TYLOR QrATERNART GRAVELS. 



85 



will give an idea of the surface -configuration of the Thames valley 

 and of the position of the fossiliferous gravels. The top bed of the 

 Chalk and the basement bed of the Thanet Sands are well seen at G, 

 Plate VIII. (fig. 18, p. 86), at a height of 46 feet, at Erith ; and at 

 B, Plate IV. fig. 6, Grays, at a height of about 47 feet above the 

 Ordnance datum-hne. 



This basement bed is 70 feet at Purfleet; and 66 feet at Crayford 

 (fig. 22) above the Ordnance datum-line. 



The supposition of Mr. SearlesWood, 

 jun., that the Thames valley is in a 

 line of fault appears to me inadmis- 

 sible, as far as Grays, Erith, and Cray- 

 ford are concerned. The sections I 

 now bring forward will show that the 

 Thames valley has been excavated by 

 river-action out of a mass of chalk 

 and Thanet sands, lying nearly hori- 

 zontally in the localities referred to. 

 Beyond the districts referred to there 

 is a flexure in the chalk which de- 

 presses the basement-bed of the Thanet 

 sands from 46 feet at G, Plate VIII., 

 Erith, to about 10 feet above the Ord- 

 nance datum-line at Erith station, 

 without any fault. It is seen again 

 at Woolwich and Lewisham, at the 

 same height as at Erith, before it sinks 

 under the London basin. 



Eeferring to Plate IV. fig. 7. The 

 chalk has been seen at A, 50 feet 

 from the surface and 36 feet above 

 the Ordnance datum-line, also in a 

 well at S, 13 feet the above datum-line. 

 It is said to be very near the surface at 

 S, and that it is visible at low water 

 for a mile between Erith and Cray- 

 ford Ness, T. Although the ground 

 along B S has not bef n excavated, like 

 the other side of the Thames at Grays, 

 it is known that the brick-earth thins 

 out, and that the lower gravel (/), 

 separated from the covering gravel (e) 

 by brick-earth (d) and sands (c), to- 

 gether 50 feet thick between A and 

 B, come together at S, just as e and / 

 touch each other in fig. 6, Plate IV., 

 as they approach the centre of the 

 valley. 



The escarpment of chalk and Thanet 

 sands has not yet been excavated 



