Page:VCH Surrey 1.djvu/56

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A HISTORY OF SURREY and hollows, which remained ever afterwards as the dominant factors in its structure. The dome of the Weald and the trough of the London Basin, referred to in the foregoing pages, are the results of this earth- movement with which we are principally concerned in Surrey. In the Wealden dome the strata were raised up in a huge oval tract extending from Hampshire on the west to the Bas Boulonnais in France on the east, now broken through towards the eastern end by the Straits of Dover. ' We must not however overlook the fact pointed out by W. Topley, that the Secondary rocks attained their maximum thickness within this dome, and therefore that its elevation may be in part the result of original irregularities of deposition. 1 But it seems highly probable, though the point seems to have escaped notice, that where the strata are thickest, there also will they be most likely to bulge upwards under lateral pressure, thus accentuating the original inequality. At any rate, there can be no doubt that considerable disturbance tending to elevation has taken place throughout the dome. Broadly speaking the Wealden uplift forms a single anticline ; but when examined more closely we find that it is made up of numerous subsidiary waves or flexures, arranged en echelon, which usually rise up gradually from the south and plunge over more steeply towards the north, and flatten and fade out longitudinally. One of these flexures or minor folds explains the sudden expansion of the Lower Greensand outcrop west of Dorking, and its crest brings up a small ' inlier ' of Atherfield Clay and Weald Clay, surrounded by the overlying formations, in the neighbour- hood of Pease Marsh two miles south of Guildford, whilst its northerly plunge gives rise to the steep dips and narrow outcrops of the Hog's Back and its vicinity. Less pronounced waves of the same kind occur farther south between Godalming and Haslemere, and to the eastward near Dorking, Reigate and Westerham. But besides these flexures, the strata are sometimes broken through by fractures or ' faults,' between the two sides of which there has been differential movement, so that a once continuous bed now occurs at different levels on the opposite sides of the dislocation. A pronounced ' fault ' of this kind is found about a mile to the eastward of Farnham, where the strata on the north-east side of the fracture are carried down from i 50 to 200 feet lower than the corresponding strata on the south- west side. This fault is sufficiently large to affect the line of outcrop considerably, the Hog's Back ridge being brought to a termination by the setting back of the Chalk escarpment on the west ; and a depression of the surface has been subsequently developed at this point, through which the railway between Farnham and Guildford is carried. Another fault has been traced running east and west from the southern outskirts of Dorking to beyond Wotton, with a downthrow to the north amount- ing in places to 100 feet ; and again at Betchworth, two miles east of Dorking, there is a line of fracture striking from south-east to north-west, 1 See Quart. Journ. Geol. Sac., vol. xxx. p. 186, and Geology of the Weald, pp. 241, 242. The result of recent deep borings in Kent has added fresh weight to Mr. Topley's argument. 22