QUATERNARY DEPOSITS OF BRITISH ISLES — BROOKS. 309 



numbers of shells of Corhicula fluminalis. Beneath the gravel, bor- 

 ings show another bowlder clay. At Grimsby, also, two bowlder 

 clays are separated in places by gravelly sand. This marine bed is 

 in fact found at a great number of localities; it was considered by 

 Wood and Rome (69) as a distinct deposit, termed by them Hessle 

 sand, between the purple clay and the Hessle clay, and C. Reid 

 states that there is no doubt whatever about its position between two 

 bowlder clays. It can be traced around the greater part of the old 

 bay of Holderness, and, where the ancient chalk cliff is low, extends 

 for some distance inland. The land sank gradually to a depth of 

 about 100 feet, and rose again gradually, so that except in parts of 

 Lincolnshire there is an almost complete absence of a cliff bounding 

 the gravels at this level. There is a distinct line of erosion at the 

 bottom of the gravels and another at the top. The only still-water 

 deposits of this horizon are those at Croxton and Kirmington. 



The marine fauna is by no means poor. Sixty-one species are 

 known, slightly northern, but not arctic. Except Corbicula pumin- 

 alis, there are no exclusively southern forms; 12 do not now range 

 so far south. The great majority are living British forms, many 

 of which do not extend far north. The fauna agrees almost per- 

 fectly with that of March in the fen land. The mammalia include 

 Elephas pHmigenius, Cervus tarandus, Bison priscus, and Rhinoc- 

 eros leptorhinus. We may safely assign the marine gravels to a 

 period of milder climate than the bowlder clays preceding and fol- 

 lowing it. The flora from the estuarine peat at Kirmington (70) 

 supports these conclusions. 



After the melting of the ice of the last glaciation the land lay about 

 40 feet lower than at present ; the beaches of this submergence have 

 all been destroyed by the encroachment of the sea, except a raised 

 estuarine beach extending for some miles near Barton, but river 

 gravels in the chalk valleys and the positions of some of the peat de- 

 posits, e. g., on Kelsey Hill, indicate a saturation level about 40 feet 

 higher than now. Most of these deposits contain a temperate 

 flora of oak and hazel, with Cervus Hfhegaceros^ Bos primiffenius, and 

 Elephas primigenius, hut at Holmpton temperate lacustrine de- 

 posits belonging to this stage are overlain by a laminated peaty bed 

 with Betula nana, which is so characteristic a northern form that it 

 seems sufficient evidence of a return of a colder climate; this arctic 

 bed agrees with similar postglacial arctic beds in the Pennine dis- 

 trict to be described. 



Just north of Holderness the glacial succession has been studied 

 by G. W. Lamplugh in the Flamborough Head district (71). The 

 equivalents of the four bowlder clays found by C. Reid in Holder- 

 ness are recognized, but at a higher level. The purple bowlder clay 

 is separated from the equivalent of the Hessle clay by unfossiliferous 



65133°— SM 1917 21 



