104 NATURAL SCIENCE. August. 



(i.) Existence of a high plateau; formation upon this of the 

 " Older Plateau Gravels " of Lane End, Penn, &c. 

 (Coloured red on the Geol. Surv. Maps.) 



(2.) Prolonged period of denudation, resulting in the formation of 

 a second plateau, lower than the last, and interrupted at 

 places by hills left undenuded. Deposition on this plateau 

 of the " Newer Plateau Gravels." (Coloured pink on the 

 Geol. Surv. Maps.) The Chalk escarpment was then in 

 existence, but it was much higher and further to the north. 



(3.) Erosion of the series of north-west to south-east valleys, 

 including those of the Thames at Goring, and of the Miss, 

 Chess and Loudwater. 



(4.) Cutting back of the Chalk escarpment to approximately its 

 present position by the erosion of the Thame and Ouse. 

 This cut off the head waters of the Miss, Loudwater, etc. 



(5.) Advance of the ice-sheet which deposited the Boulder Clay. 



The only remaining question is whether it be possible to fix any 

 definite date for this series of events. Everyone will admit that 

 no. 5 was early Pleistocene, as it is generally considered that the 

 arctic facies of the fauna in the Norwich Crag was due to the 

 refrigeration of the climate, which culminated in the formation of our 

 English glaciers. This gives us the latest date for the last member 

 of the series. Professor Prestwich includes no. i also in the Pleisto- 

 cene ; but if we do not accept his correlation of these drifts with 

 the Westleton beds, then these gravels may be of any age between 

 the Eocene and the early Pleistocene. When we remember how 

 little the structure of the country appears to have altered since the 

 time of the Boulder Clay, it is a rather heavy order to crowd such 

 a long series of events into the short time between the end of the 

 Pliocene and the beginning of the deposition of the Boulder Clay. 

 This affords further reasons against the acceptance of the Westleton 

 age of the " Older Plateau Gravels," which may be allowed an 

 extension backward into the Upper Tertiary, the exact amount of 

 which cannot now be determined. 



Let us now turn to the western end of the Berkshire — Chiltern 

 escarpment, where valuable evidence of the Pre-Pleistocene age of 

 the Chalk escarpment is afforded by the distribution of the Sarsen 

 stones. These are huge masses of siliceous rock, now lying scattered 

 over the Downs. Whence they came is not quite decided : they un- 

 questionably represent some former bed of sand, which has now been 

 denuded away and these boulders only left. They vary greatly in 

 composition. They are believed to have been derived either from the 

 Woolwich and Reading or the Bagshot beds, but probably came 

 from more than one horizon. Some of them have been found in situ in 

 the former, and thus unquestionably belong to them, while others occur 

 on the London Clay and on Bagshot beds, and thus with equal cer- 

 tainty are far more recent than the Woolwich and Reading beds. 



