24 Procccdiiif/s of the Ohio State Academy of Science 



of supplementing^ this knowledge of water areas with the history 

 of land areas. 



Base-level, peneplain and erosion cycle} 



The importance of stream erosion had long been recognized, 

 yet when Powell- in 1875 first formulated the idea of the base- 

 level, he was stating what may have been implicit in the state- 

 ments of earlier writers, but what had not been definitely recog- 

 nized and defined. A base-level may be defined as an imaginary 

 surface, the projection beneath a drainage basin of the surface of 

 the water body into which that drainage basin emoties. Towards 

 this imaginary surface the streams cut their valleys ; near their 

 mouths they may actually reach it ; but further back they cannot, 

 because of the necessary though, in the case of the large rivers, 

 small rise of the stream up-valley. After the nearest approach to 

 base-level has been made, the widening of the valleys by valley- 

 side weathering and lateral swinging of the streams still goes on, 

 the inter-stream areas are attacked, and the whole land surface 

 is slowly reduced to a low rolling plain not far above base-level. 

 Plains of this origin were recognized by Powell- in 1875, in the 

 level surface of tilted rocks on which the Carboniferous limestone 

 of the Grand Canon rested, and similar buried or "fossil" plains 

 were recognized later by \'an Hise and Walcott. McGee'^ in 1888 

 was the first to call attention to such a plain, dissected but exist- 

 ing as a land form today, in the Middle Atlantic slope; and 

 Davis followed with an accoimt of similar plains as developed in 

 Pennsylvania, Xew York and Xew England. It was Davis"* who 

 in 1889 first applied the name peneplain to this end-form of long 

 continued subaerial erosion. The time during which the land 

 surface is passing from the initial form of youth to the i)enei)lain 

 which is developed in old age is the cycle of erosion and it is 



'See Davis: Geographical Essays, (Ginn & Co.), pp. ■24.'>-513. 



■ Powell : Exploration of the Colorado River of the West. 



■' McGee : Three Foniiations of the Middle Atlantic .Slope. .\m. Jour. 

 Sci., XXXV., pp. 141-2. 



* Davis : Topographic Developtnent of the Triassic Fonnation of 

 the Connecticut J 'alley, .\m. Jour. Sci., xxxvii., p. 430. 



