536 PKOF. T. m'k. hughes on some 



Again, these pedestal blocks are not glaciated like those of similar 

 material in the drift. We must of course allow, especially in the 

 case of the limestone boulders, for the weathering of the blocks 

 exposed like the rest of the surrounding rock to the action of the 

 rain and vegetation, and might expect some amount of smoothing 

 along the surface of a boulder pushed forward even a short distance 

 by the ice-foot. But any such smoothed surfaces are rare. 



Moreover, we find that the margin of the limestone plateau or 

 terrace is perforated by numerous swallow-holes along the edge of 

 the impervious shale and clay on which the rain-water collects into 

 streams, which lose themselves on reaching the bare jointed lime- 

 stone. Often lines of such swallow-holes further out show the 

 former greater extension of the clay, as shown in the accompanying 

 section (fig. 5). 



Fig. 5. — Diagram-section from Boulder on Limestone Pedestal to 

 impervious heds resting on Mountain Limestone, Norher Brow, 

 AustivicJc, YorlcsJiire. 



B. Boulder. Y.E. Yoredale Eocts. 



B.Cl. Boulder-clay. M.L. Mountain Limestone. 



Sw.Hs. Swallow-holes. 



Had the impervious beds once enveloped the boulders and been 

 gradually cut back we ought to find such swallow-holes over the 

 surface around and among them. But this is not the case. Now 

 we know that not more than about 18 inches of the general surface 

 of the limestone has been removed since the pedestal blocks were 

 placed where we now find them, while the swallow-holes run 

 down to as many feet; so they cannot have been subsequently 

 effaced. 



The base of the Yoredale Eocks is generally overlapped by a great 

 mass of various kinds of drift, among which we find the ordinary 

 blue Boulder-clay full of scratched stones. I have elsewhere* 

 pointed out that in the case of small glaciers, where most of the 

 stones are carried on the surface, there are few scratched stones; 

 but that it is in great glaciers, where the stones have all, or nearly 

 all, had time to get down or through the crevasses to the bottom 

 of the ice, that they have been rounded in the moulins and scratched 

 in the ground-mass. There is hardly a scratched stone to be found 

 at the end of the present Khone glacier, but they become more and 

 more common as we follow the ancient drift down to the Jura. So, 

 as we might expect, we do not find these great numbers of local 

 unscratched boulders occurring together in the older blue Boulder- 



* Proc. Geol. Polytechn, Soc. W. Eiding, 1867. 



