G. W. Bulman — Glacial Geology. 343 



by which the drift-beds might be separated into geological horizons 

 — because, if, as some maintain, two glacial and one inter-glacial 

 period are represented in these beds, there ought to exist, a priori, 

 some decided distinction in the MoUuscan fauna. I have utterly- 

 failed to detect any."i 



That the limited mass and area of these sands and gravels, and 

 tlie absence of signs of denudation between them and the Boulder- 

 clay, is decisive evidence against their being the deposits of a mild 

 interglacial epoch, comparable in duration to the glacial period 

 itself, is indicated by a consideration of what would probably take 

 place during such an interval, and by a study of post-glacial sands 

 and gravels. 



For, on the melting of the ice, river action would be intensified ; 

 vast quantities of morainic matter would be carried down by the 

 swollen streams, and spread out over the boulder-clay of the plains 

 in the form of sand and gravel. At the same time there would be 

 denudation of the boulder-clay in places, and some of these denu- 

 dation hollows would be filled again with gravel. In places this 

 sand and gravel might reach the sea, and form marine deposits. 



But this would not continue for very long : the streams would 

 lose their carrying power; the gravel would no longer be carried to 

 the lower grounds, and they would commence to cut their channels 

 through the gravel, and then through the underlying boulder-clay. 



This gives a clear idea of the unconformable, denuded junctions 

 we should expect to find between the sands and gravels of an inter- 

 glacial epoch, and the boulder-clays of preceding and succeeding 

 glacial epochs. And such a course of events is indicated for the 

 close of the glacial epoch, and the incoming of the present mild 

 climate by a study of post-glacial sands and gravels. 



In Northumberland, for example, a considerable thickness of such, 

 deposits, known as the " Upper Drift Sands and Gravels," are found 

 overlying the boulder-clay. At the junction the latter shows signs 

 of denudation in places, while the overlying beds show at times 

 clear indications of being in part composed of the re-assorted 

 boulder-clay. And since the formation of the upper beds the 

 streams have, in many cases, cut down through sand and gravel 

 and boulder-clay to the rock below. 



Ought we not, then, to look for some evidence of a similar course 

 of events in the relations and characters of the beds of the supposed 

 interglacial epoch ? 



The glacial phenomena of the Vale of Eden, as described by 

 Mr. Goodchild (Q. J. G. S. 1875), point rather to the intermingling 

 and overlapping of the action of ice and water taking place more 

 or less contemporaneously, than to distinct glacial and interglacial 

 periods. 



A few sections at the foot of Stainmoor, Mr. Goodchild points out, 



" show that locally a threefold division of the drift obtains." Yet 



he adds a little further on, "It is nearly impossible to make out any 



definite order of succession in the drifts in the lower parts of the 



1 Q.J.G.S. 1874, p. 124. 



