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 



