William D. Thornbury
Principles of Geomerphology
Concept 1
The same physical processes and laws that operate today operated throughout geologic time, althought not necessarilly always with the same intensity as now.
This is the underlying principle of modern geology and is known as the principle of uniformitarianism. It was first enunciated by Hutton in 1785, beautifully restated by playfair in 1802, and popularized by Lyell in the numerous editions of his principles of Geology. Hutton taught that "the present is the key to the past," but he applied this principle somewhat too rigidly and argued that geologic processes operated throughout geologic time with the same intensity as now. We know now that this is not true. Glaciers were much more significant during the Pleistocene and during other periods of geologic time than now, world climates have not always been distributed as they now are, and, thus, regions that are now humid have been desert and areas now desert have been humid, periods of crustal instability seem to have separated periods of relative crustal stability, although there are some who doubt this, and there weretimes when vulcanism was more important than now.
Numerous other example could be cited to show that the intensity of various geologic processes has varied through geologic time, but there is no reason to believe that streans did not cut valleys in the past as they do now, that the more numerous and more extensive valleys glaciers of the Pleistocene behaved any differently from existing glaciers, that the winds which deposited the Najavo sandstone during Jurassic times obeyed any different laws from those which control wind movement today. Groundwater opened up solutional passageways in limestones and other soluble rocks and formed surface depressions which we now call sinkholes during the Permian and Pennsylvanian periods as it does today in many parts of the world. Without the principle of uniformitarianism there could hardly be a science of geology that was more than pure description.
Concept 2
Geologic structure is a dominant control factor in the evolution of land forms and is reflected in them.
Most student who have had an elementary course in physical geology or geography are likely to have been exposed to the teaching of W. M. Davis that structure, process, and stage are the major control factors in the evolution of land forms. It is with the first of this trilogy that we now concern ourselves. The term structure as it is used in geomorphology is nor applied in the narrow sense of such rock features as folds, faults, and unconformities but it includes all those ways in which the earth materials out of which land forms are carved differ from one another in their physical and chemical atributes.
It includes such phenomena as rock attitudes, the presence or absence of joints, bedding planes, faults, and folds, rock massiveness, the physical hardness of the constituent minerals, the susceptibility of the mineral constituents to chemical alteration, the permeability or impermeability of rocks, and various other ways by which the rocks of the earth’s crust differ from one another. The term structure also has stratigraphic implications, and knowledge of the structure of a region implies an appreciation of rock sequence, both in outcrop and in the subsurface, as well as the regional relationship of the rock strata. Is the region one of essentially horizontal sedimentary rocks or is it one in which the rocks are steeply dipping or folded or faulted? A knowledge of geologic structure in the narrow sense thus becomes essential.
It is common practise to speak of rocks as being ”hard” or ”resistant” or ”weak” or ”non-resistant” to geomorphic process. Such terms may be used so long as we recognize that we are using them in a relative sense and not always in a strictly physical sense, for rocks are attacked by both physical and chemical process. A rock may be resistant to one process and not-resistant to another and under varying climatic conditions may exhibit different degrees of resistance.
In general, the structural features of rocks are much older than the geomorphic forms developed upon them. Such major structural features as folds and faults may go back to far distant periods of diatrophism. Even in areas of as recent diatrophism as that of the Pleistocene it is difficult to find uneroded folds. Hence as a general principle we may assume that most rock structures were established long before the land forms which exist upon them.
We shall see in subsequent chapters many examples of how rock structure affects the characteristics of land forms. Commonly these relationships are obvious and result in striking topographic features. We should not, however, make the mistake of concluding that where the effect of geologic structure is neither obvious nor striking its influence is lacking. The effects are there but we may lack the ability to see them. Sometimes, however, the apparent lack of structural control of topography may simply indicate homogeneity of structure with resultant homogeneity of opography or may result from the large scale of the structural units. It is perhaps not going too far to say that no variation in rock structure is too slight to have significance over a sufficient span of geologic time. The increasing application of the board applicability of this principle.
Concept 3
Geomorphic processes leave their distinctive imprint upon land forms, and each geomorphic process develops its own characteristic assemblage of lands forms.
The term process applies to the many physical and chemical ways by which the earth’s surface undergoes modification. Some process, such as diastrophism and vulcanism, originate from forsec within the earth’s crust and have been designated by Penck as endogenic, whereas others, such as weathering, mass-wasting, and erosion, result from external forces and have been called exogenic in nature. In general, the endogenic processes tend to build up or restore areas which have been worn down by the exogenic processes, otherwise, the earth’s surface would eventually become featureless. The concept of geomorphic process operating upon the earth’s crust is not a new one. Even the ancients recognized it to some degree, but the idea that the individual process their stamp upon the earth’s surface is rather recent. Just as spesies of plants and animals have their diagnostic characteristics, so land forms have their individual distinguishing features dependent upon the geomorphic process responsible for their development. Floodplains, alluvial fans, and deltas are products of stream action, sinkholes and caverns are produced by groundwater, and end moraines and drumlins in a region attest to the former existence of glaciers in that area.
The simple fact that individual geomorphic process do process distinctive land features make possible a genetic classification of land forms. The recognition of this fact and his insistence upon its superiority to otehr types of landscape descrption was one of Davis’s important contributions to geomorphology, for it changed the subject from one in which land forms were classifield upon a purely morphologycal basis without regard to interpretations which could be made from them as to their geomorphic history. It doubtless would be possible to describe the multitudinous lans forms of the world in terms of perhaps a dozen o fifteen primary or elements forms such as plain, slope, scarp, mount, ridge, table, column, depression, valley, trough, col, niche, arch, hole, and cavern., but in general such terms tell little or nothing about the origins of the forms or the geologic history of the regions in which they exits. How much more illuminating are such terms as floodplain, fault scarp, sinkhole, snad dune, and wave cut bench. Even though they are in part descriptive they have genetic implications.
A proper appreciation of the significance of process in land form evolution not only gives a better picture of how individual land forms develop but also emphasizes the genetic relationships of land forms assemblages. Land forms are not haphazardly developed with respect to one another but certain forms may be expected to be associated with each otehr. Thus, the concept of certain types of terrain becomes basic in the thinking of a geomorphologist. Knowing that certain forms are present, he should be able to anticipate to a considerable degree other forms which may be expected to be present because of their genetic relationshipa to one another. Eventhough a hill or mountain may obstruct his vision, in his mind’s eye he may be able to see through it and visualize the topography on the other side. Terrain analysis, which has become so important in modern warfare and other fields, is more dependent upon this keen appreciation of land form assemblages than upon any other one thing.
Concept 4
As the different erosional agencies act upon the earth’s surface there is produced a sequence of land forms having distinctive characteristics at the succesive stages of their development.
That land forms process distinctive characteristic depending upon the stage of their development is the idea that Davis stressed most. A logical outgrowth of this principle is the concept of a geomorphic cycle, hich we may define as the various changes in surface configuration which a land mass undergoes as the processes of land sculpture act upon it. This concep, if properly conceived and applied, is one of the most useful tools in geomorphic interpretation. The basic idea is that, starting with a given type of initial surface underlain by a certain type of geologic structure, the operation of geomorphic process upon this mass result in a sequential rather than haphazard development of land forms. The methaporical terms, youth, maturity, and old age, are commonly used to designate the stages of development, and it is customaty to add such qualifying adjectives as early and late to designate substages.
Although sequence of time is also implied in a geomorphic cycle it is in a relative rather than in an absolute sense. It should be emphasized that there is no implication that these stage are of equal duration nor that separate regions which have attained the same stage of development have necessarily required the same length of time for attainment of that stage, for there are numerous factors which may cause the rate of degradation to vary from place to place. Neither is it implied that two areas in the same stage would be imilar in the detail of their topography. This could be expected only if the initial surface, the geologic structure, and the climatic and diastrophic conditions in the two areas were the same. Infinite variety may be expected in the details of topographic forms, but this does not negate the idea that there is a systematic evolution of land forms which make possible a recognition of their stage of development.
Much more progress has been made in working outthe details of the geomorphic cycle as carried out by running water than by any other geomorphic agenc, but the basic idea seems to apply to the other erosional agencies with the possible exception of glaciers. It seems to apply to a limited degree to erosion by mountain glaciers but as yet no one has seen its application to the work of ice sheets.
It should be noted that to explain land forms in terms of the three control factors, structure, process, and stage, as these terms were used by Davis, the diastrophic history of a region while undergoing degradation must be encompassed either under structure or process. Usually it is considered under process, but much might be said for considering this factor equally important as the other three. Particularly in such tectonically unstable regions as California and New Zealand this factor may be a critical one in the evolution of land forms and may actually seem to obscure any tendency toward sequantial development. Under conditions of continuing uplift orrapidly repeated intermittent uplift a landscape may be kept perpetually youthful, matre, or old without runnung thye course of a normal cycle. It is significant that it has been to a large degree from the geologist in this instable belts that skepticism or doubt as to the validity of the geomorphic cycle has arisen. Although it must be admitted that the proponents of the idea of cyclical developments of lans forms havein many instances been too enthusiastic about its ability to explain landscapes, to deny its reality is to deprive us of one of the most useful and fundamental concepts in geomorphology.
A necessary corollary to the concept of a completed of geomorphic cycle is that of a partial cycle. In fact, a partial cycle is far more likely to exist tahan a completed cycle. We should recognize that much of earth’s crust is restive and is subject to intermittent and differential movement, but inspite of this there does appear to be considerable evidence to indicate that portions of the earth’s crust do at time remain sufficiently stable for a geomorphic cycle nearly to run its course. Continued uplift may suspend cyclical development for long period of geologic time, but theri is no reason to assume that such conditions will be permanent. There good reason to belive that there are regions that were once areas of intense diastrophism that are now relatively stable. Even a partial cycle may leave its imprint upon a landscape, and geomorphologist neds to be able to recognize the effects of partial cycles because they are probably more the rule tahn the exception.
Concept 5
It is probably a fundamental human trait to prefer a simple explanation to a more involved one, but a sipmple explanation is probably rarely the correct one. Many of the great controversies in science have arisen from this preference for a simple explanation. The serious student of land forms does not progres far in his study of them before he comes to realize that little of the earth’s topography can be explained as the result of the operation of a single geomorphic process or a single geomorphic cycle of development. Usually,most of the topographic details have been produced during the current cycle of erosion, but there may exist within an area remnants of features produced during prior cycles, and, although there are many individual land forms which can be said to be the product of some single geomorphic process, it is a rare thing to find landscape assemblages which can be attributed solely to one geomorphic process, even though commonly we are able to recognize the dominance of one. It may facilitate our interpretation of landscape to group them, as Horberg ( 1952 ) did, in five major categories : ( 1 ) simple, ( 2 ) compound, ( 3 ) monocyclic, ( 4 ) multicyclic, and ( 5 ) exhumed or resurrected landscape.
Concept 6
Concept 7
Concept 8
Concept 9
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