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Show 292 EOCENE PERIOD. [Ch. XXI. O't'een-sand ('firestone, , or' ma lm rock ' ' as it is sometim.e s .c ulfl ed) ib s almost absent m• t 1l e t.u lc t het·e alluded to·. It 1s, m act, d t thin out to an inconsiderable stratum seen at Beachy Hea 0 d · · f 0 f 10 ose green-san d ., b u t farther to the west war 1t 1s. o great thickness an d con t am· s hard beds of blue chert and hmeston· e. Here, ac'c or d'm g1 Y ' we find that it produces . a corresponl'dkm g m. fluence on t 11 e s.e erl ery of the country, fot· 1t ru. ns out 1 e a step bey on d t 11 e £o o t o f the chalk-hills ' and constttutes a lower a, Chalk with flints. c, UJ1per green saud, or firestone. No.67. b, Chalk without flints. cl, Gault. fJ t terrace vary.m g 1.1 1 b1. e a dth fl'om. . a quarter of a mile. to tht*·e e mi.l es anc l 1r 0l l ow.m ocr the sinuosities of th. e chalk escatpment . It 1: s 1. mposs1' ble t o d e sire a more satisfactory proof th. at the escarpment l.S d ue t o the excavatinOo' power of wa.t er dul'lng the gradual ri.s e o f t h e s tI. a ta . For we have shown, m our account of the coast o f S l.C.J 1Y t ' in what manner the encroachm. ents of the sea ten d to e~ura ce tha' t succession of terraces whtch musdt ot1 1 erw1. se res ult from the successive rises of a coast pre1y e ttpon by t 11 e waves. DurinOo' the interval between two e evda. tory movemen t s, the lower terrace will usua.l l1y be 1d estroyetl ' wherever 1. t 1. s comp osed of incoherent matena s;h w Iereasf th1 e sea will not have time entire1 y to sw~ep away a not er part o e t .. ce or lower platform, whtch happens to be composed same eiia ' bl f ffi · firm('r of rocks of a harder texture and capa e o o enng a . ·~tance to the erosive action of water. tes~alleys u•he-re softer strata, ridges where harde< crop 01~~ -It is evident that the Gault No. 2 (see the rna~) cou not h ave opposed any effectual resistance to the denudmg force • M Murchison Geol. Sketch of Sussex, &c., Geol. Trans., 2nd .Series, vol. ii. P· 98• r. ' t See p. 111, and wood.cut No. 24, tu.t~ Ch. XXI.] DENUDATION OF WEALD VALLEY, 293 of the waves; its outcrop, therefore, is marked by a valley, the breadth of which is often increased by the Joose incoherent nature of the uppermost beds of the lower green-sand, which lie next to it, and which have often been removed with equal facility. 'rhe formation last mentioned has been sometimes entirely smoothed off like the gault; but in those districts where chert, limestone, and other solid materials enter largely into its composition, it forms a range of hills parallel to the chalk, which sometimes rival the escarpment of the chalk itself in height, or even surpass it, as in Leith Hill. This ridge often presents a steep escarpment towards the Weald clay which crops out from under it. (See the strong lines in diagram No. 63, p. 288.) The clay last mentioned forms, for the most part, a broad valley, separating the Jower green-sand from the Hastings sands, or Forest ridge; but where subordinate beds of sandstone of a firmer texture occur, the uniformity of the plain is broken by waving irregularities and hillocks*· In the central region, or Forest ridge, the strata have been considerably disturbed and are greatly fractured and shifted. One fault is known where the vertical shift of a bed of calcareous grit is no less than 60 fathoms t. It must not be supposed that the anticlinal axis, which· we have described as running through the centre of the weald, is hy any means so simple as is usually represented in geological sections. '!'here are, on the contrary, a series of anticlinal and synclinal :t • Martin, Geol. of Western Sussex. Fitton, Geol. of Hastings, p. 31. t Fitton, ibid., p. 55, t We adopt this term, first used, we believe, by Professor Sedgwick ; its signification will best be underlitood by n~ference to the accompanying diagram. a, Anticlinal lines. b, Syncllnalliues. |