Page:VCH Berkshire 1.djvu/49

 GEOLOGY forms the valley which lies between the ridge of Corallian rocks on the north and the great ridge of the Chalk on the south. This valley was long ago described by Dr. Beke as ' the remarkably fertile vale of Berk- shire which crosses the county from the parish of Shrivenham on the west to Cholsey on the eastern boundary. At present, as when Domesday Survey was taken, the western part of this vale is employed as pasture land, chiefly dairy, while the sides and eastern part are arable, and may be reckoned some of the most productive wheat land in the kingdom. The soil of this vale in general is a strong grey calcareous loam which evidently owes its excellence to the intimate mixture of vegetable mould with cretaceous earth.' l The lower 175 feet of the Selbornian is a grey clay belonging to the Lower Gault. This is overlain by some 50 to 60 feet of light grey silty marl which is darkest towards the bottom. This latter is the lower part of the zone of Ammonites rostratus, and together with the Lower Gault is marked Gault on the map. The ground is generally flat and marshy. This part of the series contains no water. The higher beds of the Selbornian are mapped Upper Greensand. They are composed of 60 to 90 feet of sandy marls and malmstone, and i o to 12 feet of grey marl with large grains of glauconite at the top of the formation. The total thickness of the Selbornian is about 315 feet. Steventon, Wantage and Didcot are situated on it. The stone beds form a broad plateau by Ardington, Hendred, Harwell, Didcot, Hagbourne, North Moreton and Brightwell, and Mr. Jukes-Browne observes that it is along this tract that the malmstone attains its greatest thickness, probably about 90 feet. The stone lies in regular beds, the central part being a fairly pure malmstone containing sponge spicules and globular colloid silica in large quantity and weathering to a very light grey, so that it might easily be mistaken for grey chalk on a cursory inspection. The beds form a ridge of high ground to the west and south-west of Wallingford. Strong springs are thrown out on the inner side of this ridge at Sotwell and Brightwell, but Mr. Jukes-Browne thinks that a considerable amount of water must find its way beneath the Chalk, a good supply having been obtained from these strata by borings at Wal- lingford and Moulsford. The water is sometimes rather hard. The fossils of the Selbornian are all marine ; the lower clayey part was probably laid down in fairly deep water, the upper part may possibly have been deposited during a pause in the depression of the sea-bottom, causing a shallowing of the sea, and the change in mineral character may be due to earth movements causing an alteration in the coast-line and a consequent change in the nature of the sediment carried out to sea in this area. Probably the sea was by degrees spreading over this part of. Europe. In the south-west corner of the county the Upper Greensand comes to the surface near Inkpen. It forms a patch, for the most part outside 1 Dr. Beke in Lyson's Mag. Brit. I (1806), 188. II