354 



ME. C. EEID ON THE PLEISTOCENE 



Opposite Thorney Coastguard Station, and also still nearer to 

 Medmerry Farm, the surface of the Bracklesham Clays is bored 

 into by that colony of Pliolas crispata which struck Godwin-Austen 

 as so remarkable, owing to the size of the individuals. I have had 

 many opportunities of examining tlie colony, and cannot help 

 thinking that the resemblance of the shells to the gigantic 

 specimens found in the Arctic seas, or in glacial deposits like the 

 Bridlington Crag, is no accidental coincidence, but shows that the 

 specimens lived under Arctic conditions. 



It will be observed that Godwin-Austen distinctly states that the 

 southern Pecten polymorplms is found within the crypts occupied 

 by the large Pholas crispata. The contents of a large number of 

 these crypts were therefore examined ; but, instead of containing 

 southern species, those nearest to Medmerry yielded nothing but 

 Balanus porcatus^ a northern cirripede not recorded from the over- 

 lying mud-bed, but always associated with the large Pliolas crispata 

 in the recent state. 



This observation seemed difficult to reconcile with the statement 

 of Godwin-Austen ; but better sections showed that in all proba- 

 bility the PAo Zas-borings belonged to an older Arctic deposit. The 

 crypts ought thus usually to contain shallow-water Littorince and 

 Balani, like those found in the glacial gravels ; but where the newer 

 marine strata happened to cut down to the Bracklesham Beds, the 

 crypts in hard clay might readily be partly scoured out and re-filled 

 with southern mollusca, indicating a greater depth of water. 



No Arctic mollusca have at present been noticed in the erratic- 

 gravel, though valves of Balanus are abundant, and Littorina also 

 occurs. The scarcity of shells is not surprising, for a shoal on 

 which ice-foot constantly forms, and ice strands in spring, must be 

 a most uncongenial habitat for littoral shells. Thus far no charac- 

 teristic Arctic mollusca have been discovered in our southern 

 counties, but the deposit at Medmerry is worth further search. 



The next question to be discussed is the stratigraphical position 

 of the gravel with erratics : is it older or newer than the marine 

 clays? If the suggestion above thrown out be correct, the 

 gigantic Pliolas crispata belongs to a cold period older than the 

 incoming of the southern mollusca, and therefore probably equi- 

 valent to a period when erratics were being transported. But, 

 unfortunately, we cannot trace the erratic-gravels till they pass 

 under or over the clays with southern mollusca, and there remains 

 a gap of about half a mile between the two deposits, l^o doubt the 

 shingle of the old raised beach can be seen to j)ass over each, but, as 

 the relation of the beach to the mild period is not perfectly clear, 

 the succession cannot be proved by direct superposition. Another 

 method is available : to observe the occurrence of material derived 

 from the one stratum and re-deposited in the other. No fragments 

 of southern mollusca have yet been found in the erratic-gravel, 

 but the clays with southern mollusca often contain re-deposited 

 erratics. The gravel with erratic blocks is therefore the older of 

 the two. 



