504 Early Physical Changes. 



channel for itself through the boulder-clay that partially 

 blocked up the original valley in which it ran. When 

 that original valley was formed through which the older 

 Severn ran is the point that I shall now attempt to 

 discover. This subject is intimately connected with the 

 origin and geological dates of the channels of many of 

 the other large rivers of England, most of which, unb'ke 

 the Severn, flow eastward to the English Channel and 

 the Oferman Ocean. 



I must begin the subject by a rapid summary of 

 certain physical changes that affected the English 

 Secondary and Eocene strata long before the Severn, 

 after leaving the mountains of Wales, took its present 

 southern and south-western course along the eastern 

 side of the Palaeozoic rocks that border that old land. 



About the close of the Oolitic epoch the Oolitic for- 

 mations were raised above the sea, and remained a long 

 time out of water ; and, during that period, those atmo- 

 spheric influences that produced the sediment of the 

 great Purbeck and Wealden delta were slowly wearing 

 away and lowering the land, and reducing it to the 

 state of a broad undulating plain. At this time the 

 Oolitic strata still abutted on the mountain country 

 now forming Wales and parts of the adjacent counties. 

 They also completely covered the Mendip Hills, and 

 passed westward as far as the mountains of Devon 

 passing out between Wales and Devonshire through 

 what is now the Bristol Channel. The whole of 

 the middle of England was likewise covered by the 

 same deposits, overlying the rocks that now form the 

 plains of Shropshire, Cheshire, Lancashire, and the ad- 

 joining areas, so that the Lias and Oolites passed out 

 to the area now occupied by the Irish Sea, over and 

 beyond the present estuaries of the Dee and the Mersey, 



