Page:VCH Sussex 1.djvu/45

 GEOLOGY folds we find higher Cretaceous deposits coming on in succession above the Weald Clay in the southern part of the county between Eastbourne and Midhurst. The earliest of these strata belongs undoubtedly to the Lower Cretaceous period ; for this Atherfield Clay, so called from the place where it is best seen, Atherfield in the Isle of Wight, contains Lower Greensand marine fossils. The Atherfield Clay has only been traced as far east as Warminghurst, where Mr. Lamplugh recently noticed about 20 feet of clay with marine fossils, below the sandy Hythe Beds, and resting with a sharp division on the blue shaly Weald Clay. It is by no means clear yet whether there is not everywhere a break between the Atherfield Clay and the Weald Clay below ; for there is a sudden change from estuarine to purely marine conditions, and near Eastbourne most of the Lower Greensand and probably much of the Weald Clay have been cut out or overlapped by deposits of somewhat newer date. Unfortunately however the junction of the two clay deposits is difficult to examine ; for it usually occurs in flat land where natural sections are wanting and artificial sections are scarce. Wells are not sunk, near the junction, for there is seldom any water to be had, and that found is not palatable. The Lower Greensand above the Atherfield Clay consists mainly of sandy deposits with subordinate beds of harder rock. When met with in wells or excavations some depth below the level of the surface, the sands are commonly tinged more or less with green (hence the name ' Green- sand ') from the presence of small grains of a dark green mineral known as glauconite. This mineral, which is an iron compound, readily oxidizes on exposure, and then the sands take the familiar bulf or rusty hue which makes people wonder why geologists ever called them Greensand. There is a remarkable change in the Lower Greensand when traced from west to east and south through Sussex. At Petersfield it has a thickness, according to Topley, of 425 feet ; seventeen miles to the east, at Pul- borough, it has decreased to 380 feet, through the thinning of the two lower divisions, the Atherfield Clay and the Hythe Beds. Another seventeen miles to the east, at Hassocks Gate, the total is only 130 feet, the Atherfield Clay having disappeared and the other three divisions having thinned considerably. Five miles or so further towards the south-east the bold pine-clad sandy ridges which characterize this forma- tion sink and seem to melt away into an almost featureless undulating plain, which stretches to the sea near Eastbourne, where the Lower Greensand is represented by a few feet of coarse sand between the Gault and the Weald Clay, all the rest of the formation having disappeared. The question of the relation of the Lower Greensand to the Wealden strata in Sussex happens to be of more than purely scientific interest ; for if Lower Cretaceous deposits can disappear so rapidly towards the south-east, it is evidently possible that the geological structure may cor- respond with that on the northern side of the Wealden anticline, and the Lower Cretaceous and perhaps the Jurassic strata may be entirely wanting around Newhaven. Palaeozoic rocks may there occur much nearer to 7