Page:VCH Surrey 1.djvu/43

 GEOLOGY throughout the county, consisting of clean sharp incoherent sand or soft sand-rock, of white, yellow, brown or reddish tints, with irregular con- cretions of hard iron-sandstone locally known as ' carstone,' which is often dug for road-mending. The sand generally shows strongly-marked ' current-bedding ' ; it is usually about 100 feet thick. It forms, in many places, heathy common land, too poor for cultivation. Its topmost layers, immediately underlying the Gault, usually contain small phosphatic concretions, probably denoting a pause in the accumulation of the sedi- ments, when fresh supplies of sand were no longer brought down by the waves and rivers into the sea, but before the quiet conditions necessary for the settling down of the overlying clay were established. Soon however the shallow sea or gulf with its strong currents and shifting sand-banks gave place to more open waters, as the renewed depression which ushered in the Upper Cretaceous period submerged more and more land, until finally, during the deposition of the Chalk, there was no longer any shore within a considerable distance of the area now constituting our county. SELBORNIAN The first deposit of this deeper sea was the Gault, a more or less calcareous mud or clay, which is so celebrated for the abundance and beauty of its fossils where exposed on the Kentish coast near Folkestone. This clay, as indicated on the map, has a continuous outcrop across the county in a narrow belt at the foot of the Chalk Downs, causing by its more rapid wasting a longitudinal depression between the Downs and the Lower Greensand hills. Good sections of the Gault are rare in Surrey, and there is some uncertainty as to its thickness, which is believed to be usually between 90 and 120 feet at the outcrop, and may be much less in places, but is said to reach 343 feet in a boring at Caterham, 1 and about 200 feet in borings in the north of the county (see p. 19). The next division of the series, the Upper Greensand, a name having little reference to the composition of the rock in Surrey, is closely associated with the upper portion of the Gault ; and in the latest scheme of classification the two are linked together as a single formation under the term Selbornian, since it is suggested that there is a lateral as well as a vertical passage between them, and that the upper part of the Gault clay of Kent was deposited contemporaneously with the Upper Greensand rocks of Surrey. 2 These Upper Greensand or ' Merstham Beds ' of Surrey consist in the lower portion of slightly glauconitic silty marl, containing large sili- ceous concretions, surmounted by beds of Malm-rock or Fire-stone, a peculiar more or less calcareous sandstone with cherty aggregations, 1 See W. Whitaker's ' Some Surrey Wells and their Teachings,' Trans. Croydon Micro- scop, and Nat. Hist. C/ub, 1886, p. 48. Britain,' vol. i. (1900) p. 93. 9
 * See A. J. Jukes-Browne in Geol. Survey Memoir, ' The Cretaceous Rocks of Great