Report of an Excursion to Claygate and Oxshott, Surrey: Saturday, June
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EXCURSION TO CLAYGATE AND OXSHOTT, SURREY. 237 After tea the members returned by footpath to Whitton, pass ing, and briefly examining on the way, Whitton Leys Brickfield, where Boulder Clay was seen lying as a valley deposit on London Clay. The Oldhaven Pebble Bed and Reading sands were also visible. It was mentioned that this brickyard was one of the exposures of Eocene Beds described by Prestwich in his 1852 and 1854 papers on the Lower London Tertiaries, and by Sir J. B. Phear in 1854. Time did not allow visits to any more sections, and the party returned from Whitton by electric car to Ipswich Station, leaving by the 7.40 p.m. train for Liverpool Street. Mr. 1. S. Double was the Excursion Secretary on this occasion. REFERENCES. Geological Survey Map, Old Series, Sheet 48 N.W. 1852. PRESTWICH, SIR J.-Quart. yourn. Geol. Soc., vol. viii, p. 241. 1854. -------.-lbid., vol. x, p. g2. 1854. PHFAR, SIR. J. B.-" Geology of Gipping Valley, etc." Trans. Cambro Ph>!. Soc. 1885. WHITAKER, W., AND DALTON, W. H._H Geology of Ipswich, etc." frlem. Geol. .sU1V. Ig03. BASS.HT, H.-" Oldhaven Beds at Ipswich." Geol. Mag., P.453. Ig03. JUKES-BROWNE, A. J.-" Zones of the Upper Chalk in Suffolk." Proc. Geol. Assoc., vol. xviii, p. 85. 1907. SLATER, G.-" Excursion to Ipswich and Claydon.' Proc, Geol. Assoc., vol. xx, p. 186. Igog. HARMER, F. W.-" Geology in the Field," Part It p. 103. II Eastern Counties." Ig10. ------.-HGlacial Deposits of Norfolk and Suffolk." Trans. Norj. Nat. Soc.. vol. ix, p. 108. IgII. SLATER, G.-" Excursion to Ipswich." Proc, Geol, Assoc., vol. xxii, p. II. REPORT OF AN EXCURSION TO CLAYGATE AND OXSHOTT, SURREY. SATURDAY, JUNE 15TH, 1912. By HENRY DEWEY, F.G.S., OF H.M. GEOLOGICAL SURVEY, Director of the Excursion. THE party of fifty-two proceeded from Claygate Station to Messrs. Sims and Son's clay-pit at Oaken Lane. A view of the eastern face of the pit is given (Plate 35.A). Here the Director remarked that the primary object of the visit of the Association to this neighbourhood was the study of the Claygate Beds or the passage-beds between the London Clay and Bagshot Sands. HENRY DEWEY, For many years it has been remarked by observers that the top part of the London Clay con tains seams of sand and loam, and in this respect differs from the main mass, which is, in the London District, a heavy homo geneous clay. The first observer to record the results of his work upon a map was Mylne, who re presents the distribution of these sandy beds in North London, especially at Hampstead and Highgate. Prestwich, however, regarded them simply as a sandy facies of the true London Clay, representing a recurrence of the conditions which prevailed when the Basement Bed was deposited, and which more or less persisted during the whole period of London Clay in the Hampshire Basin. Later, Mr. Whitaker * noted their occurrence around London and considered them as true Passage Beds. He called attention to their presence at Wimbledon, Norbiton, Gipsy Hill, and north of the Crystal Palace, and quotes Mr. Caleb Evans's account of them at Hampstead and Highgate. Mr. H. B. Woodward t also discusses the relation of these sands and loams to the London Clay and Bagshot sands, and remarks that "in a future 6-inch ,. ' survey the passage-beds will no . .."q' doubt be separately mapped: a matter of importance, as at pre sent the boundary line between Bagshot Beds and London Clay has not been systematically drawn." Many of the memhers will ~ '- \ * "Geology of London." M em. Geol, SUffJ" ~: vol, i, ,889. pp. 238-265. ~ t "The Geology of the London District." ~ M em. Geol, Surv•• '909. p, 36. ~ ~ 0 ~ .; 0 V> i 0 ~ EXCURSION TO CLAYGATE AND OXSHOTT, SURREY. 239 remember Mr. Leach's-* lucid account of the passage-beds at Shooter's Hill, given before the Association last January. Messrs. Hall and Russell t give analyses of samples taken from these beds at Claygate and Tolworth from which their sandy nature can be seen. Description 0/ the Beds.-Probably the passage-beds are nowhere so well developed and exposed as at Claygate ; for which reason and for convenience of reference they have been called the Claygate Beds. They cannot be Classed either as London Clay or Bagshot Sands, for they differ from both in their characters. They differ from the London Clay in being even-bedded in layers; in containing fine laminre of white sand between the clay beds at the base, and thin seams of clay between the thicker beds of sand in the upper part of the sequence. They differ from the Bagshot Beds in not showing current bedding, in containing sand of much finer grain, and in possessing fairly thick seams of clay. Moreover, they form a complete passage from one type of sedimentation to the other. In this respect they represent a phase of the sedimentary sequence which is not seen in Berkshire. Near Bracknell ] the junction of the London Clay and Bagshot Beds is exposed in several large sections, and everywhere the white sands rest upon the blue clay, indicating an abrupt change of sedimentation, perhaps a slight unconformity. These indications point to the different physiographical conditions which then obtained in various parts of the London Basin. These beds seem everywhere to be unfossiliferous, no fossils having been seen at Claygate nor in the sections it Esher and Chessington. In North London and Essex fossils appear to be common in the clay immediately underlying these beds, as they are also in Surrey. The beds therefore appear to be persistent over the eastern half of the London Basin-farther west erosion was occurring pari passu with deposition of sediment in the waters of the east. The fine sections exposed during the working of the beds for the manufacture of bricks were then examined (see Plate 35.A). The working face is some 70 ft. high, and the beds are worked in two stages. The following section is seen: ft. (c) Bagshot Sands 12 (b) Claygate Beds 5° (I) London Clay. 8 (a) The London Clay occurs in the floor of the pit, and for some 8 ft. up the face of the cliff. It exhibits the normal * 10 On the Geology of Shooter's Hill." Proc, Geol, Assoc.. ]912, vol. xxttt, pp, 112-124. t H Agriculture and Soils of Kent, Surrey, and Sussex." Board of Agriculture and Fisheries, IgII, p. ]82. t H Excursion to Bracknell." Proc, Geol, Assoc., IglIJ vol. xxii, PP. 176.178. HENRY DEWEY, characters, being a dark grey-blue structureless clay with occasional cement stones or septaria, nodules and slabs of grey iron pyrites and crystals' of selenite. The septaria are frequently fossiliferous, the most common fossils being Cardium nitens and Me711branipora. Pieces of fossil-wood bored by teredo also occur. There is no trace of bedding in the heavy and compact clay. (b) A few feet above the floor of the pit faint lines appear in the clay, which on closer examination prove to consist of thin partings of sand. Higher up the cliff-face these seams increase in thickness and lie closer together; the clay at the same time becoming much lighter in colour and texture; consisting essen tially of silt. A further change is noticeable in the higher parts of the section where sand preponderates and clay occurs only in narrow seams of pipe-clay, which has a shelly structure and breaks readily into horizontal flakes. Two prominent bands of ferruginous sand, formerly worked for iron, are seen in the upper working face of the pit. The bedding is here even and nearly horizontal, only a slight dip being measureable. Overlying these beds are the true Bagshot Sands (c), which are of coarser grain, and pronouncedly current-bedded. Structure of the Ground.-From this section and from the appended diagram, Fig. 38, a general notion of the stratigraphy of the country examined during the excursion, may be Obtained. The whole sequence of deposits dips gently towards the south west, the sandy series III and II reclining upon the impermeable London Clay IV. At the junction water is thrown out, giving rise to wet and boggy ground and springs. This water bubbles up carrying with it "running sand," or, as it is termed locally, "spewey sand." The constant drain of sand from. below leads to the gradual slipping of the ground, and, where aided by slope, landslides occur, large slabs of land gliding down the slippery clay slopes. If, however, adjacent ground acts as a buttress, the sliding mass tends to buckle up and is thrown into folds. Messrs. Welch's clay-pit at Claygate (Plate 35.B) exhibits a very perfect example of such folding. Here the Claygate Beds are exposed for a depth of 40 ft., and are folded throughout their entire visible thickness. The clay has been worked in the flank of an isolated hill which slopes gently east and west, the highest point being central. The folding is greatest immediately beneath the highest part of the hill, but decreases at the sides, where the beds become nearly horizontal. This section also shows the alternate beds of sand and clay, with thin laminre of white sand between the clay beds at the base and thin seams of clay between the thicker beds of sand in the upper part of the section. No septaria have been found, but box stones, pyrites and" stars" of selenite are common. PROC. GEOL. Assoc., VOL.