Definition and Fundamental Conecpts
Geomorphology, if we go by the Greek roots of the term, would mean ‘a discourse on forms of the earth's surface'. Initially, the subject was concerned with unravelling the history of landform development, but now it is also concerned with understanding the processes which create landforms and how these processes operate. In many cases, geomorphologists have tried to model these processes and, of late, some have taken into consideration the effect of human agency on such processes. Basically, geomorphology is the study of the nature and history of landforms and the processes which create them.
Geomorphology is often identified with geology, or considered a branch of geology. The systematic study of landforms, indeed, requires some fundamental knowledge of geology as the genesis and development of all types of landforms is dependent upon the materials of the earth's crust and the forces that emanate from within the earth.
Some fundamental concepts are enumerated by W.D. Thornbury which comes into use in the interpretation of landscapes. These are:
1. the same physical processes and laws that operate today operated throughout geologic time, although not necessarily always with the
same intensity as now.
same intensity as now.
This is the great underlying principle of modern geology and is known as the principle of uniformitarianism. It was first enunciated by Hutton in 1785, restated by Playfair in 1802, and popularised by Lyell. 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 were times when volcanism was more important than now. Numerous other examples 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 streams did not cut valleys in the past as they do now or that the more numerous and more extensive valley glaciers of the Pleistocene behaved any differently from existing glaciers.
2. Geologic structure is a dominant control factor in the evolution of land forms and is reflected in them
The term structure here is not 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 landforms are carved, differ from one' another in their physical and chemical attributes. 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 relationships 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.
3. To a large degree the earth's surface prossesses relief because the geomorphic processes operate at differential rates.
The main reason why gradation of the earth's surface proceeds differentially is that the rocks of the earth's crust vary in their lithology and structure and hence offer varying degrees of resistance to the gradational processes. Some of these variations are very notable while others are very minute, but none is so slight but that it affects, to some degree, the rate at which rocks waste. Except for regions of very recent diastrophism, it is usually safe to assume that areas which are topographically high are underlain by "hard" rocks and those which are low by "weak" rocks, relatively speaking.) Differences in rock composition and structure are reflected not only in regional geomorphic variability but in the local topography as well. Much of the minor topographic details, or what we may call the microtopography, is related to rock variations often too minute in nature to be readily detectable.
4. Geomorphicprocesses leave their distinctive imprint upon landforms, and each geomorphic process develops its own charnrteristic is assem6Zaje-of landLrms.
Just as species of plants and animals have their diagnostic characteristics, so (landforms 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 processes do produce distinctive land features makes possible a genetic classification of landforms. Landforms are not haphazardly developed with respect to one another but certain forms may be expected to be associated with each other. 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 the other forms that may be expected to be present because of their genetic relationships with one another.
5. As the different erosional agents act upon the earth's sur ace there is produced an orderly sequence of Zan dd forms.
Under varying conditions of geology, structure and climate, landform characteristics may vary greatly even though the geomorphic processes may have been acting for comparable periods of time. 0imilarity in the topographic details of two regions would be expectable only if the initial surface, lithology, structure, climate and diastrophic conditions were comparable. Although passage of time is implied in the concept of the geomorphic cycle, it is in a relative rather than an absolute sense. There is no implication that two areas that are in comparable stages of development have required the same length of time for their attainment. Much confusion has arisen from the fact that numerous geologists have defined a geomorphic cycle as the period of time required for reduction of an area to base level rather than as the changes through which a land mass passes as it is reduced toward the base level.
6. Complexity of geomorphic evolution is more than simplicity.
The serious student o landforms does not progress far in his study of them before he comes to realise 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 duringprior cycles, and, although there are many individual landforms 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 recognise the dominance of one.
7. Little of the earth's topography is older than Tertiary and most of it no older than Pleistocene.
Older discussions on the age of topographic features refer to erosion surfaces dating back to the Cretaceous or even as far back as the Precambrian. We have gradually come to a realisation that topographic features so ancient are rare, and, if they do exist, are more likely exhumed forms than those which have been exposed to degradation through vast periods of geologic time.
It is, of course, true that many geologic structures are very old. It has been previously stated that geologic structures are in general much older than the topographic features developed upon them. The only notable exceptions are to be found in areas of late-Pleistocene and Recent diastrophism. The Cincinnati arch and the Nashville dome began to form as far back as the Ordovician but none of the topography developed on them today goes back of the Tertiary; the Himalayas were probably first folded in the Cretaceous and later in the Eocene and Miocene but their present elevation was not attained until the Pliocene and most of the topographic detail is Pleistocene or later in age; the structural features which characterise the Rocky Mountains were produced largely by the Laramide revolution, which probably culminated at the close of the Cretaceous, but little of the topography in this area dates back of the Pliocene and the present canyons and details of relief are of Pleistocene or Recent age.
8. Proper interpretation of present-da land-scapes is impossiblywit aut_.a -full__appreciation__ of the manifold influences of thegeologic and c amatic changes during the Pleistocene.
Correlative with the realisation of the geologic recency of most of the world's topography is the recognition that the geologic and climatic changes during the Pleistocene have had far-reaching effects upon present-day topography. Glacial outwash and wind-blown materials of glacial origin extended into areas not glaci-ated, and the climatic effects were probably worldwide in extent. Certainly, in the middle latitudes the climatic effects were profound. There is indisputable evidence that many regions that are today and or semi-arid had humid climates during the glacial ages. Freshwater lakes existed in many areas which today have interior drainage. We also know that many regions now temperate experienced during the glacial ages temperatures that are found now in the subarctic portions of North America and Eurasia, where there exists permanently frozen ground or what has come to be called permafrost conditions. Stream regimens were affected by the climatic changes, and we find evidence of alternation of periods of aggradation and downcutting of valleys.
Although glaciation was probably the most significant event of the Pleistocene, we should not lose sight of the fact that in many areas the diastrophism which started during the Pliocene continued into the Pleistocene and even into the Recent.
9. An appreciation of world climates is necessary to a proper understanding o the varying importance of the different geomorphic processes.
Climate variations may affect the operation of geomorphic processes either indirectly or directly. The indirect influences are largely related to how climate affects the amount, kind and distribution of the vegetal cover. The direct controls are such obvious ones as the amount and kind of precipitation, its intensity, the relation between precipitation and evaporation, daily range of temperature, whether and how frequently the temperature falls below freezing, depth of frost penetration, and wind velocities and directions. There are, however, other climatic 'factors whose effects are less obvious, like how long the ground is frozen, exceptionally heavy rain falls and their frequency, seasons of maximum rainfall, frequency of freeze and thaw days, differences 'in climatic conditions as related to slopes facing the sun and those not so exposed, the differences between conditions on the windward and leeward sides of topographic features transverse to the moisture-bearing winds, and the rapid changes in climatic conditions with increase in altitude.
10. Geomorphology, although concerned primarily with present-am andscapes attains its maximum usefulness by historical extension.
Geomorphology concerns itself primarily with the origins of the present landscape but in most landscapes there are pi event forms that date back to previous geologic epochs or periods. A geomorphologists is thus forced to adopt a historical approach if he is to interpret properly the geomorphic history of a region. The historical nature of geomorphology was recognized by Bryan (1941) when he stated: "If landforms were solely the result of processes now current, there would be no excuse for the separation of the study of landforms as a field of effort distinct from Dynamic Geology. The essential and critical difference is the recognition of landforms or the remnants of landforms produced by processes no longer in action. Thus, in its essence and in its methodology, physiography (geomorphology) is historical. Thereby, it is a part of Historical Geology, although the approach is by a method quite different from that commonly used."