Topley — Physical Geography of East Yorkshire^ 437 



and by Dr. Foster and the author. ^ To these various piihlications I 

 would refer for a fuller discussion of the question, merely noting 

 now the general principles involved. 



The sea, wherever we now see it at work, exerts a levelling power, 

 planing off to a more or less uniform slope the various beds which 

 come within the range of its breakers. This forms what Prof. 

 Eamsay has called a plain of marine denudation.^ Vast masses of 

 rocks have thus been swept away, often far exceeding in amount 

 those subsequently removed by atmospheric agencies. Now, since 

 we know that the beds have been unequally upheaved, proved by 

 their present dip, the action of the breakers would plane off the 

 various beds as they successively came within its range. Hence the 

 resulting plain of denudation woidd present a succession of beds 

 cropping out along its surface. Of these, some would be hard, some 

 soft : the former resisting atmospheric denudation would stand out 

 as escarpments ; the latter would weather into longitudinal valleys. 

 The surface drainage of this area would collect into streams joining 

 eventually into main rivers running down the main slope. This 

 will usually be the dip slope, because the line of greatest upheaval 

 will be the line of strike and will determine the direction of the 

 marine plane of denudation. Thus originate the transverse A^alleys 

 which cut across escarpments. These two kinds of valleys can be 

 well studied in the Weald and the bordering chalk country. I think 

 the Humber is a transverse valley of this description ; it cor- 

 responds to the main channel of the Medway where the latter cuts 

 through the Greensand and Chalk on its way to the Thames. The 

 Ouse with its tributaries the Swale and the Ure, coming from the 

 Vale of York and the Lias hills, corresjjond to the Eden and its 

 tributaries in Kent. The Derwent, rising far within the Oolitic 

 country and running in a direction opposed to the general dip, 

 corresponds exactly to the tributary of the Medway which rises near 

 Ig'htham and runs down past Plaxtole and Hadlow into the Weald 

 Clay valley to join the main river. The beautiful valley of Eskdale 

 begins icitliin the Oolitic country and runs with the general dip. Its 

 river has excavated a channel through the hardfer Oolite which forms 

 the steeper part of its slopes down to the Lias below. Eskdale cor- 

 responds to such valleys as the Cray and Little Stour in Kent. 



If the present valleys were all made by the sea we might expect 

 that they would always drain into the sea by the nearest way. But 

 this is by no means the case. The upper branches of the Serwent 

 rise close to the sea near Filey, and again near Eobin Hood's Bay ; 

 but they drain inland, and after a long course reach the sea by the 

 Humber. Again the river Wiske which rises up in the moors near 

 Osmotherly runs down towards the Tees for about eight miles of its 

 course, and at one place approaches it within 1^ mile, then turning 

 south it runs past Northallerton into the Swale, and finally, ai^er 



^ On the Medway Gravels and Denudation of tlie Weald. Quart. Journ. Geol. Soc. 

 Vol. xxi. 1865. p. 470. 



- See a paper on the Physical Features of Cardiganshh-e. Brit. Assoc. Reports. 

 1847. Trans. Sects, p, 66 ;.also Op. cit. pp. 79 et seq., p. 139. 



