Hugh Millei — Escarpments and Terraces. 29 



brunt. The profiles of the longer dip- slopes — in the case of the 

 Whinsill sometimes exceeding 700 yards — would therefore be found 

 to thin towards their scarped face like the blade of a carpenter's 

 chisel towards its edge, except probably more curved in outline. 

 When quarried or cut into along the line of slope, the dip of the 

 beds might even be seen to exceed the angle of the declivity. But 

 though opportunity and observation have not been wanting, I have 

 never found this to be the case. The front of a sandstone escarp- 

 ment sometimes attests the tendency in being, not squared, but 

 rounded off, like one side of a roche moutonnee. The extent to 

 which this occurs, however, scarcely bears out the views of extreme 

 glacialists, though something must be allowed for the subsequent 

 weathering of the escarpment. 



Having, as I think, given good reasons for dating the escarpment 

 system of West Northumberland back to a period anterior to the 

 Glacial epoch, I will pass to a consideration of the proofs and ob- 

 jections advanced by Mr. Goodchild. 



In stating the order of resistance of the Yorkshire Yoredale rocks 

 to mechanical erosion, as 1 Limestone, 2 Sandstone, 3 Shale, Mr. 

 Goodchild's grounds are probably indisputable. But it seems to me 

 quite otherwise with his conclusions regarding their durability under 

 atmospheric disintegration. In the cliffy faces bared at the angles 

 of a winding stream, strata of sandstone, limestone, and shale are 

 sometimes exposed in such a manner as to exhibit their respective 

 relations to weathering. The face of the beds of shale is invariably 

 concave, while sandstones and limestones of equal thickness project 

 about equally.^ Their durability is usually directly in proportion 

 to their thickness. A limestone compared with a sandstone double 

 as thick is found to have shrunk further back, but is in advance of 

 sandstones thinner than itself. So far as this evidence can be relied 

 on, it distinctly negatives Mr. Goodchild's proposition that sandstone 

 is notably the most durable, that second to it comes shale, while lime- 

 stone, subject at once to disintegration and dissolution, is more 

 destructible than either. 



The two kinds of reducing atmospheric agents, classed as mechani- 

 cal and chemical, can scarcely be assumed as mutually complementary 

 in their working, especially in regard to blocky limestones such as 

 those of Wensleydale. Chemical action corrodes the outsides of the 

 cuboidal masses, and by widening the joints retards the great agent 

 of mechanical disintegration — frost — in its work of sundering en bloc. 

 For, from the non-granular texture of these limestones, frost, as de- 

 taching particle by particle, can gain little hold, and hence the chief 

 planes open to it are those of jointage and stratification, which 

 together determine the rock into squared masses. A stage may be 

 reached at which frost is impotent to achieve much, and the limestone, 

 built up of loose blocks, may be compared, not inaptly, to a " dry 

 stone dyke," said to be the most lasting kind of wall for upland 



1 In many cases the two harder beds are so rapidly undermined, owing to the hollow- 

 ing out of the shale, as to have little chance of becoming weathered to any great 

 extent. 



