174 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



bourhood, regarded by us as an estuarine portion of the Hessle clay, 

 might much contribute towards the sohition of that point. 



The presence of Cyrena fluminalis in the Hessle gravel, assuming 

 the Kelsea-Hill bed to belong to that formation, suggests an iden- 

 tity between that gravel and those earlier Postglacial formations 

 occurring at Erith, Crajford, Ilford, West Hackney, and Grays in 

 the Thames valley, and at Gedgrave and Sutton in Suffolk, Clacton 

 in Essex, Chislet in Kent, and along the Cam, which have yielded 

 this shell. The negative evidence afforded by its absence in numerous 

 other Postglacial beds partakes of the unsatisfactory character always 

 attaching to that description of evidence; but in looking at the sub- 

 ject in the broader light of the sequence of Postglacial denudation, 

 that identity becomes strengthened ; for since these earlier Postglacial 

 deposits are, by the first-named of us, traced in this way as belonging 

 to that portion of the period which was anterior to the denudation 

 of the Weald valley, and was occupied in the Thames region by the 

 denudation which descended from the Glacial clay through the 

 Lower Tertiaries to the chalk, so the Hessle gravel obviously suc- 

 ceeded a period of lengthened Postglacial denudation, during which we 

 see, from the condition of the Wold-brow, that the land had emerged 

 at least 800 feet from the Glacial sea. On the other hand, the 

 formation of this gravel was succeeded by the time necessary to 

 produce a local depression of not less than from 150 to 200 feet 

 (which introduced the Hessle clay), by the formation of the quies- 

 cent deposit of that clay possessing an original uniform thickness of 

 nearly 20 feet, by a reelevation coequal in extent, by the formation of 

 the deposits of the / series, and, finally, by the changes which have 

 elevated the low land of much of the western Wold-foot above the sea, 

 and depressed part of that on the east, on which grew the Hull and 

 Grimsby forest, as much as from 33 to 62 feet beneath it. Reason- 

 ing in this way, and having regard to the probably greater rapidity 

 with which marine denudation during emergence proceeds, over 

 that of deposit during subsidence, we seem to have evidence here of 

 a period following the Hessle gravel, not inferior in duration to 

 that which the fii^st-named of us traces as having followed the for- 

 mation of the Thames-valley deposits*, and which, he considers, 

 was in that area occupied by the denudation of the Weald valley, 

 by the reversal of much of the drainage, and by the introduction of 

 the Thames river over a forest — a period to which the gravels of 

 several great river- valleys and other late Postglacial deposits appear 

 to us to belong. 



In Yorkshire and Lincolnshire we possess the greatest sequence 

 of deposits, from the climax of the Glacial period to the present time, 

 which any part of England has yet afi*orded. So far as it is a guide, 

 there appears to have been a gradual, but unbroken, ameliora- 

 tion of cHmate, and no indications of anything affording ground of 

 inference that any recurrence or alternation of severe conditions of 

 cold took place subsequently to the elevation of the country above 

 the Glacial sea. 



* Quart. Journ. Geol. Soc. vol. xxiii. p. 411 et seq. 



