312 PROF. J. PRESTWICH ON THE RAISED 



of the Kennet, where a rubble derived from the plateau of gravel 

 capping the adjoining Bucklebury range of hills (446 feet) skirts 

 the base (210 feet) of the hills, and the drift-gravel has clearly been 

 driven into the London Clay at the time of deposition. The section 

 is as follows : — 



feet 



Brick-earth — vei*y irregular to 4 



Flint-gravel, with Tertiary debris, indenting on the clay . 2 to 8 

 London Clay, with Septaria. 



In the lower part of the Thames Valley a rubble-drift overlies 

 the river-drift at Chislet, Crayford, and Grays Thurrock ; while 

 elsewhere around London it lies on an old land-surface of the 

 same period. One such instance I have described at West Hackney, 

 where I showed that on the plain, 60 feet above the Thames, there 

 were two drift-gravels separated by an "interval of dry-land 

 surface," the lower bed being sandy and stratified, and the upper 

 one loamy, unstratified and capped by brick-earth.^ 



(9) Chilton. — We will now pass on to a section I described a few 

 years ago ^ at Chilton, south of Oxford. An angular local drift there 

 fills a depression on the plaiu of Lower Chalk, at a level of about 290 

 to 400 feet above sea-level, and of 160 feet above the Thames at 

 Moulsford. It consists of chalk-rubble with very sharply angular 

 fragments of flint, mostly of small size, and generall}^ very closely 

 packed together. On the south side, the Chalk hills rise to between 

 400 and 600 feet, the higher ones being capped by Tertiary strata. 

 With the exception that there is no precipitous cliff, this drift with 

 mammalian remains is an exact counterpart of the Elephant Bed at 

 Brighton, only that the materials are finer, and there is a larger 

 proportion of chalk-rubble and marl, the Chalk-with-flints being 

 more distant. Its structure is precisely similar — not stratified in the 

 usual sense, but sweeping into an old gully in broad lenticular 

 masses, giving a false appearance of bedding. The flints are neither 

 stained nor worn, and the chalk-rubble is in places almost angular, 

 and in others is reduced to the state of a cream-coloured paste or 

 marl. It is in one of these latter that the land-shells were most 

 abundant. The whole of the materials become finer as the drift 

 recedes from south to north, and they are all of local origin, being 

 derived from the Lower Chalk, Chalk-with-flints, and overlying 

 Tertiary Beds. In the lower half of the drift there were found, 

 irregularly dispersed in various parts of the cutting, a few blocks of 

 very hard and compact sarsenstone (Lower Tertiary sandstone). 

 Two of the largest measured 2 ft. x 10 in. x 9 in. and 1| ft. x 1^ ft. 

 X 10 in. The angles were rounded, but otherwise the blocks were 

 not much worn. 



The shells are all of terrestrial species except two, and these 

 are so rare that they constitute the exceptions which prove the rule. 

 Besides, Gwyn Jeffreys informed me that the PlanorUs albus, which 



Quart. Jo urn. Geol. Soc. vol. xi. (1855) p. 107. 

 Ibid. vol. xxxviii. (1882) p. 127. 



