EROSION AND ITS SIGNIFICANCE. 1 19 



It was evident after the production of this note on cor- 

 rasion that although the main plateau level of New England 

 and the broad mature valleys in the main plateau level and 

 the mighty canons dissecting them might be referred to the 

 forces of erosion, nevertheless the various levels lying 

 above the great lower plateau surface must be explained 

 in some other manner. It may be stated, however, that 

 when the earlier papers on New England were written, 

 such explanation of origin as is there outlined was in 

 harmony with the conceptions of modern physiographers. 

 Land Sculpture by Streams. 



Scope of note. — An attempt is here made to present in 

 briefest outline the various steps in the formation of the 

 peneplain, and then to make several important applications 

 of such reasonings to geological problems. 



The following notes do not conflict with the published 

 views on erosion by Gilbert, Powell, Davis, Penck, Dutton, 

 Lawson, Tarr, Salisbury, and others, but seek simply to 

 add to them and to call attention to the important conse- 

 quences of accepting such principles. 



The forces of erosion. — Let us consider the reduction 

 of a high mountain or plateau mass by the forces of erosion, 

 the action of the sea being neglected in this connection. 1 



On the one hand we have a plateau or uplifted plain, 

 either high or low, either simple or complex in rock struc- 

 ture, either simply warped or intensely faulted, either 

 resistant or weak. Such a feature is evidently a challenge 

 to the forces of erosion. On the other hand as destructive 

 forces we have the action of gravity in bringing streams 

 down to the lowest points of a region, and we have still 



1 The action of the sea appears to be very limited unless helped by 

 relative subsidence of the land. This is easily seen from a consideration 

 of the slight depth only at which wave base can be developed below sea- 

 level, and the slope of the profile of the shore thence to the shore-line, 

 along which the sea must possess strength sufficient to transport its 

 burden as a whole. 



