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. 



* See A. J. Jukes-Browne in Geol. Survey Memoir, ' The Cretaceous Rocks of Great 

 Britain,' vol. i. (1900) p. 93. 



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