640 STEATIGKAPHICAL GEOLOGY 



also the allied species V. clactonensis ; Neritina grateloupiana, 

 found in the Thames valley, is also extinct though allied to the 

 modern N. danubialis. A. common shell in the older river gravels 

 is Unio littoralis, which is extinct in Britain but still living in 

 Europe. Among land shells there is Helix fruticum, now only 

 found alive in Central Europe, and Clausilia pumila, found at 

 Barn well, near Cambridge, but now only living in parts of Northern 

 Europe and in Silesia. Hydrobia marginata and Pisidium astartoides 

 are other species found in British Pleistocene deposits but not now 

 living in Britain. These and some of the above are shown in 

 Fig. 209, for the use of which I am indebted to Mr. B. B. Woodward. 



2. British Deposits 



As it would be impossible within the limits of this volume to 

 give anything like a comprehensive account of the clays, gravels, 

 cave - earths, raised beaches, and other non-Glacial deposits in 

 Britain, only a few examples will be described. 



Clay-with-Flints. This is a deposit or accumulation of 

 indefinite age, occurring at high relative levels and mainly composed 

 of materials derived from the Eocene clays and sands ; hence it 

 only occurs in the south of England and the north of France. 



Its typical form is that of a stiif reddish-brown clay containing 

 a large number of flints, the majority of those in the lowest layer 

 being entire unworn Chalk-flints, while those in the higher part 

 are angular pieces of broken flints, with some quartz pebbles and 

 fragments of ironstone. In some districts there is also a large 

 percentage of sand in the clay. The whole deposit lies irregularly 

 011 the Chalk, from which the unworn flints have been derived, and 

 by the solution of which it has been let down into hollows, funnels, 

 and pipes. This clay is often overlain by, and sometimes it seems 

 to pass laterally into, thick deposits of loam, brick-earth, sand, and 

 gravel, all of which may have been derived from the destruction of 

 the Eocene Beds. 18 



Plateau Gravels. This name has been given to various 

 unstratined gravels in the southern, eastern, and midland parts of 

 England, because they occur on the plateaus out of which the 

 modern river-valleys have been excavated. In Norfolk and Suffolk 

 they generally rest on the Chalky Boulder-clay, and consist mainly 

 of large rolled and battered flints whence they have been called 

 " Cannon-shot Gravels." 



In the Midlands they rest on Jurassic rocks and contain a 

 mixed collection of stones, most of which have been derived from 

 more or less distant sources. Thus around Oxford the Plateau 

 drift contains quartzite -pebbles from the Bunter, Carboniferous 



