500 STRATIGRAPHICAL GEOLOGY 



In this area Pecten asper does not occur in the Selbornian, 

 but is found in the overlying arenaceous representative of the 

 Lower Chalk. 



At Sidmouth and in the Blackdown Hills the lower sands have 

 yielded a rich fauna, the shells being preserved in chalcedony, and 

 the following are some of the commonest species, Sch. varicosa, 

 Aporrhais calcarata, Turritella granulata, Protocardia hillana, 

 Oucullcea glabra, Oytherea (Oallista) plana, Dosiniopsis caperata, 

 Lucina orbicularis, Trigonia aliforrnis, and Tr. scabricola. 



An outlier of the Selbornian sands occurs on the Haldon Hills 

 about 12 miles south-west of Sidmouth. Here they rest on the red 

 Permian breccias, and are reduced to a thickness of 70 or 80 feet. 



Passing now to the main outcrop of the Selbornian from 

 Wiltshire northward, we may briefly indicate the lithological 

 variations which it exhibits in that direction. In Berkshire and 

 Oxfordshire the facies is similar to that of Hampshire, but the 

 total thickness is greater, being about 300 feet. The zone of Sch. 

 rostrata is partly represented by marls and partly by malmstones, 

 and the bed of unfossiliferous greensand at the top is a continuous 

 band 15 or 20 feet thick. But both the greensand and the malm- 

 stone die out in Buckingham, leaving a purely argillaceous facies 

 like that of Folkestone, the Lower Gault being about 150 feet 

 thick and the Upper about 80 feet, a notable fact being tKe 

 occurrence of a layer of phosphatic nodules at the base of the 

 Upper Gault, embedded in glauconitic clay and lying on an eroded 

 surface of the lower clays. 



Traced to the north-east both subdivisions become thinner and 

 the Upper Gault has also suffered erosion, with the result that a 

 layer of nodules and fossils derived from it occurs at the base of the 

 Lower Chalk. Near Cambridge the Gault is from 140 to 120 feet 

 thick, including a variable thickness of the rostrata zone, but at 

 Soham, where it passes below the level of the Fenland, it is only 

 90 feet. 



When it emerges at Stoke Ferry in Norfolk it is still 60 feet 

 thick, but at Eoydon and Grimston it is less than 20 feet and 

 consists of red and grey marls with two beds of grey limestone. 14 

 Farther north it passes into a red limestone, which is generally 

 called the " Red Chalk " and is so strikingly exposed in the 

 cliffs at Hunstanton. This red rock is only 3| feet thick, and 

 consists of three layers, each passing into the other. The lowest is 

 sandy and of a deep brick-red colour full of Bel. minimus and 

 Terebratula biplicata ; the central part is a red nodular limestone 

 with the same fossils and many others, including Hoplites lautus, H. 

 splendens, H. tuberculatus, Inoceramus sulcatiis, and Inoc. concentricus. 



