Gregory — Progress in Interpretation of Land Forms. Ill 



Contributions to the Journal between 1850 and 1870 

 reveal a tendency to accept greater degrees of erosion 

 by rivers, but the necessary end-product of subaerial 

 erosion — a plain — is first clearly" defined by Powell in 

 1875. 23 In formulating his ideas Powell introduced the 

 term "base-level," which may be called the germ word 

 out of which has grown the "cycle of erosion," the 

 master key of modern physiographers. The original 

 definition of base-level follows : 



"We may consider the level of the sea to be a grand base- 

 level, below which the dry lands cannot be eroded ; but we may 

 also have, for local and temporary purposes, other base-levels of 

 erosion, which are the levels of the beds of the principal streams 

 which carry away the products of erosion. (I take some liberty 

 in using the term 'level' in this connection, as the action of a 

 running stream in wearing its channel ceases, for all practical 

 purposes, before its bed has quite reached the level of the lower 

 end of the stream. What I have called the base-level would, in 

 fact, be an imaginary surface, inclining slightly in all its parts 

 toward the lower end of the principal stream draining the area 

 through which the level is supposed to extend, or having the 

 inclination of its parts varied in direction as determined by 

 tributary streams.) " 



Analysis of Powell's view has given definiteness to the 

 distinction between "base-level," an imaginary plane, 

 and "a nearly featureless plain," the actual land surface 

 produced in the last stage of subaerial erosion. 



Following their discovery in the Colorado Plateau 

 Province, denudation surfaces were recognized on the 

 Atlantic slope and discussed by McGee (1888) , 24 in a paper 

 notable for the demonstration of the use of physiographic 

 methods . and criteria in the solution of stratigraphic 

 problems. Davis (1889) 25 described the upland of 

 southern New England developed during Cretaceous 

 time, introducing the term "peneplain," "a nearly fea- 

 tureless plain." The short-lived opposition to the 

 theory of peneplanation indicates that in America at least 

 the idea needed only formulation to insure acceptance. 



It is interesting to note that surfaces now classed as 

 peneplains were fully described by Percival (1842), 26 

 who assigned them to structure, and by Kerr (1880), 27 

 who considered glaciers the agent. In Europe "plains 

 of denudation" have been clearly recognized bv Ramsav 

 (1846), Jukes (1862), A. Geikie '(1865), Foster and Top- 



