162 



Fig. 10. 



Diagram shewing the relation of the present contour of the limestone terraces 

 and scars (A. B.) in Wensleydale to the contour they would assume under atmos- 

 pheric denudation. The rock with the vertical shading represents limestone ; 

 that with the horizontal shading, shales ; and the stippled part sandstones. 

 The supposed preglacial contour is shewn by tlie dotted outline on the right. 

 The inclination of the beds shewn is not intended to have any signification. 



confined to the inner edge of the terrace, as shewn in Fig. lo, 

 at B : swallow holes over the rest of the terrace being, as a rule, 

 of much less common occurrence ; and sometimes are absent 

 almost entirely. Swallow holes, in such cases as the present, 

 are, of course, due to the solvent action of surface waters flowing 

 from the impervious beds that form the part of the hillside imme- 

 diately above the outcrop of the limestone ; and they are, almost 

 necessarily, initiated along that line where the outcrop of the 

 impervious shales comes on above that of the pervious and easily- 

 dissolved limestone below. As the face of the impervious beds is 

 weathered back, and new areas of limestone are exposed, the point 

 where the water sinks into the limestone recedes pari passH; and 

 so the swallow hole is enlarged and lengthened in the direction of 

 the hill. Now, it must be obvious that, if the development of the 

 limestone terrace is really due to the long-continued exposure of 

 the rocks to the action of the weather, that there must have been 

 a time when the shale overlying the limestone extended at least as 

 far outward as the scar now forming the edge of the terrace, as 

 shewn by the dotted line in the diagram ; and as a consequence, 

 swallow holes must have been formed there, just as they are now 



I 



