Denudation by Rivers
The general recognition of the competency of streams to form valleys was a necessary prelude to the broader view expressed by Jukes (1862)24
“The surfaces of our present lands are as much carved and sculptured surfaces as the medallion carved from the slab, or the statue sculptured from the block. They have been gradually reached by the removal of the rock that once covered them, and are themselves but of transient duration, always slowly wasting from decay.”
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 subaërial erosion—a plain—is first clearly defined by Powell in 1875.25 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 subaërial erosion.
Following their discovery in the Colorado Plateau Province, denudation surfaces were recognized on the Atlantic slope and discussed by McGee (1888),26 in a paper notable for the demonstration of the use of physiographic methods and criteria in the solution of stratigraphic problems. Davis (1889)27 described the upland of southern New England developed during Cretaceous time, introducing the term “peneplain,” “a nearly featureless 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),28 who assigned them to structure, and by Kerr (1880),29 who considered glaciers the agent. In Europe “plains of denudation” have been clearly recognized by Ramsay (1846), Jukes (1862), A. Geikie (1865), Foster and Topley (1865), Maw (1866), Wynne (1867), Whitaker (1867), Macintosh (1869), Green (1882), Richthofen (1882), but all of them were looked upon as products of marine work, and writers of more recent date in England seem reluctant to give a subordinate place to the erosive power of waves. Americans, on the other hand, have been thinking in terms of rivers, and the great contribution of the American school is not that peneplains exist, but that they are the result of normal subaërial erosion. More precise field methods during the past decade have revealed the fact that no one agent is responsible for the land forms classed as peneplains; that not only rivers and ocean, but ice, wind, structure, and topographic position must be taken into account.
The recognition of rivers as valley-makers and of the final result of stream work necessarily preceded an analysis of the process of subaërial erosion. The first and last terms were known, the intermediate terms and the sequence remained to be established. A significant contribution to this problem was made by Jukes (1862).34
“... I believe that the lateral valleys are those which were first formed by the drainage running directly from the crests of the chains, the longitudinal ones being subsequently elaborated along the strike of the softer or more erodable beds exposed on the flanks of those chains.”
Powell’s discussion of antecedent and consequent drainage (1875) and Gilbert’s chapter on land sculpture in the Henry Mountain report (1880) are classics, and McGee’s contribution30 contains significant suggestions, but the master papers are by Davis,31 who introduces an analysis of land forms based on structure and age by the statement:
“Being fully persuaded of the gradual and systematic evolution of topographical forms it is now desired... to seek the causes of the location of streams in their present courses; to go back if possible to the early date when central Pennsylvania was first raised from the sea, and trace the development of the several river systems then implanted upon it from their ancient beginning to the present time.”
That such a task could have been undertaken a quarter of a century ago and to-day considered a part of everyday field work shows how completely the lost ground of a half century has been regained and how rapid the advance in the knowledge of land sculpture since the canyons of the Colorado Plateau were interpreted.