Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/181

Rh 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.

Fig. 18 is a section exhibiting the excavation of the chalk and Thanet sands by the old River Thames. The chalk (a) reaches a height (fig. 18) of about 45 feet above the Ordnance datum-line; and the Thanet sands (b) have been partly denuded, so that only 15 feet, out of 60 feet seen in the Station ballast-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 (a) 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



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 incoherent 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 (b) are so sharply cut in this transverse section of a river-bed that it would be difficult to believe it was not an