1. Davisian Theory:
The most popular theory of landform development was given by American geomorphologist William Morris Davis. His concept of geographical cycle (or commonly known as cycle of erosion) provided a genetic classification and systematic description of landforms.
According to Davis, geographical cycle is a period of time during which an uplifted landmass undergoes its transformation by the process of land sculpture ending into low featureless plain or peneplain (which Davis called peneplane), Davis’s theory was the outcome of a set of theories and models presented by him during the 1880s and 1890s.
He propounded the model of ‘complete cycle of river life’ in his essay on The Rivers and Valleys of Pennsylvania (1889), and that of ‘geographical cycle’ (1899) and ‘slope evolution’. He, under the concept of ‘complete cycle of river life’, postulated the cycle concept of progressive development of erosional stream valleys, and through the ‘geographical cycle’ described the sequential development of landforms through time.
However, Prof. Savindra Singh says that the general theory of landform development of Davis is not the ‘geographical cycle’ as many of the geomorphologists believe. Davis’s theory may be expressed as: “There are sequential changes in landforms through time (passing through youth, mature and old stages) and these sequential changes are directed towards well defined end product development of peneplain”.
2. Penck’s Theory:
According to German geomorphologist Walther Penck, the characteristics of landforms of a given region are related to the tectonic activity of that region. Contrary to the Davisian concept that “landscape is a function of structure, process and time (stage)”, Penck put forward his view that geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation, where it is assumed that interaction between the two factors, uplift and degradation, is continuous. According to Penck’s view the landforms observed at any given site give expression to the relation between the two factors of uplift and degradation that has been or is in effect, and not to a stage in a progressive sequence.
The main premises of Penckian model of landscape development are the following:
1. The morphological characteristics of any region of the earth’s surface is the result of competition between crustal movement and denudation processes.
2. On the basis of morphological characteristics tectonic movements can be explained and their causal factors ascertained.
3. Development of landscape is not time- dependent.
4. The shape of the hillslope depends on the relative rates of valley incision by rivers and removal of debris from the hill-slope.
5. Three crustal states are witnessed: (a) state of crustal stability with no active displacement; (b) state of initial domed uplift in a limited area followed by widespread unlift; and (c) state of extensive crustal upliftment.
6. Upliftment and erosion are always coexistent.
7. Three states of adjustment between crustal movement and valley deepening are observed: (a) if for a longer time crustal upliftment remains constant, the vertical erosion by the river is such that there is balance between the rate of upliftment and erosion; (b) if the rate of uplift is more than the rate of valley deepening, then the channel gradient continues to increase till the rate of valley deepening matches with the rate of upliftment and the state of equilibrium is attained when both become equal; and (c) if the rate of valley deepening is more than the rate of crustal uplifment, then the channel gradient is lowered to such an extent that the rates of upliftment and erosion become equal and the state of equilibrium is attained.
However, it must be noted that there was certain misunderstanding in the interpretation of Penck’s work as it was published in obscure German language. Penck’a morphological system was severely criticised in the United States in the same way the ‘geographical cycle’ was criticised in Germany. Penck’s concepts of parallel retreat of slope and continued crustal movements were subjected to severe criticism by many geomorphologists and geologists.
However, despite lack of support for Penck’s concept of long continued upliftment and tectonic speculations, his concepts of slope development and weathering processes are of immense geomorphological significance.
3. Gilbert’s Theory:
On the basis of his investigation of landforms and the processes associated with their formation in different parts of the United States, Grove Karl Gilbert formulated a set of principles to explain geomorphic features. The concepts and principles propounded by Gilbert provided the base for the development of the dynamic equilibrium theory involving time- independent development of landforms and it subsequently became the pivot of drastic methodological shift in geomorphology.
According to Gilbert, “the landscape is the result of two competing tendencies i.e. tendency towards variability (when driving force exceeds resisting force) and tendency towards uniformity (when driving force equals resisting force)”. Gilbert says landscapes remain in equilibrium condition, their history is rhythmic punctuated by oscillatory changes and their forms are punctuated by frictional rhythms arising out of the mechanism of driving and resisting forces. The three major components of Gilbert’s geomorphic principles are: the concept of quantification, the concept of time and the concept of equilibrium.
Gilbert’s concept of equilibrium, also known as the principle of least force, envisages that in the final form of any functional system “the sum of the forces acting on the final form equaled zero”. There are two types of forces: driving force and resisting force. He tried to explain the formation of laccoliths resulting from vulcanicity during his field studies by applying the concept of equilibrium.
According to him the formation of laccolith depends on the competence of during force (rise of magma) and resisting force (overlying pressure of superincumbent load). The formation and growth of laccoliths continue so long as the driving force of rising magma is not countered by resisting force of equal magnitude acting downwards. When the driving force is balanced by the resisting force, the growth of laccolith becomes static. A state of equilibrium is achieved and thus the principle of least work becomes operative wherein the sum of driving and resisting forces becomes zero.
4. Theory of L.C. King:
L.C. King’s theory of landform development is based on his studies of landforms in arid, semi-arid and savanna regions of South Africa. He formulated a set of cyclic models (such as landscape cycle, epigene cycle, pediplanation cycle, hillslope cycle, etc.) and asserted that these are practicable in other parts of globe as well. The reference system of King’s model says “there is uniform development of landforms in varying environmental conditions and there is insignificant influence of climatic changes in the development of fluvially originated landforms.
Major landscapes in all the continents have been evolved by rhythmic global tectonic events. There is continuous migration (retreat) of hillslope and such retreat is always in the form of parallel retreat.” For King, the profile of an ideal hillslope consists of all four elements of slope, viz., summit, scarp, debris slope and pediments and such hillslopes develop in all regions and in all climates where there is sufficient relief and fluvial process is the dominant agent of denudation.
Pointing out that the Davisian model of arid cycle of erosion was inadequate to explain all types of landscapes, King, in the 1940s, propounded a new cyclic model of pediplanation (or pediplanation cycle) to explain the unique landscapes that he observed in the arid, semi-arid and savanna parts of Africa. According to King, the African landscape consisted of three basic elements: (a) rock pediments flanking river valleys and having concave slope varying in angle from 1.5° to 7° cut into solid rocks; and (b) scarps having steep slopes bounding the uplands and varying in angle from 15° to 30° and experiencing parallel retreat due to backwasting by weathering and rainwash; (c) steep sided residual hills known as inselbergs (bornhardts) which vary in size and shape. The size of the inselbergs is dependent on the magnitude of erosion and their shape on the nature of underlying structure.
It is worth noting that King’s concept of upliftment and crustal stability is similar to the concept of Davis. The cycle of pediplanation is performed by twin processes of scarp retreat and pedimentation. Each cycle begins with rapid rate of upliftment followed by long period of crustal (tectonic) stability. The cycle of pediplanation begins with the uplift of previously formed pediplains and not of any structural unit. The pediplanation cycle passes through the stages of youth, mature and old as in the Davisian cycle of erosion.
However, there are certain differences between the models of King and Davis. Davis’s peneplain is formed due to down wasting while King’s pediplain is formed due to coalescence and integration of several pediments which are formed due to parallel scarp retreat. Once formed, Davis’s peneplain does not experience further growth until it is reuplifted. When uplifted, new erosional cycle is initiated and the rivers are rejuvenated.
On the other hand, King’s pediplain once formed further grows headward. New scarp is initiated at the far end of the previously formed pediplain which is progressively consumed by the retreat of new scarp and thus second pediplain is formed while the former pediplain experiences decrease in its extent. The process continues and a series of intersecting pediplains are formed which extend headward. Hence, King’s pediplains, so formed, are analogous to Penck’s piedmont treppen.
Critique of King’s Model:
King’s model was subjected to many criticisms:
(a) King’s model was limited to the African experience.
(b) It is doubtful to assert that there is uniform development of landscapes in different environmental conditions.
(c) King’s concept of antique pediplanation remains questionable.
5. Theory of J.T. Hack:
American geomorphologist J.T. Hack made a serious attempt to fill the conceptual vacuum created by the criticism and rejection of Davisian evolutionary model of geographical cycle and Penck’s morphological system. Hack pointed out that multi-level landscape (polycyclic relief) cannot be explained in terms of multiple erosion cycle (Davisian notion), rather these landscapes can be explained in terms of dynamic equilibrium theory.
According to Hack, geomorphic system is an open system and so long as energy remains constant in the geomorphic system, landscape remains in the steady state condition despite the lowering in the landscape by denudational processes. Hack’s model envisages time-independent development of landscape. In other words, “the shape of the landforms reflects the balance between the resistance of the underlying materials to erosion and the erosive energy of the active processes”.
The main assumptions of the Hackian model of landscape development are:
(a) There is balance between denudational processes and rock resistance.
(b) There is uniform rate of downwasting in all components of landscapes.
(c) Differences and characteristics of form are explicable in terms of spatial relations in which geologic patterns are primary consideration.
(d) The denudational processes which operate at present have been carved out of the earth’s surface landscapes.
(e) There is lithologic adjustment to landforms.
6. Theory of Morisawa:
American geomorphologist Marie Morisawa formulated a geomorphic model based on tectonic movements and changes. She analysed the results of goemorphological studies pertaining to erosion and reliefs undertaken by different geomorphologists in different parts of the world and concluded that “there is high rate of erosion on uplifted landmass because potential energy required for erosion increases due to greater height and high potential energy results in high kinetic energy due to increased channel flow velocity which ultimately accelerates erosion”. She said that the rate of denudation and basin reliefs were highly positively correlated and 90 per cent of the total differences in erosion rates in different drainage basins were due to average reliefs of the basins.
The main premises of Morisawa’s tectono- geomorphic model are:
(a) Landforms are the result of inequality of force or inequality of resistance or of both.
(b) The variations in landforms are due to inequality of rates of operation ‘of exogenetic processes acting on different geomaterials of the earth’s surface and inequality of the rates of endogenetic processes.
(c) Nature tends to attain balance or equilibrium between force (of processes) and resistance of geomaterials. However, this balance is not always maintained since the earth is unstable and dynamic. The isostatic feedback also affects the rates of upliftment and erosion, and deposition and subsidence.
(d) The current landforms are the result of difference of ratios of the actions of endogenetic and exogenetic processes.
(e) When uplifted or newly created, a landmass undergoes rapid transformation of its form through exogenetic (denudational) processes. The rate of transformation is dependent on the nature of force and resistance.
(f) Some morphological features can be explained in terms of plate tectonics.
7. Theory of S.A. Schumm:
Schumm’s theory is based on the episodic erosion model. He stated that denudation is not gradual and continuous, rather it is episodic. The geomorphic history of landscape development, according to Schumm, includes numerous periods of rapid erosion (period of instability) and deposition. Period of rapid erosion is followed by long period of deposition. There is repetition of periods of erosion and deposition and thus there is complexity in the evolution and development of landforms.
Schumm says the complexity of landscape can be explained in terms of two geomorphic concepts: (a) concept of geomorphic thresholds, and (b) concept of complex response. Explaining these two concepts Prof. Savindra Singh writes, “The concept of geomorphic thresholds suggests that changes may occur in the fluvial system but these changes are not occasioned by external factors such as isostatic upliftment, but are effected by inherent geomorphic controls of eroding fluvial system (say drainage basin).
For instance, if there is deposition of eroded sediments in a fluvial system, these deposited sediments become unstable at a critical threshold slope i.e. channel slope gradient increases due to sedimentation and a limit (threshold) is attained when no further sediments may be accommodated. Consequently, the channel gradient becomes such (due to deposition) that erosion of deposited sediments begins due to increased channel flow velocity. It is evident that such changes (deposition and erosion) have not been effected by external variables of the fluvial system but have been caused by the internal geomorphic controls.
According to the concept of complex response, when a fluvial system is rejuvenated (say, drainage basin), then the response of the fluvial system to rejuvenation is not just renewed accelerated rate of valley deepening but the response is in the form of attainment of new equilibrium (it may be stated that the equilibrium is disturbed due to rejuvenation) through down cutting, aggradation and renewed erosion. If the effects of external variables of the fluvial system (isostatic upliftment) is combined with geomorphic thresholds and complex response then at least during the initial stage (youth), the geomorphic cycle of erosion cannot be progressive; rather, there would be complex response of events of relative periods of stability separated by periods of episodic erosion. In other words, there is repetition of periods of erosion and erosionless periods (periods of stability), the response (result) of which is that, the fluvial system and the resultant landscape become very complex. The main reason of the resultant complexity of landscape is the fact that if any event occurs in any segment of a river, there is no instantaneous impact of such event on the entire channel length.”
Though some of Schumm’s ideas did not find favour with many geomorphologists, modern studies of thresholds and complex response have suggested to synthesise the Davisian cyclic decay model and the steady state model of Gilbert into an organic vision of landform evolution.