TYLOR QUATEENARY aRAVELS. 



87 



is exposed at 45 feet above the Ordnance datum-line. I have found 

 the Cyrena at about the same height at D ; and this is the highest 

 level at which the Cyrena is found either in the Thames valley or in 

 the Humber or Somme. * 



Pig. 18 is a section exhibiting the excavation of the chalk and 

 Thanet sands by the old Eiver Thames. The chalk (a) reaches a 

 height (fig. 18) of about 45 feet above the Ordnance datum-line; and 

 the Thanet sands (6) have been partly denuded, so that only 15 feet, 

 out of 60 feet seen in the Station baUast-pit at Erith, remain here. 

 The gravel (c) reposes conformably on the top of the Thanet sands at 

 G, the upper part of the section ; but the gravel follows the denuded 

 surface of the chalk, filling up the concavities. It contains much 

 rearranged Thanet sand ; and a large lump (5) is conspicuous in it. 

 The 6 -feet step in the chalk is well marked and sharply cut out by 

 the action of the river flowing at right angles to the line G K. 



Fig. 19 is a section in the same direction as fig. 18 ; but steps from 



Fig. 19. — Section in Erith Pit. (l^atural scale.) 



1 to 2 feet deep, cut by water in the Thanet sands, are distinctly 

 seen. The gravel is then deposited in the concavities formed by 

 the water. Brick-earth, with veins of gravel {d), follows on for 

 15 feet, and the covering gravel (e) succeeds, sloping to the river at 

 a gradient of 1 in 200 only. The Thanet sand is a somewhat inco- 

 herent mass ; and the action of the water must have been very gentle 

 to have formed such perfect steps. The deposition of gravel and 

 brick-earth must have at once followed the denudation. 



Fig. 20 is a section along part of the line G E. The chalk, with 

 a basement-bed of Thanet sands and green-coated flints, is seen at d, 

 in situ. The Thanet sands, in fig. 20, were excavated by one of the 

 side-streams which formerly flowed into the ancient Thames. The 

 edges of the sand (6) are so sharply cut in this transverse section 

 of a river-bed that it would be difficult to believe it was not an ar- 



