A HISTORY OF ESSEX including 703 feet of Chalk (base not reached). It is probable that the thickness of the Eocene strata was here to a certain extent duplicated by a reversed fault or fold. 1 At Loughton a boring was made to a depth of 1,100 feet into Gault ; and at South Shoebury another was carried 1,048 feet, probably into Lower Chalk. An old boring at Saffron Walden was carried to a depth of a little over 1,000 feet, but the details of the strata cannot now be interpreted with certainty. In all of these borings, if we except the Palaeozoic rock, the oldest formation whose age has been proved is the Gault. GAULT AND UPPER GREENSAND The Gault consists of stiff blue and grey clay and marl, which was proved to have a thickness of 172 feet in the deep boring at Loughton. It was found below Harwich and Weeley, and is probably everywhere present beneath the Chalk under Essex ; but it evidently becomes thinner towards the north-east, as it was only 50 or 60 feet thick at Harwich. There is evidence of the presence of Upper Greensand beneath a portion of Essex along the Lea valley. Mr. Whitaker has remarked that this formation was found beneath the Chalk at Cheshunt, and was met with in the deep well at Loughton, the thickness being about 30 feet. Below the Gault at Loughton there was sand, which may possibly represent Lower Greensand. 2 CHALK The Chalk appears at the surface over a comparatively small portion of Essex, but where it does occur its presence is usually indicated by pits and lime-kilns. The Lower Chalk in Cambridgeshire includes at its base the well- known bed of phosphatic nodules and phosphatized fossils termed the ' Cambridge Greensand,' but whether or not this bed occurs underground in the northern part of Essex is unknown. Above is the Chalk Marl, a mass of grey marl 50 or 60 feet thick, which yields Ammonites variant, Inoceramus latus and other fossils. Still higher we come to the Tottern- hoe Stone, a hard band of somewhat sandy rock, which north of Heydon is a noted water-bearing stratum, strong springs being thrown out at its junction with the Chalk Marl. The Totternhoe Stone is characterized by Rbynchonella mantelliana, Kingena lima, Pecten orbicularis and P. Jissi- costata. The uppermost portion of the Lower Chalk comprises a mass of grey chalk or clunch which yields Belemnitella (Actinocamax) plena and Ho/aster subglobosus? These portions of the Chalk do not appear at the surface in Essex, but they no doubt occur everywhere below ground. The Middle Chalk, which is exposed north of Heydon, at Great 1 See Whitaker, Trans. Essex Field Club, vol. iv. pp. 149, 1 68. 2 Ibid. vol. iv. pp. 158, 159 ; and Quart. Journ. Geol. Soc., vol. Ivi. p. Ixxv. ; and Jukes-Browne, Cretaceous Rocks of Britain, vol. i. p. 370. 3 See W. Hill and A. J. Jukes-Browne, Quart. Journ. Geol. Sue., vol. xlii. p. 216. 4
