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Davis Theory -William Morris Davis- Geomophology Chapter

William Davis proposed the geographical cycle model to describe the development of landforms over time. The model outlines three stages - youth, maturity, and old age - through which uplifted land passes as erosion shapes the landscape. In the youth stage, steep valleys form through rapid vertical erosion. In maturity, vertical erosion declines as valleys widen, and by old age erosion is primarily lateral as the landscape develops into a low-relief peneplain. Davis' model was influential as a general theory of landform evolution driven by erosion following crustal uplift.

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William Morris Davis, an American geomorphologist, was the first geomorphologist to present a general theory of landform development. Infact, his theory is the outcome of a set of theories and models presented by him time to time. (1) complete cycle of river life in 1889- cyclic concept of progressive development of erosional stream valleys (2) Geographical cycle in 1899- sequential development of landforms through time. According to him: “there are sequential changes in landforms through time (passing through youth, mature and old stages) and these sequential changes are directed towards well defined and end product-development of peneplain.” 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 as follows:
• 1. It is a period of time during which an uplifted landmass undergoes its transformation by the process of landsculpture ending into low featureless plain or peneplain (Davis calledpeneplane) . • 2. Three factors-structure, process and time play important roles in the origin and development of landforms of a particular place. –Landscape is a function of structure Lithology-rock types, folding, faulting joints etc), process ( agents of denudation including both weathering and erosion-running water in the case of geographical cycle). and time (change of landforms). • Assumptions: • Landforms are the evolved products of the interactions of endogenetic (diastrpohic) forces originating from within the earth and the external forces originationg from the atmosphere (denudation)
The evolution of landform takes place in an orderly manner in such a way that a systematic sequence of landforms is developed through time in response to an environmental change. Streams erode their valleys rapidly downward until the graded condition is acieved. There is a short period rapid rate of upliftment in landmass. It may be pointed out that Davis also described slower rates of upliftment if so desired. Erosion does not start untill the upliftment is complete. In other words, upliftment and erosion do not go hand in hand. Graph: The cycle of erosion begins with the upliftment of landmass.
There is a rapid rate of short-period upliftment of landmass of homogenous structure. This phase is not included in the cyclic time as this phase, in fact, the preparatory stage of the cycle of erosion. The model of geographical cycle wherein UC (upper curve) and LC (lower curve) denote the hill-tops or crests of water divides (absolute relief from msl) and valley floors (lowest relief from msl) respective. The horizontal line denotes time whereas vertical axis depicts altitude from sea-level. AC represents maximum absolute relief whereas BC denotes initials average relief. Initial relief is defined as difference between UC (summits of water divides) and LC (valley floors) of a landmass. Relief is defined as the difference between the highest and the lowest points of a landmass. ADG is base level of erosion which represents sea-level. No river can erode its valley beyond base level (below sl).
Thus, base level represents the limit of maximum vertical erosion (valley deepening) by the rivers. The upliftment of the landmass stops after point C as the phase of upliftment is complete. Now erosion starts and the whole cycle passes through the following three stages. 1. Youth 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 tributeries are are engaged in headward erosion due to which they extend their length. 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.
Long profile of a typical stream.
C A B D E F G H Base Level UC LC Old ageYouth Maturity AC-absolute relief BC- Initial average relief (UC-LC)
• 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) increases transporting capacity of the rivers allows them to carry big boulders of high calibre (more angular boulder) 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 interstream areas) remain almost parallel to the horizontal axis (AD) 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) is attained. • In nutshell: • CF is parallel to AD • UC is not affected by erosion • LC falls rapidly • Relief continues to increase • Valleys are V shape and convex • Valley form is gorge or canyon • Long profiles of the rivers are characterized by rapids and waterfalls which gradually diminish with march of time and these practically disappear by the end of late youth. The main river is graded.
• Graded river • A stable stream where slope and channel characteristics are such that the sediment discharge is enough to provide transport for the load, but where there is no energy for erosion. A graded river is in a state of equilibrium. If any of the factors controlling discharge change, the river would respond in such a way as to re- establish grade. A river may establish grade over one section of its course; this is a graded reach. •
• Mature stage • The early mature stage is marked by lateral erosion and well integrated drainage network. • The graded conditions spread over larger areas 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 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 the youthful stage into wide valleys with uniform valley sides.
C A B D E F G H Base Level UC LC Old ageYouth Maturity AC-absolute relief BC- Initial average relief (UC-LC)
• The marked reduction of in valley deepening is because of substantial decrease in channel gradients, flow velocity and transporting capacity of the rivers. • 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. Water divided are reduced in dimension by both, downwasting and backwasting. • Upper curve falls more rapidly, thereby, 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 markedly 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 divided crests, broad, open and gently sloping valleys having extensive floodplains, well developed meanders, residual convexo-concave monadnocks and extensive undulating plain of extremely low relief. Thus, the entire landscape is transformed into peneplain.
• Evaluation of the model: Positive aspects of Davis model • Davis model of geographical cycle is highly simple and applicable. • He presented his model in a very lucid, compelling and disarming style using very simple but expressive language. • Davis based his model on detailed and careful field observations. • Davis, model came as a general theory of landform development after Hutton’s “cyclic nature of the earth history” (present is the key to the past’ and no vestige of a beginning,—no prospect of an end,“) • 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 engineer’s concept of “profile of equilibrium” in his model.
Graded profile is the term given to the long profile of a river in equilibrium, when inputs and outputs are balanced. A graded profile shows a (gradient that decreases progressively down valley;) graded to sea level.
• His model is capable of both predictions and historical interpretation of landform evolution. • Negative aspects: • 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 slow and long continued process. • 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-(1) to make the model simple (2) erosion is insignificant during the phase of upliftment.
• 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. • 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 experienced progressive and sequential changes through time. He pleaded for deletion of time (stage) from Davis’ function 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 “.
• A.N. Strahler, J.T. Hack and Chorelyand several others have rejected the Davisian concept of ‘historical evolution’ of landforms. They have forwarded the dynamic equilibrium theory (non-cyclic) for the explanation of landform development. • Though Davis has attempted to include structure, process and time in his model but he overemphasized time. His interpretation of geomorphic processes was entirely based on empirical observation rather on field instrumentation and measurement. Though Davis described the structural control on landforms but he failed to build any model of lithological adjustment of landforms. • Davis attempted to explain the concept of grade in terms of ability to work (erosion and deposition) and the work that needs to be done.
• It is evident from the essays of W.M.Davis that the initial stage of landform development (in terms of cycle of erosion) the available energy is more than needed to transport the eroded sediment. Thus, the river spends additional available energy to erode its valley. As the river valley is deepened, the sediment supply (the work needed to be done increases) for transportation increases but available energy decreases. Ultimately, required energy and available energy become equal and a condition of eqilibrium is attained. But the critics maintain that the concept of balance between available energy and the work to be done has not been properly explained by Davis.
• It is apparent from the writings of Davis that ‘the work to be done’ refers to transportation of debris by the rivers and energy is spent in two ways e.g. in transportation of debris and in valley deepening. Such division of expenditure of energy is not justified. Thus, there are two shortcomings-(1) erosion in itself depends on the mobility of sediments and erosion is never effective in the absence of sediments, (2) such condition when the whole energy is spent in transporting the sediments and erosion becomes totally absent is particularly not possible.