184 GEOLOGY OF THE ISLE OF WIGHT. 



CHAPTER XII. 



OLIGOCENE— con^mMerf. 



Hamstead Beds. 



In 1853 Forbes pointed out that a thick series of beds overlies 

 the Bembridge Marls, and yields in its upper part a marine fauna, 

 which includes a large number of characteristic species. These 

 beds attracted great interest, for they were the highest of our 

 older Tertiary series, and were separated by many writers from 

 the rest of the Fluvio-Marine Beds and were referred to the Miocene 

 period. Though no longer classed as Miocene the interest of these 

 deposits has not decreased, for nearly all the recent additions to 

 the fauna are characteristic of the upper part of the Hamstead 

 group. 



Not only has our knowledge of the fauna increased since 

 Forbes' time, but the deposits prove to be both thicker and more 

 extensive than was originally imagined. When Forbes' Memoir 

 was published the only known strata of this age occurred in 

 Hamstead and Bouldnor cliffs, with a doubtful outlier in Parkhurst 

 Forest. Now it has been ascertained that they cover a much 

 larger area, for they extend over about half the Tertiary basin in 

 the Isle of Wight, stretching continuously from Yarmouth to 

 Brading, and occupying the greater part of the wide trough of 

 the Isle of Wight syncline. 



In thickness also Forbes under-estimated the importance of this 

 group. Instead of only reaching about 170 feet in Hamstead 

 Cliff, new measurements prove that the Hamstead Beds are there 

 256 feet thick (probably rather more), and that at Parkhurst 

 Forest, and at Woottoii, in the East Medina, they also exceed 

 200 feet. 



Forbes divided the Hamstead Series into : — 

 Corbula beds. 

 Upper freshwater and estuary marls (full of Cerithium 



plicatuni). 

 Middle freshwater and estuary marls (full of Mclania 



fasciata), with the "White Band " at the base. 

 Lower freshwater and estuary marls (with Melania 



mvricata and Mclanopsis carinatd). 

 " Black Band." 



