CHAP. XII.] ICENIAN PERIOD. 247 



It was doubtless a period during which the action of rain 

 and rivers effected great changes on the surface of the 

 land, and it is possible that some of the geological features 

 of western and central England were initiated at this time. 

 By this expression I do not mean that the present physical 

 features of these districts were then developed, but that large 

 areas of Chalk and Eocene were then removed by erosion and 

 detrition from the borders of Wales and from the Midland 

 counties, and that the ridge or outline of the great Jurassic 

 escarpment may then have been developed, so that the 

 watershed between the valleys of the Severn and the 

 Thames may have been formed at this period ; but as the 

 recession of the Mesozoic escarpments was continued 

 throughout the Pliocene epoch, we must assume that in 

 Miocene times the escarpment of the Chalk lay much 

 nearer that of the Oolites than it does now, and that both 

 were to the westward of their present lines. The valley of 

 the Thames, as first pointed out by Sir A. Ramsay, must 

 be older than the development of the Chalk escarpment 

 through which it passes between Wallingf ord and Reading. 

 Its course must have been determined when the watershed 

 lay over the Jurassic escarpment, and when a sheet of 

 chalk covered by Eocene strata sloped away eastward from 

 the summit of the Cotteswold Hills. 



From the position of the early Pliocene beds on the 

 North Downs overlooking the area of the Weald, we may 

 infer that the anticlinal dome or uplift of this area was 

 much less marked in Miocene time than it is now, and that 

 its central portion was not then relatively lower than the 

 tracts occupied by the Chalk, but that the whole area 

 formed a plateau of low elevation, from which perhaps the 

 Upper Cretaceous rocks had not yet been entirely removed. 

 The outer parts, and in all probability the tracts where 

 Upper Chalk is now at the surface, were then covered by 

 Eocene strata, and the greater part of what we call the 



