MIDDLE TERTIARY. 1 73 



period of time termed Oligocene^ which is the only 

 part of the middle Tertiary represented in Great 

 Britain. 



CHAPTER XXI. 

 MIDDLE TER 1 1ARV. 



The Middle Tertiary period, usually termed 

 Miocene, makes a more striking approximation in 



it> life to the animals and plants which exist at 

 the present day. In the 1 European area it is a 

 record <>t" terrestrial and lacustrine Conditions, 

 alternating with the deposits oi shallow seas. 

 In Great Britain only a porti n of the earlier 



part of the Miocene period is represented by de- 

 posits winch now cover the northern part of the 



Jit, much of the New Forest, and are 

 I in the cliffs at Honlwell, Tollands Bay, 



at Bern Lnd Hempstead. These .strata are 



grouped together as OligOCene. 



Headon Bt 



The oldest ol beds, known as the 



Headon series, are 130 feet thick at Ileadon Hill, 

 in the ls!c of Wight, are in the main fresh-water 

 strata. They comprise first, about 70 feet of 

 brackish water marls, and fresh-water limestoi 

 superimposed upon the marine sands above the 

 Barton series. This proves that the shallow sea, 

 with the Upper Bagshot Sands for its floor, had 

 become converted into dry land, upon which 

 lakes were formed by fresh waters draining into 

 the bottom of the trough from a limestone region 

 such as the chalk or the oolites. 



