172 



Froceedings of the Royal Physical Society. 



1. The larger boulders throughout the till, as I have 

 already stated, are glaciated generally in the direction of 

 ice-flow. 



2. To this glaciation they have been subjected "in the 

 position they now occupy." 



3. The till was, as a formation, slowly built up, and " pre- 

 sented surface after surface to the great grooving and polishing 

 agent." 



4. The glaciating agent was the same that had already 

 acted more or less upon the rocks below. 



The applications of these general facts may be summarised 

 in a few words. We have seen that the glaciated boulder- 

 surfaces correctly register the direction of ice-movements. 

 They ought, therefore (1), to serve as guides to the glaciation 

 of drift-covered districts, and (2) to register some of the 

 changes of ice-movement during the progress of the forma- 

 tion, and throw better light upon the intercrossing of erratics. 

 The presence of pavement-boulders may also ultimately prove 

 (3) a distinguishing mark of those formations that are to be 

 correlated with the true till. And if it be, as it certainly 

 seems, impossible for icebergs to glaciate, in fixed directions 

 and uniformly upon one cheek, so many blocks lying loose 



Fig. 6. 



Two Boulder-clays, with derivation marked by the striation of boulders, 

 Northumberland. A, Till, 8 to 10 feet : b Till, 20 feet. 



at the sea-bottom, they may add (4) another and very strong 

 proof of its origin as a ground-moraine. 



