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 Gravels.
The section from the surface of the Thanet Sands, 86 feet above the Ordnance datum-line, to the river Thames, through the celebrated Cyrena-pits (Pl. 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, Pl. IX. Pl. VII. is a copy of part of the Ordnance Parish Map of Grays reduced to half size.
An excavation into the chalk between B and C offers an opportunity 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.—Section at Grays, Essex. | |
W. D 50. E.
|
e. Covering graveld. False-bedded muds 18 feet thick.d′. 4-feet pebble-bed, with schells, Cyrena &c.c′. Elephant-bed.c. Cyrena-bed 1 foot thick. c″. Mottles clays, with Cypris living now in the Thames,
and impressions of plants, 10 feet. |
Fig. 16 is a transverse section near the point D, showing about 18 ft. of yellow false-bedded sands, d, under the covering gravel, e, and lying upon the 4-feet pebble-bed, composed principally of pebbles 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
