1894- 



THE EVOLUTION OF THE THAMES, 



107 



The dates of these various episodes cannot be fixed very precisely. 

 There are two outside limits: the valleys are earlier than the Boulder 

 Clay. They are later than the Newer Plateau Gravels, but it is not 

 possible to determine the exact age of these, for if we reject Professor 

 Prestwich's hypothesis of their correlation with the Westleton beds, 

 then they may extend back indefinitely into the Tertiary. The 

 nearest approach to a determination of a maximum age of the great 



Fig. 3. — Sketch-map of the South-East of England, showing the rivers and 

 principal escarpments (black lines) ; the dotted rivers are now dry. 



central plateau which fixed the direction of the rivers of the south- 

 eastern quarter of England is that it is Post- Eocene ; how much 

 later, one cannot be sure. The evidence quoted from the Berkshir 

 area shows that the erosion of the main valley between the Chalk 

 and the Oolites had commenced before the period of the later set of 

 Sarsen stones, which are probably of Upper Bagshot age. So that 

 one can only conclude that the main outlines of the river systems in 



