Page:VCH Kent 1.djvu/52

 A HISTORY OF KENT of the county; but in the south the sea never lost its grip of the district nor ceased to build up the framework of the present land. Atherjield Clay. — The first deposit of this sea was the Atherfield Clay, so named from a locality on the coast of the Isle of Wight where it is typically exposed, a brown or greenish blue clay, somewhat sandy in places, containing numerous marine fossils such as Ammotiites, Nautilus, Exogyra (an extinct oyster) and many other shells, with small crustaceans, fish teeth and other remains. Owing to its soft perishable material, its comparatively slight thick- ness, and the position of its outcrop on steep foundering slopes capped by the harder overlying strata, it is rare that natural sections of this clay are visible in Kent, though it was passed through in the railway tunnel at Sevenoaks, and was in part recently exposed by artificial excavations near the railway station at Hythe and at Bastead Mills near Plaxtole. The mining shafts at Dover have however provided the most favourable opportunity for studying this deposit in Kent ; its thickness here was 40 feet, the clay yielding large numbers of the characteristic fossils, and its base resting with a sharp line of demarcation upon the Weald Clay. Hythe Beds. — As shown in Table II., the term Lower Greensand is generally extended to include the Atherfield Clay as well as the over- lying sandy deposits, but is more strictly applicable to the latter. It has reference to the prevalence of disseminated grains of glauconite, a green silicate of iron, in the series. Near the surface however this mineral is usually decomposed by weathering, giving rusty red or yellow tints to the sandy rocks. These deposits, being less readily erosible, form bold terraces or ' features ' at their outcrop ; fringing the northern border of the low tract of Weald Clay, they constitute most of the rolling country between this lowland and the North Downs, and include much of the pleasantest inland scenery of the county. They represent the accumula- tions of a shallow current-swept sea at a time when the land was not far distant. The series is admirably exposed where intersected by the present coast in the cliffs between Folkestone and Hythe, and the names of its subdivisions are founded on these sections. The Hythe Beds, which constitute the lowest subdivision above the Atherfield Clay, are composed of irregular alternations of slightly loamy glauconitic semi-indurated sand (' hassock ') and hard sandy limestone. These harder beds, known as ' Kentish Rag,' are extensively quarried for building purposes and road mending at Hythe, Maidstone, Sevenoaks and other places. They are frequently associated with thin layers of chert, representing the nodular concentration of silica derived from the tiny spicules composing the netted framework of the sponges that lived on the old sea-floor. This chert, which is especially valued as a road material, is most abundant in the upper part of the division on the high ground south-west of Maidstone. Fossils are occasionally abundant in the Hythe Beds, though rare in many localities. At Hythe the series has yielded many echinoderms. Ammonites of several species. Nautilus, Belemnites of 10