Page:The Journal of geology (IA journalofgeology21894univers).pdf/46

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 cañons. 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 cañons widen, and the divides become narrow and sharp, with rugged peaks showing the stream's maturity, but the work of the fluvial sculptor still continues, 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 Baselevel of Erosion. Thus in a complete cycle of a river's history the cañon 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 cañons have then disappeared, and the land reduced by long continued erosion approximately to sea level.

The development of the baselevel begins upon the seashore