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 GEOLOGY GLACIAL DRIFT Glacial Drifts and especially Boulder Clay occupy a large portion of the surface of Essex and form some of the more fertile agricultural areas. The Boulder Clay is a tough unstratified clayey deposit of irregular thickness, containing numerous rounded fragments of Chalk of all sizes from that of a pea to large blocks ; hence it is known as the Chalky Boulder Clay. It contains also numerous unworn and angular fragments of flints, and pebbles of various rocks, fossils and rocks derived from different formations, together with schists, granites and greenstones. 1 Essentially however it is a chalky clay, the soil being a ' strong loam ' which forms good land for wheat, barley and beans. This loam is a de- calcified portion of the Boulder Clay which in places resembles the soil on the London Clay, but is usually stony and very much thicker. Among the more abundant fossils are Gryphcea dilatata derived from the Oxford Clay, Belemnites abbreviatus from the Corallian, and Lucina minuscula from the Kimeridge Clay. The Boulder Clay lies somewhat irregularly on the strata beneath, chiefly London Clay and Chalk ; and there are remarkable instances of glacial erosion which took place prior to or during the Glacial epoch. Thus a deep gorge filled with Drift occurs along the Cam valley, partly in Essex and partly in Cambridgeshire. The evidence derived from well-borings has been published by Mr. Whitaker,* who notes the extraordinary thickness of Glacial Drift at Newport (340 feet), Wenden (272 feet), Littlebury (214 feet), and Great Chesterford (156 feet). It appears probable that this great trench was excavated and subsequently filled up during the time of most intense glaciation, and that possibly the disturbed Chalk near Chishall (previously noticed) may then have been tilted like the Chalk in several localities in Norfolk. Boulder Clay occurs on the Chalk uplands in the north-western part of the county in the neighbourhood of Heydon, Chrishall, and also around Debden and Broxted. Further south it forms a more or less continuous sheet at Hatfield Forest, Hatfield Broad Oak, High Easter and the Rodings. Eastwards it is broken up by valleys which divide the high grounds near Thaxted, Dunmow, Braintree, Halstead and Coggeshall. To the south the Boulder Clay extends to Chelmsford and Chipping Ongar and in patches to the heights, as at Havering-atte-Bower, over- looking the Thames valley. Its presence in this valley was unsuspected until in the making of the railway near Hornchurch a mass of tough Chalky Boulder Clay fifteen feet thick was found by Mr. T. V. Holmes to underlie one of the higher tracts of Thames valley gravel. 1 This dis- covery is of the greatest interest as it was the first instance where Boulder Clay had been seen in connection with the Thames valley gravel. As Mr. Holmes points out it does not prove that the Thames valley was Sec Rev. A..W. Rowe, 'On the Rocb of the Essex Drift,' Quart. Jour*. Geol. Sec., vol. xliii. p. 35 1. 3 Ibid. vol. xlviii. p. 365 ; and Euex Nat., vol. vii. p. I. 13
 * Quart. Journ. Gtol. Sac., vol. xlvi. p. 333.