CHAPTER IX 

 THE OLIGOCENE 



WE pass on to strata which used to be called Upper 

 Eocene, but are now generally classified as a period by 

 themselves, and called the Oligocene. They are also 

 known as the Fluvio-marine series. Large part was 

 deposited in fresh water by rivers running into lagoons, 

 or in the brackish water of estuaries, while at times the 

 sea encroached, and beds of marine origin were laid down. 

 The west of the Island is much the best locality for the 

 lower strata, those which take their name from Headon 

 Hill between Alum and Totland Bays. There are three 

 divisions of the Headon strata, marine beds in the middle 

 coming between upper and lower beds formed in fresh 

 and brackish water. Light green clays are very character- 

 istic of these beds, and at the west of the Island thick 

 freshwater limestones, which have died out before the 

 strata re-appear in Whitecliff Bay. The strongest masses 

 of limestone in Headon Hill belong to the Upper division. 

 The limestones are full of freshwater shells, nearly all the 

 long spiral Limnsea and the flat spiral disc of Planorbis, 

 perhaps the most abundant species being L. longiscata 

 and P. euomphalus. The limestones themselves are almost 

 entirely the produce of a freshwater plant Chara, which 

 precipitates lime on its tissues, in the same manner as the 

 sea weeds we call corallines. On the shore round the base 

 of Headon Hill lie numerous blocks of limestone, the 

 debris of strata fallen in confusion, in which are beautiful 

 specimens of Limnaea and Planorbis. The shells, however, 

 are very fragile. The marine beds of the Middle Headon 



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