THE TERTIARY ERA 57 



beginning of the succeeding syncline. The strata now 

 dip under the Solent, and rise into another anticline in 

 the Portsdown Hills. North and south of the great 

 anticline of the Weald of Kent and Sussex are two syn- 

 clinal troughs known as the London and Hampshire 

 basins. Nearly the whole of our English Eocene strata 

 lies in these two basins, having been denuded away from 

 the anticlinal arches. The Oligocene only occur in the 

 Hampshire basin, the higher strata only in the Isle of 

 Wight. 



Above the Chalk we come first to a thick red clay called 

 Plastic clay. It is much slipped, and the slip is over- 

 grown. The only fossils found in the Island are fragments 

 of plants ; larger plant remains on the mainland show a 

 temperate climate. This clay was formerly worked at 

 Newport for pottery. The clay is probably a freshwater 

 deposit formed in fairly deep water. On the mainland 

 we find on the border shallow water deposits called the 

 Woolwich and Reading beds. (The clay is 150 to 160 ft. 

 thick at Whitecliff Bay, less than 90 ft. at the Alum Bay.) 

 We come next to a considerable thickness of dark clay 

 with sand, at the surface turned brown by weathering. 

 This is the London clay, so called because it underlies the 

 area on which London is built. At the base is a band of 

 rounded flint pebbles, which extends at the base of the 

 clay from here to Suffolk. In it, as well as in a hard 

 sandstone 18 inches higher up, are tubular shells of a 

 marine worm, Ditrupa plana. The sandstone runs out 

 on the shore. About 35 ft. above the basement bed is a 

 zone of Panop&a intermedia and Pholadomya margari- 

 tacea, at 50 ft. another band of Ditrupa, and at about 

 80 ft. a band with a small Cardita. In the higher part of 

 the clay are large septaria, rounded blocks of a cal- 

 careous clay-ironstone, with cracks running through them 

 filled with spar. Pinna affinis is found in the septaria. 

 The thickness of the clay in Whitecliff Bay is 322 feet. 



