NOTES ON CERTAIN TECHNICAL TERMS USED IN GEOMORPHOLOGY

Although there is general agreement on the meaning of most of the terms used in geomorphology, a good deal of confusion does exist as to their exact definition. A study of three texts, one American, one English, and one German, and a dictionary in French suggested that a comparative analysis of their usage might make it possible to pare away some of the differences between them, particularly where processes are concerned. The three texts were Principles of Geomorphology by W. D. Thornbury, Principles of Physical Geology by A. Holmes, and Morphological Analysis of Landscape by W. Penck. The dictionary is Vocabulaire Franco‐AngloAllemand de Géomorphologie by H. Baulig. A summary of the writer's conclusions is as follows: Starting with the general processes at work in, or at the surface of, the lithosphere, there is a basic division into two kinds: those of internal origin, and those of external origin. To the former can be given the general name of crustal movements, while the latter are referred to as the processes of gradation. Crustal movements can probably be best defined by adapting Thornbury's definition of diastrophism, and are the processes which elevate or depress portions of the lithosphere and thereby prevent the gradational processes from reducing it ultimately to a mean level. The processes of gradation can still be defined most economically in the words of Chamberlin and Salisbury as being “all those processes which tend to bring the surface of the lithosphere to a common level.” In their turn they can be divided into the processes of degradation and those of aggradation. It is with regret that the latter term is used in place of deposition, but the great advantage which it has through its sense of opposition to degradation outweighs its ugly sound. Degradation can best be defined as the whole ensemble of external processes which tend to lower the surface of the lithosphere– thus combining the concepts of Baulig with those of Thornbury. Aggradation is obviously the opposite process. Under the general heading of degradation there are, accepting Holmes's view, three separate processes: weathering, erosion, and transportation. Weathering cannot be defined better than it was by Thornbury as “the disintegration and decomposition of (the) rock in place.” Erosion on the other hand was best defined by Holmes as “all the destructive processes due to the effect of the transporting agents.” It should be noted that this definition is only all‐inclusive if gravity is looked on as being a transporting agent. Transportation was left undefined by Holmes though it was he who said that it was a separate element of degradation (denudation). Thornbury classes it as part of erosion (i.e. degradation) but it must obviously play a part in aggradation also. The best definition seems to be the removal, by whatever means, of weathered material. Erosion itself can be subdivided into the processes of corrosion, corrasion, and attrition. All three are defined by both Holmes and Thornbury in similar ways with Holmes being generally the more wordy of the two. Following Thornbury, therefore, we can say that: Corrosion is the removal of material by solution and corrasion is the removal of bedrock particles by “the tooling action” of the transported material. This definition of attrition seems unnecessarily complex, however, and can be shortened with advantage to the “wear and tear rock particles undergo in transit.”