GEOLOGY bed in our area the Gault is followed there is in that bed evidence that its lower layers at least were formed chiefly from the waste of the Gault underlying them, for they contain phosphatic nodules and worn fossils derived from it. The so-called ' coprolite bed ' which has mainly re- sulted from the denudation of the upper beds of the Gault, does not there- fore mark a distinct geological horizon, but may at one place be of Upper Greensand age and at another may represent the lower, or even sometimes the higher beds of the Chalk Marl. When the Upper Greensand is present it appears as a thin bed of soft marly sandstone passing up in places into a chloritic marl, which may represent the higher beds of the same formation, but is generally the lowest bed of the Chalk Marl there represented. This bed of phosphatic nodules and worn Gault fossils, whether it be of Upper Greensand or of Chalk Marl age, is of considerable economic value, being extensively worked for the production of artificial manure, but the only ' coprolite ' pits within the county are those at Ashwell. While there is a decided physical break between the Gault and the Upper Greensand, and a change of conditions took place from a rather deep and quiescent sea to comparatively shallow water, probably with shifting currents, there is no distinct line of demarkation between the Upper Greensand and the Chalk Marl, and it is sometimes difficult to say to which a certain bed should be assigned. The Upper Greensand is about 40 feet thick in the south-east of the county, but thins out to nothing in the north. In the extreme west only does it come to the surface, in a thin band between Marsworth and Buckland, just beyond the Wilstone reservoir. It is 44 feet thick at Cheshunt and 40 feet at Ware, its position at the former place being from 675 to 719 feet below Ordnance datum, and at the latter place from 478 to 518 feet below this datum. It consists of fine greenish sands with hard calcareous sandstone and chert, the typical green beds usually being charged with glauconite, and it contains sponge-spicules, Foraminifera, and other fossils. It ap- pears to have been laid down in a sinking sea-bed, the deposit in which gradually changed from one of mechanical origin, from erosion of adjacent land, to one of almost entirely organic origin, the great Chalk formation having been formed by and from the remains of the animals, mostly of microscopic size and lowly nature, which lived and died in and on the surface of the deep Cretaceous seas. The Lower Chalk rests upon the eroded surface of the Gault, or gradually takes the place of the Upper Greensand. It usually has for its basement-bed either the Chloritic Marl or the Cambridge Greensand, homotaxial deposits the lower beds of which contain phosphatic nodules and numerous fossils mostly derived from the Gault, many being phos- phatized. Whichever is present is followed by the Chalk Marl, a soft marly chalk with no flints but a considerable amount of silica. The Chalk Marl varies from about 20 to 60 feet in thickness, and the whole of the Lower Chalk from about 100 feet (Great OfHey) to 180 feet or thereabouts (Cheshunt). At Bushey its thickness has been given as 255 7
