88 
least some 300 feet higher than at present, for a rise of some 
200 feet would push the sea much further off, and yet only 
bring the bottom of the Wash at Newcastle about to sea- 
level. 
The depth of these old effaced river-beds gives us surely 
the best index of land elevation in old times, and several 
interesting conclusions follow from it. 
Rock stretches, I believe, close to the surface of the 
ground all across the mouth of the Tees estuary. If then we 
raise the land to pre-glacial elevation,' the Tees valley here 
should be sunk some 200 feet below the surface. The Tees 
therefore, we may assume, in pre-glacial times flowed south- 
ward into the Ouse, as the Wear flowed northward into the 
Tyne. 
The York plain and the vale of Malton represent the 
filling in of valleys whose rock bottoms, in the case of the 
plain of York, may be 200 or 800 feet below the present 
surface. 
The section of the ravine excavated by the Wear since 
the glacial period is at least, I judge, forty times less in 
area than that excavated before. How old, then, are our 
English rivers ? 
It is therefore, I think, a tolerably safe rule that all nar- 
row rocky valleys are post-glacial, — e.g., I have no doubt 
the Nidd at Knaresbro’, the Derwent at Castle Howard, 
Swale at Richmond, Esk at Whitby harbour, are cases of 
post-glacial section. In several of these cases we can see 
clearly the old filled-up valleys. The Derwent apparently 
formerly flowed out by Gilling, on to Pilmoor, &c., &c. 
It is stated that scratched flints occur at the top of 
boulder-clay a little to the north of Sunderland. If this be 
so it proves that at the close of the glacial period there were 
chalk beds to the east, off Sunderland, in continuation of the 
York wolds, at such a height as to send glaciers backwards 
when the great ice-sheet had so far lessened as to permit 
the free play of ice on minor slopes. 
