TYLOE QUATEENAKY GEAVELS. 



83 



at the present time ; but the physical conditions under which a 

 gravel- deposit like that at Sangatte could occur do not exist at the 

 present time in France. It would require the rainfall of a pluvial 

 period to furnish the fresh water to lay down such a thickness of gravel 

 as that of Sangatte in such a position on the sea-coast. 



Thames-Valley Geavels. 



The section from the surface of the Thanet Sands, 86 feet above the 

 Ordnance datum-line, to the river Thames, through the celebrated Cy- 

 rena-pits (PI. IV. fig. 6), is drawn to an exaggerated scale of heights, 

 in proportion to lengths, through the points A E I, approximately 

 laid down on the map, PI. IX. PI. VII. is a copy of part of the 

 Ordnance Parish Map of Grrays reduced to half size. 



An excavation into the chalk between B and C offers an opportu- 

 nity of observing the Thanet Sands quite undisturbed at a distance 

 from the surface of 27 ft. at C, and of 35 ft. at B ; the dip of the 

 chalk is there slightly south, the thin covering of gravel e dipping 

 south. There is no opportunity of examining the structure of the 

 ground between C and D, as it is not excavated ; but I expect the 

 section resembles that at Crayford, on the opposite side of the river. 



Fig. 16 is a transverse section near the point D, showing about 



w. 



Fig. 16. — Section at Grays, Essex. 



D50. 



E. 



e. Covering gravel. 



d. False-bedded muds 18 feet thick. 



d'. 4-feet pebble-bed, with shells, Cyrena 



&c. 

 c'. Elephant-bed, 



c. Cyrena-heA. 1 foot thick. 



e". Mottled clays, with Cypris living now in 

 the Thames, and impressions of plants, 

 10 feet. 



18 ft. of yellow false-bedded sands, d, under the covering gravel, e, 

 and lying upon the 4-feet pebble -bed, composed principally of peb- 

 bles from the Woolwich beds of the Eocene series. The Cyrena^ is 

 abundant in the sand of this pebble-bed, and occurs, associated with 

 other shells, in the shell-beds below at c, which are from 1 to 4 ft. 

 in thickness, varying much in character, but dipping north to the 



(t 2 



