Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/179

Rh 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-line.

This basement bed is 70 feet at Purfleet; and 66 feet at Crayford (fig. 22) above the Ordnance datum-line.

The supposition of Mr. Searles Wood, jun., that the Thames valley is in a line of fault appears to me inadmissible, as far as Grays, Erith, and Crayford 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 horizontally in the localities referred to. Beyond the districts referred to there is a flexure in the chalk which depresses the basement-bed of the Thanet sands from 46 feet at G, Plate VIII., Erith, to about 10 feet above the Ordnance 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.

Referring 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 Crayford Ness, T. Although the ground along B S has not been excavated, like the other side of the Thames at Grays, it is known that the brick-earth thins out, and that the lower gravel (f), separated from the covering gravel (e) by brick-earth (d) and sands (c), together 50 feet thick between A and B, come together at S, just as e and f 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