In this article we will discuss about the Davisian model of geographical cycle of erosion.
William Morris Davis, an American geomorphologist, was the first geomorphologist to present a general theory of landform development.
In fact, his theory is the outcome of a set of theories and models presented by him from time to time e.g.:
(i) ‘complete cycle of river life’, propounded in his essay on. ‘The Rivers and Valleys of Pennsylvania’ in 1889,
(ii) ‘geographical cycle’ in 1899,
(iii) ‘slope evolution’ etc.
He postulated the cyclic concept of progressive development of erosional stream valleys under the concept of ‘complete cycle of river-life’, while through ‘geographical cycle’ he described the sequential development of landforms through time.
The general theory of landform development of Davis is not the ‘geographical cycle’ as many of the geomorphologists believe.
His theory may be expressed as follows:
‘There are sequential changes in landforms through time (passing through youth, mature and old stages) and these sequential changes are directed towards a well-defined end product-development of peneplain.’
The basic goal of Davisian model of geographical cycle and general theory of landform development was to provide basis for a systematic description and genetic classification of landforms. The reference system of Davisian general theory of landform development is ‘that landforms change in an orderly manner as processes operate through time such that under uniform external environmental conditions an orderly sequence of landform develops’.
Various models were developed on the basis of this reference system e.g., normal cycle of erosion, arid cycle of erosion, glacial cycle of erosion, marine cycle of erosion etc. Thus, ‘geographical cycle’ is one of the several possible models based on Davis’ reference system of landform development.
Davis postulated his concept of ‘geographical cycle’ popularly known as ‘cycle of erosion’ in 1899 to present a genetic classification and systematic description of landforms.
His ‘geographical cycle’ has been defined in the following manner:
‘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 (Davis called peneplane).”
According to Davis three factors viz. structure, process and time play important roles in the origin and development of landforms of a particular place.
These three factors are called as ‘Trio of Davis’ and his concept is expressed as follows:
“Landscape is a function of structure, process and time” (also called as stages by Davis’ followers).
Structure means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Time was not only used in temporal context by Davis but it was also used as a process itself leading to an inevitable progression of change of landforms. Process means the agents of denudation including both, weathering and erosion (running water in the case of geographical cycle).
The basic premises of Davisian model of ‘geographical cycle’ included the following as- sumptions made by Davis:
(1) Landforms are the evolved products of the interactions of endogenetic (diastrophic) forces originating from within the earth and the external orexogenetic forces originating from the atmosphere (denudational processes, agents of weathering and erosion-rivers, wind, groundwater, sea waves, glaciers and periglacial processes).
(2) The evolution of landforms takes place in an orerly manner in such a way that a systematic sequence of landforms is developed through time in response to an environmental change.
(3) Streams erode their valleys rapidly downward until the graded condition is achieved.
(4) There is a short-period rapid rate of upliftment in land mass. It may be pointed out that Davis also described slower rates of upliftment if so desired.
(5) Erosion does not start until the upliftment is complete. In other words, upliftment and erosion do not go hand in hand. This assumption of Davis became the focal point of severe attacks by the critics of the cyclic concept.
Davis has described his model of geographical cycle through a graph below (fig. 16.1):
The cycle of erosion begins with the upliftment of landmass. There is a rapid rate of short-period upliftment of landmass of homogeneous structure. This phase of upliftment is not included in the cyclic time as this phase is, in fact, the preparatory stage of the cycle of erosion.
Fig. 16.1 represents the model of geographical cycle wherein UC (upper curve) and LC (lower curve) denote the hill-tops or crests of water divides (absolute reliefs from mean sea level) and valley floors (lowest reliefs from mean sea level) respectively.
The horizontal line denotes time whereas vertical axis depicts altitude from sea level. AC represents maximum absolute relief whereas BC denotes initial average relief. Initial relief is defined as difference between upper curve (summits of water divides) and lower curve (valley floors) of a landmass. In other words, relief is defined as the difference between the highest and the lowest points of a landmass. ADG line denotes base level which represents sea level. No river can erode its valley beyond base level (below sea level).
Thus, base level represents the limit of maximum vertical erosion (valley deepening) by the rivers. The upliftment of the landmass stops after point C (fig. 16.1) as the phase of upliftment is complete.
Now erosion starts and the whole cycle passes through the following three stages:
(1) Youthful stage:
Erosion starts after the completion of the upliftment of the landmass. The top- surfaces or the summits of the water divides are not affected by erosion because the rivers are small and widely spaced. Small rivers and short tributaries are engaged in head-ward erosion due to which they extend their lengths.
The process is called stream lengthening (increase in the lengths of the rivers). Because of steep slope and steep channel gradient rivers actively deepen their valleys through vertical erosion aided by pothole drilling and thus there is gradual increase in the depth of river valleys. This process is called valley deepening. The valleys become deep and narrow characterized by steep valley side slopes of convex plan.
The youthful stage is characterized by rapid rate of vertical erosion and valley deepening because:
(i) The channel gradient is very steep,
(ii) Steep channel gradient increases the velocity and kinetic energy of the river flow,
(iii) Increased channel gradient and flow velocity increases the transporting capacity of the rivers,
(iv) Increased transporting capacity of the rivers allow them to carry big boulders of high calibre (more angular boulders) which help in valley incision (valley deepening through vertical erosion) through pothole drilling.
The lower curve (LC valley floor) falls rapidly because of valley deepening but the upper curve (UC summits of water divides or interestream areas) remain almost parallel to the horizontal axis (AD, in fig. 16.1) because the summits or upper parts of the landmass are not affected by erosion. Thus, relative relief continues to increase till the end of youthful stage when ultimate maximum relief (EF, in fig. 16.1) is attained.
In nutshell, the youthful stage is characterized by the following characteristic features:
(i) Absolute height remains constant (CF is parallel to the horizontal axis) because of insignificant lateral erosion.
(ii) Upper curve (UC) representing summits of water divides is not affected by erosion.
(iii) Lower curve (LC) falls rapidly because of rapid rate of vally-deepening through vertical erosion.
(iv) Relief (relative) continues to increase.
(v) Valleys are of V shape characterized by convex valley side slopes.
(vi) Overall valley form is gorge or canyon.
(vii) Long profiles of the rivers are characterized by rapids and water falls which gradually diminish with march of time and these practically disappear by the end of late youth. The main river is graded.
(2) Mature stage:
The early mature stage is heralded by marked lateral erosion and well integrated drainage network. The graded conditions spread over larger area and most of the tributaries are graded to base level of erosion. Vertical erosion or valley deepening is remarkably reduced. The summits of water divides are also eroded and hence there is marked fall in upper curve (UC) i.e., there is marked lowering of absolute relief.
Thus, absolute relief and relative relief, both decrease. The lateral erosion leads to valley widening which transforms the V-shaped valleys of youthful stage into wide valleys with uniform or rectilinear valleys sides. The marked reduction in valley deepening (vertical erosion or valley incision) is because of substantial decrease in channel gradient, flow velocity and transporting capacity of the rivers.
(3) Old Stage:
Old stage is characterized by almost total absence of valley incision but lateral erosion and valley widening is still active process. Water divides are more rapidly eroded. In fact, water divides are reduced in dimension by both, down-wasting and back-wasting. Thus, upper curve falls more rapidly, meaning thereby there is rapid rate of decrease in absolute height. Relative or available relief also decreases sharply because of active lateral erosion but no vertical erosion. Near absence of valley deepening is due to extremely low channel gradient and remarkably reduced kinetic energy and maximum entropy.
The valleys become almost flat with concave valley side slopes. The entire landscape is dominated by graded valley-sides and divide crests, broad, open and gently sloping valleys having extensive flood plains, well developed meanders, residual convexo-concave monadnocks and extensive undulating plain of extremely low relief. Thus, the entire landscape is transformed into peneplain. As revealed by fig. 16.1 the duration of old stage is many times as long as youth and maturity combined together.
Evaluation of the Davisian Model of Geographical Cycle:
Davisian model of geographical cycle received world-wide recognition and the geomorphologists readily applied his model in their geomorphological investigations. The academic intoxication of Davis’ model of cycle of erosion continued from its inception in 1899 to 1950 when the model had to face serious challenges though his model was being criticised from the very beginning of its postulation.
S. Judson (1975) while commenting on Davis’ geographical cycle remarked, “his grasp of time, space and change; his command of detail and his ability to order his information and frame his arguments remind us again that we are in the presence of a giant.” C.G. Higgins (1975) admitted that “Davis system came to dominate both teaching and research in the descriptive and genetic-historical aspects of geomorphology. Its continued validity is attested in part by continuing objections to it by recent critics such as R.C. Flemal (1971) and C.R. Twidale (1975) that such an obviously flawed doctrine could have enjoyed such prolonged popularity among large segment of the geomorphic community suggests that there must be compelling reasons for its appeal”.
Positive Aspects of Davis’ Model:
(1) Davis’ model of geographical cycle was highly simple and applicable.
(2) He presented his model in a very lucid, compelling and disarming style using very simple but expressive language. Commenting on the language of Davis used in his model Bryan remarked, “Davis rhetorical style is just admired and several generations of readers became, slightly bemused by long though mild intoxication of the limpid prose of Davis remarkable essay.”
(3) Davis based his model on detailed and careful field observations.
(4) Davis’ model came as a general theory of landform development after a long gap after Hutton’s cyclic nature of the earth history.
(5) This model synthesized the current geological thoughts. In other words, Davis incorporated the concept of ‘base level’ and genetic classification of river valleys, the concept of ‘graded streams’ of G.K. Gilbert and French engineers’ concept of ‘profile of equilibrium’ in his model.
(6) His model is capable of both predictions and historical interpretation of landform evolution.
Negative Aspects of Davis Model:
(1) Davis concept of upliftment is not acceptable. He has described rapid rate of upliftment of short duration but as evidenced by plate tectonics upliftment is exceedingly a show and long continued process.
(2) Davis’ concept of relationship between upliftment and erosion is erroneous. According to him no erosion can start unless upliftment is complete. Can erosion wait for the completion of upliftment? It is a natural process that as the land rises, erosion begins. Davis has answered this question.
He admitted that he deliberately excluded erosion from the phase of upliftment because of two reasons:
(i) To make the model simple, and
(ii) Erosion is insignificant during the phase of upliftment.
(3) The Davisian model requires a long period of crustal stability for the completion of cycle of erosion but such eventless long period is tectonically not possible as is evidenced by plate tectonics according to which plates are always in motion and the crust is very often affected by tectonic events. Davis has also offered explanation to this objection. According to him if crustal stability for desired period is not possible, the cycle of erosion is interrupted and fresh cycle of erosion may start.
(4) Walther Penck objected to over emphasis of time in Davis’ model. In fact, Davisian model envisages ‘time-dependent series’ of landform development whereas Penck pleaded for time- independent series’ of landforms. According to Penck landforms do not experience progressive and sequential changes through time. He, thus, pleaded for deletion of ‘time’ (stage) from Davis’ ‘trio’ of ‘structure, process and time’. According to Penck “geomorphic forms are expressions of the phase and rate of upliftment in relation to the rate of degradation”.
(5) A.N. Strahler, J.T. Hack and R.J. Chorley and several others have rejected the Davisian concept of ‘historical evolution’ of landforms. They have forwarded the dynamic equilibrium theory for the explanation of landform development.
It may be pointed out that non- cyclic concept of ‘dynamic equilibrium’ as valid substitute of Davis’ cyclic concept of landform development and other so called ‘open system’ and non-cyclic models of landform development could not arouse any enthusiasm among the modern geomorphologists.
It may be concluded in the words of Charles Higgins (1975) that “If the desire for a cyclic, time- dependent model stems from an unacknowledged fundamental postulate that the history of the earth is itself cyclic, then no non-cyclic theory of landscape development can win with general acceptance until this postulate is unearthed, examined, and possibly rejected.”