Dr. James Geilde — Preservation of De230sits under ' Till.' 75 



example as those of the Forth and the Tweed, the whole body of 

 the ice would flow with a slower and more equable motion. As the 

 ice-sheet approached its termination, more especially if that terminus 

 chanced to be upon a broad and comparatively flat region, like East 

 Anglia, the erosive power of the ice would become weaker and 

 weaker for two reasons : first, because of its gradual attenuation, and 

 secondly, because of its constantly diminishing motion. These, in a 

 few words, are the varying effects which one might a priori infer 

 would be most likely to accompany the action of a great ice-sheet. 

 And an examination of the glacial phenomena of this and other 

 countries shows that the actual results are just as we might have 

 anticipated, had it been previously revealed to us that a large part of 

 our hemisphere was, at a comparatively recent date, almost entirely 

 smothered in ice. In places where, from the nature of the ground, 

 we should look for traces of great glacial erosion, we find rock- 

 basins ; in broken hilly tracts, where the ice-flow must have been 

 comparatively rapid but irregular, and the giaciation severe, we meet 

 with roches moutonnees in abundance, but with very little Till ; in 

 the open lowlands and in the broad valleys where the ice-sheet 

 would advance with diminished but more equable motion, we come 

 upon widespread and often deep glacial deposits, and now and again 

 with interglacial beds ; while over regions where the gradually de- 

 creasing ice-sheet crawled slowly to its termination, we discover 

 considerable accumulations of Till, often resting upon apparently 

 undisturbed beds of gravel, sand, and clay. 



The distribution of interglacial deposits, therefore, is really in itself 

 a proof that they have been overridden hy ice. When they occur in 

 highly glaciated regions, it is only as mere patches, which, occupying 

 sheltered places, have been preserved from utter destruction. In 

 the opener low grounds they are found in greater force, although 

 in such places they almost invariably afford more or less strong 

 evidence of having been subjected to much erosion and crumpling. 

 But the further we recede from the principal centres of giaciation, 

 and the nearer we approach the extreme limits reached by the 

 ice-sheets, the more extensive and the less disturbed do interglacial 

 deposits become. In a word, they occur in best preservation where 

 the erosive power of the ice was weakest ; they are entirely wanting 

 where we have every reason to believe that the grinding force was 

 strongest. 



If we look at the interglacial beds themselves with any attention, 

 it is very rarely indeed that we shall not find proof of their having 

 been subjected to more or less crushing and erosion. The overlying 

 Till cuts into them again and again — they are often caught up and 

 involved with the Till — and crumpling and contortion are frequently 

 conspicuous. No one who has paid much attention to glacial matters 

 will doubt that all this powerful erosion and confusion are due to 

 the passage of ice over the beds. It may be taken as proved, 

 therefore, that an ice-sheet does under certain conditions ride over 

 incoherent deposits of gravel, sand, silt, clay, and peat, without 

 entirely obliterating them. But all interglacial beds, even in highly- 



