Excursion to Epsom and Dorking May 24th, 1884 - 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) |
Directors—W. H. Dalton, F.G.S., and H. H. French, F.G.S. ( Report by W. H. Dalton.) (Proc. Vol. viii. p. 396.)
Note: [On May 5th, 1888, there was an excursion to view the sections on the new line of railway between Wimbledon and Putney. See Proc., vol. x. p. 471. The Directors were W. Topley and Oswald M. Prouse., On May 1st, 1886, an excursion to Ewell and Epsom took place, H. Hutchins French directing; and on May 29th Walton Common and St. George's Hill were visited under the leadership of W. H. Hudleston. Both excursions are fully described in Proc., vol. ix., p. 532 and p. 537; for convenience the two figures relating to Walton are here given.]
On arriving at Epsom, shortly before noon, the party proceeded to the Downs. Here, at the back of the rifle-butts, they examined a pit in the Upper Chalk, the beds of flint in which served as indications of the amount of dislocation of a fault which throws down the beds about eight feet to the south-east. Several slickensided faces were observable on joint-planes of parallel direction; and, although such surfaces have been attributed to pseudomorphism of aragonite, &c., it was agreed that no such hypothesis was requisite in such cases as the present, where relative movements of the different portions of rock were in evidence. At one point of the fault a vertical sheet of flint had been formed, probably by replacement of chalk in water occupying a temporarily open fissure. Traversing the race-course the party ascended the hill on which stands the village of Walton. The summit of the hill consists of a thin sheet of Thanet Sand, overlaid by crimson mottled brick-earth and gravel. An old brick-yard here was open in 1860, when Whitaker saw a section in the Thanet Sand, eight feet deep, under the brick-earth. Speculations were rife among the party as to the course of the river which formed this gravel and as to the cause of the survival of the Thanet Sand outlier, some maintaining its probable protection by beds locally harder than usual (although the several outliers reach to different horizons of the Lower Tertiary Series), whilst others preferred the hypothesis of a slight synclinal undulation parallel to the general strike of the Chalk and Tertiaries, patches of the latter being preserved in a discontinuous trough ranging for about eight miles, Walton being nearly midway between the extremities. On Walton Heath some exposures of " clay with flints " were visited, and the Directors of the party were pressed, but in vain, to commit themselves, jointly or severally, to one of the many theories of the origin of this deposit. That which attributes it to the solution of Chalk by percolating water plus relics of Tertiary beds is the theory which in these days of compromise and tolerance is perhaps the safest. The thickness of the deposit and its continuous occurrence along the escarpment attest the great denudation which the Chalk has undergone.
From the heath a fine view was obtained of Pebble Hill and Leith Hill, the former being part of the Chalk escarpment close by, whilst Leith Hill is on that of the Lower Greensand several miles to the south-west. Descending the hill by a deep-cut road, whose high banks only showed the Chalk, decomposed into amorphous marl, at intervals, a large pit in the Upper Greensand was reached, where, under a head of Chalk rubble, the Chloritic Marl was seen, highly fossiliferous, overlying the Hearthstone beds, for which the pit is opened. The two divisions are of very similar lithological character, calcareous clayey sand, with abundant glauconite grains, which on a surface bruised by a frictional stroke of the hammer form green streaks. The absence of fossils from the Hearthstone appears to be the only difference noticeable. The Chloritic Marl and the Chalk rubble are carried across the pit on staging, and the Hearthstone is raised vertically by a wind_ lass from the floor of the pit. At a lower level occurs the Firestone, used for furnace-lining, &c., to obtain which slanting tunnels have been carried far in under the hill. This stone has also been used for building, for instance, in Windsor Castle and in Henry VII.'s Chapel, Westminster Abbey.
Passing through Betchworth, the party next entered a large pit in the Lower and Middle Chalk, the Upper Chalk having been denuded back from the escarpment. The local name here is "grey stone," in contradistinction to the white Upper Chalk. Many nodules of iron-pyrites were noticed in the lower part of the pit, generally unaltered within, though oxidised to a rich rust-colour on the surface : in the proximity of joints, however, the oxidation extended deeper, and in small nodules was complete. The chemical action consists in the absorption of oxygen dissolved in percolating water, forming ferric sulphate, which by contact with the surrounding Chalk forms sulphate of lime and carbonate of iron, the latter speedily passing, by addition of further oxygen, into the red peroxide of iron.
Few fossils were obtained from this pit; but in another close by some fragments of fish remains, and some of the characteristic shells, Terebratula, &c., were purchased. The elaborate kilns here used for burning the chalk into lime were examined with much interest, and the party then defiled along the narrow winding path known as the Pilgrim's Way (which extends from Alton, in Hants, to Canterbury), to the Brockham brick and firestone-pits, where the lower beds of the Upper Greensand, and nearly the highest of the Gault Clay, are exposed, though not in continuous section., Neither are fossiliferous, but it may be noted that the upper part of the Gault section was crowded with nodules of "race," segregations of carbonate of lime from the surrounding clay, and that thin veins of fibrous gypsum are formed by the decomposition of pyrites and neutralisation by carbonate of lime, as described above. The Pilgrim's Way on the Gault and Upper Greensand is here bordered almost continuously by yew-trees, whilst east of Reigate, where it passes into the Lower Greensand, the ash replaces the yew, a striking example of the relation between geological structure and vegetation.
The firestone is worked by tunnel, the rock at the mouth appearing to differ in no respect from the Hearthstone previously examined.
After half an hour's walk the Lower Greensand was reached at a mill turned by the Mole, and the coarse ferruginous sandstone was briefly examined. This uppermost division is known as the Folkestone Beds, and its colour is due to peroxidation of its contained iron. A fine section of similarly tinted sand and sandstone is exposed on the railway a short distance further west, but time did not permit of its examination, so the party pushed on to Dorking station.