64 GEOLOGY OF THE ISLE OF WIGHT 



are best seen in Colwell Bay, where a few yards north 

 of How Ledge they descend to the beach, and a cliff 

 is seen formed of a thick bed of oysters, Ostrea velata. 

 The oysters occupy a hollow eroded in a sandy clay 

 full of Cytherea incrassata, from which the bed is known 

 as the " Venus " bed, the shell formerly being called 

 Venus, later Cytherea, at present Meretrix. The marine 

 beds contain many drifted freshwater shells as Limnaea 

 and Cyrena. The How Ledge limestone forms the top 

 of the Lower Headon. It is full of well-preserved 

 Limnsea and Planorbis. 



The Upper and Lower Headon are mainly fresh or 

 brackish water deposits. The purely freshwater beds 

 contain Limncea, Planorbis, Paludina, Unio, and land- 

 shells. In the brackish are found Potamomya, Cyrena, 

 Cerithium (Potamides), Melania and Melanopsis. Palu- 

 dina lenta is very abundant throughout the Oligocene. 

 A large number of the marine shells of the Headon beds 

 are species also found in the Barton clay. Cytherea, 

 Valuta, Ancillaria, Pleurotoma, Natica are purely marine 

 genera. 



In White Cliff Bay the beds are mostly estuarine. Most 

 of the fossils are found in two bands, one about 30 ft. 

 above the base of the series, the other a stiff blue clay, 

 about 90 feet higher, which seems to correspond with the 

 " Venus Bed " of Colwell Bay. Many of the fossils are of 

 Barton types. 



The Headon beds are about 150 feet thick at Headon 

 Hill, 212 ft. in Whitecliff Bay ; and are followed by beds 

 varying from about 80 to no ft. in thickness, known as 

 the Osborne and St. Helens series. They consist mainly 

 of marls variously coloured, with sandstone and lime- 

 stone. In Headon Hill is a thick concretionary lime- 

 stone, which almost disappears northward. The Oligo- 

 cene strata often vary considerably within short distances. 

 The Osborne beds are exposed along the low shore between 



