EEVIEW OF GEOLOGICAL 'STEUCTUEE. 35 



grooved, and carved, as it could only be where the ice fitted closely to it, 

 and since two solid bodies can not occupy the same space at the same 

 time, the clay could only have accumulated in the places where it is 

 found, after, or as, they were abandoned by the ice. The facts which 

 sustain this view of the origin of the Bowlder clay are so numerous and 

 significant that it is difficult to imagine that any one carefully review- 

 ing them should not be convinced of its truth, and yet, as there is per- 

 haps no subject on which all men think alike, there are differences of 

 opinion here. Prof. Jas. Geikie, speaking of the North American glacial 

 deposits, in the last edition of his " (Jreat Ice Age," p. 461, says : 



"Dr. J. S. Newberry maintains that the deposit in question (the Bowlder clay) mnst 

 have accumulated at the margin of the glacier. This is the same view as that held by 

 Mr. S. V. Wood and some other geologists in our own country. It fails, however, to 

 account for many of the facts. The Bowlder clay has evidently been subjected to great 

 pressure, and has been pushed and rolled forward under the ice." 



And in a note he says farther : 



" I have often tried to conceive how one hundred feet of tough Till could have been 

 slowly excluded in the form of loose clay and stones from the foot of an ice sheet so as 

 to cover a wide, flat country, but I have never been able to realize the process. Let me 

 ast those geologists who hold the opinion that the Bowlder clay has really been depos- 

 ited in this way, why it is that along the limit of the 'Northern Drift' that formation 

 consists almost exclusively of more or less loose accumulations of bowlders, gravel, and 

 sand, while to the north, Bowlder clay is present as the basement Drift, with bowlders, 

 gravel, and sand lying upon it." 



That the Bowlder clay should exhibit marks of great pressure is inev- 

 itable from the fact that it has been thrust out at the margin of the gla- 

 cier, and crushed against itself or any barrier behind it, with a force 

 almost inconceivably great. It should be remembered that the broad, 

 fiat, almost continental glacier which formed it, must have been affected 

 by the seasons of the year and alternations of warmer and colder years, 

 just as modern glaciers are, and that in its great retreat there were 

 thousands of temporary advances, and thus the materials which it ground 

 up and slowly excluded was from time to time pressed up into a ridge 

 or heap against which every advance of the glacier impinged. In the 

 gradual withdrawal of the glacier, these terminal moraine clay ridges 

 coalesced to form a plateau or sheet, such as we find it. It is also proba- 

 ble that the comparatively thin terminal edge of the glacier in its tem- 

 porary advances in some degree over-rode the great moraine sheet it had 

 thrust out. Indeed, this was inevitable, for the excluded clay would 

 form a slope which would receive the pressure of the advancing ice-sheet, 

 and thus it might with its diminished weight shoot far up on and over 

 the mass of clay it had before deposited. 



