feet, but at 

 the northern end of the Bay it suddenly expands in the shape of a 

 boss or ridge rising to the top of the cliff and attaining a thickness 

 of about 60 feet, while the shell-bed on both sides thins out against 

 it (see PI. yil. fig. 7). 



The lower rubble there contains some large worn blocks of a quartz 

 conglomerate, and the shell-bed, which has swelled out to a thick- 

 ness of 20 to 25 feet, consists of large pebbles of limestone, red sand- 

 stone of local origin, with others of porphyry, jasper, Coal Measure 

 rocks, conglomerates, tfec, derived from the adjacent Boulder Clay 

 Series, in a matrix of greyish sand. The only shells I found in this 

 bed on my two visits were — Mi/a (in fragments), Tiirritella terebra, 

 Nassa reticuUita, and N. incrassata (?). They are numerous in 

 places and in a good state of preservation. 



The red rubble which overlies the shingle is from 10 to 30 feet 

 thick, and is spread out in great sheets or lenticular masses, alter- 

 nately fine and coarse, but without regular stratification. I saw 

 no fossils either in the upper or lower bed of rubble. 



The points in which this shell-bed and overlying rubble resemble, 

 and those in which they differ from, the Beaches on the southern coast 

 of Gower are so evenly balanced that it is difficult to say whether 

 the Beaches are synchronous or not. Their close proximity, sej)a- 

 rated only by the ridge of the Worm's Head promontory, ^ mile 

 wide, combined with the facts that they are on the same level, and 

 are both covered by a like local Head, leads to the presumption of 

 their synchronism. But the shells are diflerent. None of the 

 ordinary Baised Beach species occur in the Bhos Sili shingle, 

 whereas Turritella terebra and Nassa reticulata are amongst the 

 most common shells of the Boulder Clay Series, but very rare in 

 the Beaches. 



Either the beds are synchronous, the difference in their fauna 

 being due to the circumstance that the one lived on exposed 

 rocky coasts and the other off the shore in a sheltered bay ; or the 

 shingle-bed of Bhos Sili belongs to the Boulder Clay Series, and is 

 older than the Eaised Beaches. The presence alone of old-rock 

 pebbles in the shingle is not decisive, for they may have come from 

 the Glacial gravels, which are spread over the adjacent dis- 

 trict. Nor is the presence of boulders conclusive, for transported 

 boulders occur in both, although they are rare in the Beaches of the 

 Bristol Channel. 



On the whole, I am disposed to think that, though the lower 

 rubble-bed may be Glacial — possibly an old talus carried down from 

 the adjacent high range of Old Bed Sandstone, — the overlying 

 shell-bed is synchronous with the Beaches of the Gower coast. At 

 the same time this shell-bed, especially where most largely deve- 

 loped, is so singularly like many of the ordinary sections of the 

 shelly shingle-beds of the Boulder Clay that it is impossible to avoid 

 the suggestion that in these sections we may have an instance of 

 the synchronism of the Baised Beaches of the south with the later 

 Glacial beds of the more northerly counties. 





