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A HISTORY OF ESSEX pre-glacial, but that an old hollow or valley was subsequently cut into during the excavation of the Thames valley. River gravel also overlies Boulder Clay in the Blackwater valley near Kelvedon. Underlying the valley gravel at Romford, Mr. Holmes has observed a deposit of silt and sand with pebbles of Chalk, and flint, evidently of subsequent date to the Boulder Clay from which it was largely derived. 1 He looked upon it as evidence of an ancient silted-up stream whose course he would trace between Warley, Billericay and Maldon on the north and west, and Laindon, Rayleigh and Althorne on the south and east, and into the valley of the Blackwater below Maldon. In any case this silted-up channel, like that in the Cam valley, belongs to a more ancient period than the present Thames valley ; but whether these old valleys which are occupied by Boulder Clay were pre-glacial valleys, or were scooped out by ice-action during the Glacial period may fairly be questioned. There is however no doubt that the main features of the country were formed prior to the glaciation, and therefore we should expect here and there to find traces of old valleys. When the Ice age was brought about the surface of the land had long been subject to subaerial waste, the Chalk-tracts were covered with clay-with-flints, and there was generally much weathered material or soil over the land. Before any great movement of ice took place, the accu- mulation of snow led to the formation of much ice, and to the base of this the soil and weathered sub-strata were frozen. Eventually, when movement set in and there was coalescing of great sheets of ice which traversed regions of Jurassic rock and Chalk, the base of the ice tore off the frozen soil and debris^ and in some cases great strips of the strata ; in other cases impinging against higher ground the formations were locally disturbed, as may have been the case near Heydon. The debris thus removed would rise by overthrusts into higher hori- zons in the ice, and be then carried forward and widely distributed and commingled with local detritus during alternate recessions and readvances of the ice-margin ; the Boulder Clay being deposited, to a large extent, by the melting of the ice, as indicated many years ago by Mr. J. G. Good- child in his account of ice-work in Edenside. The abundant chalky detritus was no doubt carried along minor planes of movement in the ice, the chalk lumps being scored by frac- tured flint, and the material being transported far and wide at higher levels in the ice than the bulk of the more local material. In certain instances the soil frozen to the base of the ice-sheet was little if at all moved, being overridden by subsequent ice-movements ; and much Boulder Clay must also have been overridden after deposition, owing to its exceedingly tough character. 2 1 Quart. Journ. Geol. Sue., vol. 1. p. 443 ; and ' Notes on the Ancient Physiography of South Essex,' Essex Nat., vol. ix. p. 193. 2 See H. B. Woodward, Geol. Mag., 1897, p. 485 ; J. E. Marr, ibid. 1887, p. 262 ; and J. Geikie, The Great Ice 4ge.