REVOLUTION IN THE TOPOGRAPHY OF THE PACIFIC 



COAST SINCE THE AURIFEROUS 



GRAVEL PERIOD/ 



INTRODUCTION. 



It is now generally recognized that rivers are the architects 

 and sculptors of their own valleys. The land is everywhere 

 shaped largely by its streams, and the forms developed are serial^ 

 beginning with the river's youth and changing in the progress of 

 time until finally the stream attains old age, and its topographic 

 work is completed. In their early life, when rivers have their 

 highest grade, they wash away their beds more than their banks, 

 and cut canons. Their beds are a succession of gentle flows, 

 rapids, and falls, over the softer and harder beds. When by 

 deep cutting the fall of the stream is reduced, it tends to spread 

 out and erode its banks, the canons widen, and the divides 

 become narrow and sharp, with rugged peaks showing the 

 stream's maturity, but the work of the fluvial sculptor still con- 

 tinues, and the mountains are reduced to hills and the hills to 

 knolls so low that the general aspect of the country is that of a 

 plain. The streams are powerless to erode the land below the 

 level of this gentle plain, which has been appropriately named 

 by Powell the Baselfevel of Erosion. Thus in a complete cycle 

 of a river's history the canon and the broad divide, or plateau, 

 are features of its youth ; narrow, sharp, more or less rugged 

 divides of its maturity, and the baselevel of its old age. The 

 cafions have then disappeared, and the land reduced by long 

 continued erosion approximately to sea level. 



The development of the baselevel begins upon the seashore 



' Published with the permission of the Director of the United States Geological 

 Survey. Abstract from a paper upon the same subject which will appear in the 14th 

 Annual Report of the United States Geological Survey. Read before the Geological 

 Society of Washington, April, 1893. 



32 



