614 STRATIGRAPHICAL GEOLOGY 



principally such as result from the action of moving masses of ice 

 (boulder-clays), or from the action of strong currents of water (coarse 

 gravels), but interstratified with these there are sometimes sands, 

 brick-earths, and laminated loams which have been deposited in 

 quiet water and in a more gentle manner. 



The chief product of the Glacial epoch is the material known 

 as boulder-clay. There are several varieties of such clay, and it is 

 by no means certain that they have all been formed in the same 

 manner. Some boulder-clays are stiff compact masses of unstratified 

 clay containing numerous stones and boulders, and they seem to 

 have been compacted by the passage of heavy masses of ice over 

 them. It does not follow, however, that they were dragged along 

 beneath the ice in the form of what has been called "ground- 

 moraine." The prevalent view at the present day is that the 

 materials of boulder-clay were enclosed in the moving ice, princi- 

 pally no doubt in the lower portion of it, but embodied in and 

 moving with the ice. Wherever the glacier or ice-sheet melted 

 this " englacial " material thawed out, and was dropped to form 

 an unstratified clay full of stones and boulders. It might even 

 accumulate by the thawing of the under surface of stationary ice 

 during a temporary amelioration of climate, and if climate conditions 

 again became severe a renewed movement of ice might take place 

 in such a manner as to pass over such boulder -clays without 

 removing them. 



Other boulder - clays are of a looser texture, and are more 

 frequently associated with sands and gravels which contain marine 

 shells, and such clays must either have been formed where ice 

 invaded the sea, or must be the product of an ice-sheet which had 

 passed over a sea floor, or was moving from the sea over the retreat- 

 ing shore of a sinking land. 



These shells occur at various heights above the present sea-level 

 from 100 to over 1300 feet, and also far inland, often many miles 

 away from the present coast -line. The question naturally arises 

 whether they are in situ or whether they have been transported 

 to their present positions by the agency of ice. When they were 

 first discovered andx described they were regarded as proofs of a 

 great submergence, diiring which the British Isles sank to an extent 

 of more than 1300 feet. Later it was perceived that this was 

 highly improbable, because no evidence of such a subsidence exists 

 in the south of England nor in the north of France ; marine 

 deposits in these areas being restricted to coastal belts, and not 

 occurring at more than 130 feet above sea-level. 



Latterly glacialists have tried to explain the occurrence of all 

 marine relics at high levels on the supposition that they were 



