Canons of Landscape Evolution
Total Page：16
File Type：pdf, Size：1020Kb
BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 64. PP. 721-762.10 FIGS. 4 PLS. JULY 1963 CANONS OF LANDSCAPE EVOLUTION BY LESTER C. KING ABSTRACT The manner in which epigene landscapes evolve is examined and discussed. Slope flattening as a general process of landscape evolution is rejected, and with it Davis' concept of the peneplain. Landscape evolu- tion by scarp retreat and pedimentation is accepted, and several of its consequences are examined. The opinions derived are expressed at the end of the paper as a series of canons of landscape development CONTENTS TEXT Figure Page Page 4. Elements of a hillside slope 728 Historical approach 721 5. Effect of structure on hillslopes 729 Acknowledgment 722 Statement of the problem 723 6. Development of rock floors under humid Nature of hillslopes 725 climate 737 Evolution of hillslopes according to W. M. 7. Continued development of rock floors under Davis 725 humid climate 737 Hillslope elements 728 8. Late stage of the cycle of erosion under Pedimentation 730 humid controls according to W. M. Present hypothesis of hillslope evolution 732 Davis 738 Convexity of upper slopes 735 9. Standard conception of a peneplain 739 Hornologies between tie landscapes of humid- 10. Landscape profiles under arid, semiarid, temperate, semiarid, and arid regions 736 and humid influences 741 Peneplain versus pediplain 738 Plate Facing page "Normality" in landscape 740 Epigene cycle of erosion 742 1. Erosional surfaces and scarps in South Africa 744 Dating of land surfaces 745 2. Erosional features of granite terrains, Quantitative methods of landscape study 746 Southern Africa 744 Canons of landscape evolution 747 3. Residual hills in Southern Africa 745 References cited 750 4. Pedimented landscapes in Southern Rhodesia 745 ILLUSTRATIONS TABLES Figure Page 1. Contrasted slope profiles 723 Table Page 2. Slopes of a Natal monadnock 724 1. Typical gradients upon pediments 730 3. Cyclic erosion scarps in Natal, South 2. Characteristics of mature peneplain and Africa 727 pediplain 738 HISTORICAL APPROACH of the trio—William Morris Davis. Davis had The scientific study of landscape began in the a remarkably analytical mind, he reasoned latter half of the nineteenth century and is lucidly and logically; moreover he wrote with associated especially with the names of J. W. facility, grace, and copiousness, so that his Powell, G. K. Gilbert, and W. M. Davis in ideas were easily and pleasantly assimilated, America and Albrecht Penck in Europe. The became widely disseminated, and carried con- work of the American trio has always over- viction. He was an inspired teacher, and his shadowed in the public regard that of Penck ideas found a ready audience and ready ac- senior, and modern thought still follows closely ceptance. the pronouncements and opinions of the last Particularly was this so with his concept of 721 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/64/7/721/3416501/i0016-7606-64-7-721.pdf by guest on 25 September 2021 722 L. C. KING—CANONS OF LANDSCAPE EVOLUTION the "Cycle of Erosion" under which a land- and the transportation of material by the agency of fluidity come in to assist the slow processes of scape uplifted and subjected to the forces of mechanical disintegration, and finally perform the subaerial erosion was first vigorously dissected chief part of the task". by the deeply cut valleys of the rejuvenated river systems, with consequent increase of re- The role of solution as a significant agent of lief, and later reduced by weathering and lower- reduction in the later stages of landscape de- ing of the interfluves to a lowland of faint re- velopment is open to question. Many senile lief which he termed a peneplain. The type land surfaces bear a residuum of insoluble mat- areas cited for these changes were the areas in ter as a subsurface crust—laterite, silcrete, which Davis worked in his early professional caliche are examples—but all these hinder life, chiefly from Pennsylvania to New England; degradation in a marked degree, and it is and thus something of a parochial standard of doubtful whether solution is of any real sig- comparison was instituted for other regions of nificance in the final reduction of noncalcareous the globe. Landscapes which did not conform terrains. We shall ignore it in our further dis- to this standard, such as desert landscapes, cussion. were regarded as "abnormal", "accidental", Davis' specific addition to Powell's theory or exceptional. was to visualize and describe the landforms In his later years, Davis wrote many fine that, under the action of erosional processes, papers, yet none to my mind was more truly were evolving in the landscape toward a final philosophical nor more important than his stage. contribution of 1930 wherein the earlier view The contribution of Walther Penck (1924), was revised and many true homologies demon- in which the forms of hillslopes are related to strated between the landscapes of arid and of continuing earth movements, has not found humid regions. This harmony calls for revision any enduring acceptance. Nevertheless, Penck of a number of the earlier concepts, and par- junior understood certain classes of landforms ticularly the standard of "normality" in land- (e.g., hillslopes) better than Davis did, and scape types, but the necessary revisions have several of his viewpoints such as the parallel not been undertaken, with any degree of com- retreat of scarps and the difference between pleteness, either by Davis or by others. senile and initial landscapes are real in nature The review of what Davis called "the Cycle and make an advance on the earlier Davisian of Erosion" is thus broadly the theme of the technique. present paper; but before proceeding with our Landscape studies have now advanced to argument we shall adduce a little more of the the stage where statistical analysis is being ap- historical background. plied to certain classes of data (Strahler, The cyclic concept did not originate with 19SOb). Used critically, so that it does not lead Davis; it was stated first in Powell's Explora- to erroneous conclusions that become estab- tion of the Colorado River of the West (1875), in lished in the minds of scientists, this method the following terms (we quote from Geographical should tend to much more exact definition of Essays) landforms, processes, and principles. Un- doubtedly, if the basic data can be supplied " 'aerial forces carried away 10,000 feet of rocks from observation and measurement of land- by a process slow yet unrelenting until the sea again rolled over the land' and the evenly denuded sur- scapes, the method is ideal for disentangling face is referred to as 'the record of a long time when the complexities of forms and processes gov- the region was land' ". erned respectively by many factors or variables. Powell recognized very clearly that the later What other equally satisfactory approach stages of reduction in the landscape would be could there be, for instance, to the complexities of stream flow and grade? very slow: "The degradation of the last few inches of a broad ACKNOWLEDGMENT area of land above the level of the sea would require a longer time than all the thousands of feet that The script has benefited from a critical might have been above it, so far as this degradation depends on mechanical process—that is by driving reading by my colleague Dr. T. J. D. Fair, or flotation; but here the disintegration by solution whose contributions are far more important Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/64/7/721/3416501/i0016-7606-64-7-721.pdf by guest on 25 September 2021 ACKNOWLEDGMENT 723 than the few footnotes to his name indicate. Bryan accepted parallel retreat of scarps Several phrases and sentences have been re- (q.v.). Some authorities have indeed rejected framed at his suggestion, and to him I am in- the cyclic concept altogether (Penck, 1924); debted for the canons numbered (1), (12), and others, with whom we align ourselves, have (22). Several of the opinions expressed have accepted the general concept of a cycle of land- also been derived from his careful field studies forms developed under erosion, while consider- FIGURE 1.—CONTRASTED SLOPE PROFILES Widening of valleys and change in slope profile during the cycle. Right, youth to old age, according to W. M. Davis. Left, with parallel scarp retreat, according to W. Penck. (From Davis, 1930) and published works. All his assistance is ing that the detailed forms and sequences de- gratefully acknowledged. part considerably from those visualized and adduced by Davis. The essential differences of STATEMENT or THE PROBLEM viewpoint lie in the interpretation of slope forms and the manner of hillslope development. Davis' original conception of landscape evo- In summary, this opinion, after beginning lution under the subaerial processes of rainfall, the cycle of erosion with stream incision into an running water, and weathering has often been uplifted land surface exactly as in the Da- summed up in a series of progressive sections visian model, would place limits to the amount in which an uplifted landscape is depicted as of hillslope flattening, regarding the slopes as first dissected under stream incision with the attaining a stable gradient (denned for local production of narrow valleys. After the streams conditions) after which the upper parts of the have attained grade (defined as the condition slopes retreat parallel to themselves. A well- in which all major irregularities have been known example is due to Kirk Bryan, 1922, p. eliminated from the thalweg), the rate of river 42-46. At the foot of the slope is left a pediment incision is reduced to negligible proportions, sloping gently down to the river.