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 A HISTORY OF ESSEX ford. Elsewhere in Essex there are sheets of valley gravel and brick- earth that have yielded interesting fossil remains, to which attention has been directed by the late John Brown of Stan way, by the Rev. Osmond Fisher, and others. Such deposits occur at Great Clacton, at Lexden, Copford and Marks Tey to the west of Colchester, at Kelvedon, Wit- ham and Chelmsford, and again at Great Yeldham. They comprise brickearth, peaty clay, and shell marl, with remains of mammalia, land and freshwater mollusca, and occasionally of beetles and plants. Among the mollusca the more noteworthy are Corbicula fluminalis which has been found at Clacton, Grays and Ilford, Unto littoralis found at Clacton and Grays, and Paludestrina marginata found at Copford, Clacton and Grays. 1 These are extinct in Britain ; the majority however belong to species still existing in the district. A few plant-remains have been obtained at Grays : these include the ivy, and as remarked by Mr. C. Reid, they indicate a temperate climate with mild winters. 2 In places the brickearth, as at Grays, exhibits a disturbed not to say contorted surface, recalling features that are seen in glaciated districts where the Chalky Boulder Clay rests on beds which have been disturbed during the process of its formation. The discovery at Hornchurch (previously mentioned) where Boul- der Clay underlies the Thames Valley Drift, proves that these superficial disturbances are of later date than the main glaciation. They may have arisen in part from the effects of river-ice, in part from the later influ- ences of freezing and thawing and slipping of the soil. 3 At Wanstead Mr. Martin Hinton has observed in the ' High-Ter- race Drift ' much contorted gravel overlain by undisturbed gravel, affording ' evidence of the rivers having been frozen in winter ; and, on the breaking up of the ice, of huge ice-rafts floating down, contorting the deposits in process of formation wherever they grounded.' In the ' Low-Terrace Deposits ' at Ilford the brickearth and overlying gravel and sand are also in places remarkably contorted, evidently by ' the crushing action of ice.' 4 RECENT DEPOSITS The Alluvium is the tract of marshland bordering the rivers, and is in some cases six or seven feet below the level of high-water at spring- tides, It comprises some of the most recent deposits, and is a variable series of muds or silts, clays, shell-marls and peaty deposits with sand and gravel, attaining a thickness of fifty feet at Thames Haven and more 1 See B. B. Woodward, ' On the Pleistocene (Non-Marine) Mollusca of the London District,' Proc. Geol. Assoc., vol. xi. p. 335 ; and A. S. Kennard and B. B. Woodward, 'The Post-Pliocene Non- Marine Mollusca of Essex,' Essex Nat., vol. x. p. 87, and Proc. Geol. Assoc., vol. xvi. p. 282. In these articles references are given to other papers. See also J. P. Johnson and G. White, Essex Nat., vol. xi. P- '57- 8 Quart. Journ. Geol. Sue., vol. liii. p. 464. 3 See O. Fisher, ' On the Warp (of Mr. Trimmer),' Quart. Journ. Geol. Soc., vol. xxii. p. 553 ; and 'On the Ages of the "Trail" and "Warp,"' Geol. Mag. 1867, p. 193 ; see also F. C. J. Spurrell, ' History of Rivers and Denudation,' Proc. West Kent Nat. Hist. Soc., 1886. 4 M. A. C. Hinton, Proc. Geol. Assoc., vol. xvi. p. 271. 18