360 J. G. Goodchild — On Glacial Erosion. 



The flow of the higher strata of the ice seems to have been mainly 

 guided by the position of the adjoining higher fell tops, while the 

 lower strata seem to have been guided in their course by the sides 

 of the old preglacial valleys and to have flowed steadily outwards 

 nearly like a modern glacier. 



Whatever difference of opinion there may be amongst geologists 

 with regard to the theory that I have elsewhere put forward, that the 

 great ice-sheet was charged with detritus throughout, there can 

 hardly be any with regard to the existence of stones between the ice 

 and the rock surface that it covered. It must be borne in mind that 

 when the Glacial Period set in the ice must have had to work at a 

 surface that had been exposed for ages to the attacks of subaerial 

 forces, and that, in consequence, all the rocks were weathered to a 

 great depth. "What that depth was has never yet been determined ; 

 but, if we may judge by the rapid rate which many rocks weather in 

 a single lifetime, we should be prepared to find that the amount of 

 weathering effected between the close of the next older Glacial 

 Period and the commencement of the latest was far in excess of 

 anything of the kind that we can now point to in these islands. 

 I have long held the opinion that it was this preglacially -weathered 

 rock that formed the bulk of the materials of the drift. 



When, therefore, the early glaciers began to invade the lower parts 

 of the valleys, the removal of the surface rock, loosened as it was by 

 long-continued weathering, must have been a comparatively easy 

 matter. At the outset, on account of the deeply- weathered joints in 

 the harder kinds of rock, their rates of erosion would be nearly 

 equal. But as soon as the weathered outer portions of the limestones 

 and sandstones were removed, the unweathered rock beneath would 

 be better able to withstand the grinding of the ice than would the 

 associated softer flags and shales. As a consequence, the softer beds 

 were eroded much faster than the interbeddecl harder rocks, which 

 would thus be left in relief as terraces. The remarkable capability 

 of the ice to adapt itself to every form of the surface, which the 

 glaciated surfaces of the North of England plainly show the ice must 

 have possessed, must have helped greatly to produce such a result. 

 Wherever the ice passed over a soft bed that had one much harder 

 immediately beneath it, the overlying bed was removed with com- 

 parative ease ; while the newly bared upper surface of the harder 

 rock beneath offered much greater resistance to the grinding of the 

 ice and consequently suffered much less erosion. In the case of some 

 of the more compact and thickly-bedded rocks, it seems that the 

 highest strata of the hard bed even yet form the upper surface of the 

 terrace a hundred yards or more from the outcrop of the overlying 

 soft bed. 



In connexion with this part of the subject there is one point that 

 seems to have been overlooked by many of those who have written 

 about the vertical limit of the ice-sheet. It has been assumed, 

 seemingly upon insufficient grounds, that the rough and craggy form 

 of the higher parts of districts that are well glaciated in their valleys 

 is good proof that these higher parts were never overridden by the 



