270 ROLLIN D. SALISBURY 



responding lowering of its surface, an appropriate extension of land, 

 and an increase in its height above the sea. 



Though this great body of rock new-laid upon the lands was 

 temporary in its character, primarily because of the low temperature 

 at which its substance assumed the liquid form, it was of great 

 importance, from a geologic point of view, in more ways than one. 



1. In the first place, the loading of millions of square miles of land 

 with such a weight must have had an appreciable effect upon crustal 

 movements, if the doctrine of isostasy has validity, and its disappear- 

 ance, under climatic conditions which developed later, must have 

 produced movements of the same class, but of opposite phase. 



2. Again, the development of the ice-sheets put a virtual stop to the 

 processes which had been in operation over six millions of square 

 miles of the land, and set other processes into operation in the same 

 places. Thus the normal phases of river work were suspended, most 

 rivers within the ice-covered area ceasing to flow altogether. The 

 usual phases of rock weathering and decay were practically stopped 

 over the same areas, areas which, in the aggregate, were a very con- 

 siderable fraction of the surface of the land. On the other hand, a 

 new process of erosion was substituted for the old — erosion not 

 restricted chiefly to the removal of decayed rock. 



3. The changes in erosion were hardly greater than those in 

 sedimentation, for instead of the assortment and separation of 

 decayed material into its several physical classes before deposition, 

 fine sediments and coarse, largely of undecayed material, were left 

 promiscuously commingled. Thus on a large scale and over enor- 

 mous areas deposits were made which were unhke those of comparable 

 extent at any other stage of the earth's history, unless at times when 

 climates were similar. 



4. It should be noted further that the changes in the processes of 

 erosion and sedimentation — changes in kind as well as in rate — were 

 not limited to the areas actually covered by the ice, or even to the 

 areas affected by drainage from it, or by icebergs which floated out 

 beyond its edge. Modifications of erosion and sedimentation were felt 

 in all areas affected, directly or indirectly, by the change of cHmate. 



The great ice-sheets, with the recurrent disturbance which they 

 probably occasioned in the crust of the earth and the lesser changes 



