xvin. LOW LEVEL GRAVEL. 465 



horse occur at Wytham, and in many places within a small space 

 round Oxford. 



Lower down, at Culham, is a broad plain of gravel on gault, 

 greensand, and Kimmeridge clay. The composition of the gravel 

 is much like that at Oxford, and it yielded similar bones of 

 quadrupeds, Elephas antiquus being represented by a gigantic 

 tooth. About Radley and Abingdon it is very extensive ; and we 

 follow it by Clifton-Hampden, Wittenham, Dorchester, Wallingford, 

 Henley, Hurley, Marlow, and Maidenhead. But as we proceed 

 the character of the gravel is found to change ; in the upper part 

 of the valley it is 'for the most part oolitic, in the middle part 

 mixed oolites and flint, in the lower part flints prevail ; but every- 

 where some pebbles appear of a hard reddish grit, like that of the 

 Lickey Hill and that of Harthill near Nuneaton. 



At almost every place of observation from Cricklade down to 

 Wallingford we observe a curious defect of calcareous stones in the 

 upper part of the cutting, in the soil and subsoil, whether the depth 

 be one, two, or three feet. In that upper part flints, quartz, and 

 red grits abound, and on many fields appear the only stones. In 

 the gravel pits the lower stones of whatever sort are commonly 

 coated by incrusting carbonate of lime ; and in some cases, espe- 

 cially in the admirable river-bank sections at Long Wittenham, 

 curved and implicated layers of white soft carbonate of lime lie 

 in some abundance for short spaces. Hence we infer that long 

 atmospheric and vegetative agency, the rains of some hundreds 

 or thousands of years, aided often by processes of cultivation, have 

 dissolved the carbonate of lime in the parts near the surface, and 

 carried it down to make the soft white sheets or the harder 

 semicrystalline incrustations referred to. 



The principal deposits of gravel are above the level of the river 

 Thames, 10, 20, 30 feet : except in the Oxford ridge, they are 

 on a long and broad surface sloping toward the great valley, 

 and appearing to have been brought down by active streams, 

 which now transport no material of the sort, except in gushes, 

 and then merely displace small quantities. 



The composition of the deposit varies much, as already observed ; 

 the arrangement of the materials less so, being always such as to 

 indicate partial ' sorting" of the small pebbles and sand, and dis- 

 tribution in layers with occasional false-bedding. In the numerous 



Hh 



