194 ON CERTAIN SURFACE FEATURES OF THE 



these concavities are as much as twenty feet in depth below the 

 general terrace-level, but it is obvious that the farther they are 

 from the cliff-face — i.e. from the point of outlet for the water — 

 the slower must be the rate of sinking. Should the efferent 

 channels become choked, as they often must, one would think, 

 actual stoppage of the action may easily take place. Then, should 

 the clayey constituents of some of the beds render them locally 

 sufficiently impervious, the formation of peat-bogs or even of 

 ponds would under favourable circumstances no doubt follow in 

 these small lake-basins. 



In the above description I have limited myself strictly to what 

 is seen and to what may be with certainty inferred from the 

 facts observed. The bearings of both facts and inferences will 

 be evident to all who have paid attention to recent " Glacial" 

 literature. It seems undeniable that underground erosion acting 

 in the manner indicated is quite capable of producing hollows in 

 superficial deposits having all the characters of " Kettle-holes." 



True "Kettle-holes" are held by glacialists to be highly char- 

 acteristic of kames and of terminal moraines. They are regarded 

 as original surface features left when the ice under which they 

 were moulded last melted. 



That the bowl-shaped depressions of the surface of the Sand 

 and Gravel Drift of the Tyne Valley, as exhibited near Cor- 

 bridge, are now in process of constant formation and destruction, 

 and that they therefore cannot be in any way attributed to ice 

 action — this, I think, is sufficiently proven. The point is not 

 unimportant, since on it hinges to some extent the question of 

 the amount of denudation which has taken place since Glacial 

 times and the date of last Great Ice Age. I cannot regard any 

 pfirt of the present surface-contour of the Drift that so largely 

 fills our larger Northumbrian valleys as original — i.e., as practi- 

 cally unchanged in form since the disappearance of the glaciers. 

 On the contrary, all such features must, I believe, be referred to 

 very extensive pcst-Glacial denudation. 



Incidentally I think a second point has been established, viz. : 

 that an anticlinal or arched bedding in Drift sands and gravels, 

 such as is often stated to be typical of kames or eskers, may also 



