Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/184

90 of what is there called boulder-clay, just as the stratified fossiliferous gravels at Erith are close to and interstratified with the non-remanie but moved Tertiaries at Erith.

Fig. 22.—Section in Crayford Pit. This section is to natural scale, but is reversed.

The basement-bed of the Thanet sands and a few feet of the sands are exposed along the face of the cutting, A C, Plate VI. The surface of the chalk is seen to be eroded by chemical action into pipes and basin- shaped cavities. The Thanet sands are broken through and moved into the pipes in the usual manner, and are covered by a top bed of sand and gravel, which fills up all the hollows of the underlying sands and chalk, and smooths over all previous inequalities.

The beak of chalk under a has protected small masses of Thanet sands (B and B') below and behind it from being remanie and mixed up with the gravel. The covering bed of sand and gravel is smooth and nearly horizontal, c dips at 10° towards the Thames, and in one point at 12°. The chalk near A is 70 feet above the Ordnance datum-line, although only 50 feet is shown here. At P, Plate VI., 400 feet to the south-east, the chalk is seen in a pond only 10 feet above the Ordnance datum-line.

The inclination of the escarpment between C and P is 1 in 6, or 15° on the average; and this is entirely due to the excavation of the channel by the ancient River Thames. The lowest bed exposed in fig. 22 consists of from 3 to 4 feet of coarse gravel lying on the eroded chalk.

Fig. 23 represents rather higher ground; and 15 feet of Thanet sands are seen in situ (b). The gravel and brick- earth are well seen, dipping as much as 22° east at the steepest point.

The covering bed of sand and gravel, 5 feet thick (a), lies on the edges of the gravel (c) and brick-earths, and slopes to the river at a