Excursion to Knockholt and Sevenoaks. Saturday, May 24th, 1879. (Second of the Weald Series) - Geologists' Association excursion

From: A record of excursions made between 1860 and 1890. Edited by Thomas Vincent Holmes , F.G.S. and C. Davies Sherborn, F.G.S. London: Edward Stanford [For the Geologists’ Association], 1891. Source: Cornell University copy on the Internet Archive (Public domain work)

Director—J. Logan Lobley, F.G.S. (Report by—W. Fawcett, B.Sc.) (Proc. Vol. vi. p. 194).

Starting from halstead station, the route lay through Halstead Hlace Hark, by Hnockholt Beeches, through Chevening Park, Chevening, Chipstead, Riverhead, and Sevenoaks. At Halstead Mr. Lobley showed how much more complete had been the denudation than about Cudham; here there is but little soil, or valley gravel. The Chalk has bands of flints, characterising it as Upper Chalk. From Knockholt Beeches there is an easy descent through the woods of Chevening Park, down the Chalk escarpment.

At Chipstead Tile-yard, Pleistocene Brick-earths are seen overlying the Gault clay. They are both worked and often mixed. Here in the Gault are found Ammonites interruptus, A. auritus, A. lautus, Belemnites minimus, and Inoceramus concentricus; but Am. splendens was not seen, though it is a characteristic fossil of the Gault elsewhere. From the blue Gault clay are made red bricks and dark tiles. The redness is due to the peroxidation of the iron of the clay by the burning, and the dark colour is produced by the oxide of manganese being mixed with the clay. Iron compounds are plentiful in the Upper and Lower Greensand and Gault, chiefly in the form of sulphuret or iron-pyrites, but not in the Chalk. From the tile-yard the way lay across the bottom of the valley, and over the infant Darent to Chipstead village, where the Lower Greensand, which forms the hilly and beautiful ground to the south, begins to rise and form the southern side of the valley. In half-a-mile the village of Riverhead is reached.

At Riverhead the Folkestone Beds of the Lower Greensand (of a foxy-brown colour) are exposed; but they are better seen in the quarry at the Bat and Ball Station, Sevenoaks. In this latter place the sand is loose, the grains only being cemented together here and there in thin irregular layers of ferruginous sandstone. These layers at first sight give the appearance of false bedding, but Mr. Lobley was of opinion that they were not due to that cause. He pointed out the " Box-stones," large irregular ferruginous concretions, or rather shells with loose light-coloured sand in the middle, many of them being of very considerable size. They are very dark in colour, and so hard that they are used for road-metal.

[On June 1st, 1889, some gravels at and near Ightham were visited, Prof. Prestwich and W. Topley being directors. See Proc., vol. xi., p. lxvi.; and Prestwich, Quart. Journ. Geol. Soc., vol. xlv. (1889), p. 270.]