90 



PEOCEEDINaS OP THE GEOLOGICAL SOCIETY. 



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. 



Eig. 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, AC, 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 YI., 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. 



Eig. 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 



