DRIFTS OF THE VALE OF CLWTD. 



87 



on in the same line, as far again, we find the Hessle beds con- 

 taining the same fauna. 



In all these beds there are flints somewhat rolled, subangular, as 

 they call them, but never worn to pebbles. 



Round the south coast to Pembrokeshire we find the same, and 

 the low-lying plateau at St. David's is covered by a gravel containing 

 flints ; but I have not as yet found traces of shells in it there. 



This is part of a wide submergence, and of course, when the 

 higher hills were submerged to the extent that I have shown above, 

 the lower regions and the hollow places were all below the sea, the 

 lowest going down first ; so, as the climate was growing milder at 

 the close of the age of glaciers, the more southern and temperate 

 forms of life kept following on the receding ice, but the earlier 

 deposits would still contain many of the Arctic types. This may be 

 the reason why, in Mr. Shone's lists of shells from the drifts in 

 Cheshire and South Lancashire, there are so many more northern 

 species than appear among the shells in the Yale of Clwyd, or Colwyn 

 Bay, or the higher levels of Moel Tryfan, Caerhug, or Macclesfield. 

 Or it may be that the Scandinavian shells are derived from an older 

 boulder-clay. 



But we must not attach too great importance to this point ; for 

 the persistence of a few northern forms does not justify our referring 

 even these beds to the glacial age. IS earer the mountains we have 

 abundant evidence that the deposits we have called the St. Asaph 

 Drift are Postglacial. The striated stones are all such as might be 

 derived from the preexisting Arenig Drift ; none of the stones 

 peculiar to the St. Asaph Drift are glaciated. Broken glaciated 

 boulders, balls of Arenig boulder-clay, and, with very few exceptions, 

 shells not of Arctic type occur in this St. Asaph Drift. 



Deposits of a submergence which succeeded the age of great 

 glaciation have been recognized round the north and east of Wales 

 (Mackintosh, Quart. Journ. Geol. Soc. vol. xxxviii. 1882, p. 184). 

 Along the " Severn Straits " and beyond into the Midland counties, 

 relics of the material washed from the older drift are recognized * 

 (Rev. W. Lister, Q. J. G. S. vol. xviii. 1862 ; Davies, Proc. Geol. 

 Assoc. vol. iv. 1876, p. 423 ; Crosskey, Proc. Birmingham Phil. Soc. 

 vol. iii. 1882, p. 209). 



Marine deposits of this age occur over the plains of Cheshire and 

 Lancashire, as may be seen from the references I give below with 

 the lists of shells (see also : — De Ranee, Quart. Journ. Geol. Soc. 

 vol. xxvi. 1871, p. 641, Mem. Geol. Survey, " Superficial Geology of 

 S.W. Lancashire " ; Mackintosh, Chester Soc. Nat. Sci. Feb. 1876, 

 Quart. Journ. Geol. Soc. vol. xxv. 1869, p. 407, vol. xxxiii. 1877, 

 p. 732 ; Ricketts, Proc. Liverpool Geol. Soc. 1876-7, p. 245 ; Morton 

 and Shrubsole, Proc. Liverpool Geol. Soc. 1875-6 ; Morton, G. H. 

 ib. 1876-7, p. 294, &c, Bep. Brit. Assoc. 1876, p. 110, Geol. 



* In a paper just published, Mr. Deeley says of the Pleistocene succession in 

 the Trent Basin, "all the deposits I have described as belonging to the two 

 previous epochs [Older and Middle Pleistocene] were formed during one con" 

 tinuous period of submergence " (Q. J. Gr. S. vol. xlii. 1886, p. 467). 



