Yol. 6S.^ GLACIAL OEIGIN OP THE CLAY-WITH-F LINTS. 199 



11. On the Glacial Origin of the Clay- with -Elints 0/ Bucking- 

 hamshire and on a former Course of the Thames. By 

 • Eobert Lionel Sherlock, D.Sc, A.H.C.S., F.G.S., and Arthur 

 Henry ^N'oble, B.A., F.G.S. (Eead March 13th, 1912.) 



[Plates XII-XIY.] 



I. Introduction. 



During the years 1910 and 1911 we have surveyed on the 6-inch 

 scale some 260 square miles in the counties of Buckinghamshire, 

 Berkshire, Hertfordshire, and Middlesex, and have obtained some 

 results which we believe to be new. Perhaps we can now explain 

 the mode of formation of the Clay-with-Flints and superficial 

 gravels, so long a puzzle to geologists. 



This paper falls naturally into two parts — first, the origin of 

 the Clay-with-Flints and associated gravels of our area ; and, 

 secondly, the origin of the gravels which have sometimes been 

 called Plateau Gravels,^ and are high above the ordinary river- 

 gravels, though at lower levels than those associated with the Clay- 

 with-Flints. These Plateau Gravels we believe to be of Fluvioglacial 

 origin, and their formation is inseparabl}' connected with the former 

 course of the Thames, at a time when the ice had not yet reached 

 its southern limit of extension. 



Our classification of the superficial deposits is somewhat difi'erent 

 from that of the Old Series Survey Map. The brick-earth accom- 

 panying the clay with fiints is usually a clay-gravel, and differs 

 only from the deposits coloured as Gravel in that it is worked for 

 brickmaking. We have, therefore, in these cases mapped it as a 

 gravel. Also, the Pebble Gravel which caps certain hills does not 

 differ in some cases from the Glacial Gravel. On the other hand, 

 the Glacial Gravel of the old map has been divided into two 

 divisions — one of Gravelly Drift, and the other of Fluvioglacial 

 Gravels. 



II. The Clay-with-Flints and the Accompanying Gravels. 



(a) The Clay-with-Flints. 



The irregular accumulation of clay, loam, and brick-earth with 

 abundant fiints, which covers large areas of the Chalk, received 

 its name of Clay-with-Flints in 1861 from Mr. Whitaker,^ who 

 considered it to be derived in great part from the slow decomposition 



^ As, for instance, H. J. O, White, Proc. Geol. Assoc, vol. xv (1897-99) 

 p. 159. 



2 'Geology of Parts of Oxfordshire & Berkshire' Mem. Geol. Surv. 1861, 

 p. 54. 



