Factors controlling landform development

Factors controlling landform development deals with processes and theories of exogenetic processes.

Order of relief

1. First-order relief - Oceans and continents
2. Second-order relief - Features on oceans and continents as a result of endogenetic processes
3. Third-order relief - Features on oceans and continents and on second-order relief due to exogenetic processes.

Landforms are the products of many processes. Processes create and shape landforms.

Landforms and landscape are influenced by.

1. Processes
   1. Exo - Climatic processes - decide the type and the intensity of the exogenetic process
   2. Endo - Internal heat
2. Rock structure - the presence of folds fault joints slopes, rock composition mineral composition
3. Relief and altitude - decide the effectiveness of the denudation process
4. Time
5. Biota - Man is a deciding factor

The way the factors combine and express themselves have been modelled through theories in geomorphology.

WM Davis

One of the earliest theory in landform development is WM Davis's. He is considered the father of geomorphology. His theory is called "the theory of landform evolution" or "the theory of geomorphic cycles".

Historical approach

Davisian approach was a historical approach with the objective of reconstructing the geological past of the landforms.

The historical approaches by nature will be speculative and based on certain simplifying assumptions. Davis and his historical approach were influenced by three concepts.

1. Hutton's uniformitarianism - It is a principle in any historical study.
2. Darwin's concept of gradual evolution - The concept of gradualism and evolutionary processes challenged the the concept of catastrophic revolutionary changes.
3. Powell's concept of base level

Hutton's uniformitarianism

According to Hutton, "The processes that operate today with their consequences were also the processes that operated earlier and with the same consequences".

In other words, "The present is the key to the past". If we understand the processes that operate today, we can infer about the processes earlier and reason the consequences accruing them.

Charles and Lyell modification

later Charles and lyall modified the concept of uniformitarianism and and suggested that the processes that operate today also operated area but not necessarily with the same intensity or with the same degree of effect

uniformitarianism is also expressed as the processes operate as to there is no vistage of a beginning and no prospect of an end.

The influence of Darwin.

Darwin's gradualism and his concept of evolution was an alternative to catastrophism. Where he asserted things don't happen suddenly. Every event and its consequences is a part of very slow evolutionary processes with changes that operate through predictive and definite stages.

Davis while using darwinian concept of evolution asserted that time is an active factor. So division idea in the historical context is a time-dependent approach.

Davisian idea of 'cycle' as an orderly and sequential set of stages that operates gradually through time was influenced by Darwin.

Powell's concept of base level.

The base level of powell is essentially the sea level. According to Davis, Base level is the tendency and drive of the river to reach the base level that energizes the river that makes it an agent of erosion. The base level therefore is what operates the cycle of erosion of Davis.

Davis focuses on denudation and erosional processes. His model of landform evolution is based on the assumption that the base level is constant and cannot change. Namely, the cycle of erosion operates in conditions of prolonged crustal stability. Any possibility otherwise was considered as abnormal by Davis.

There are different types of base level.

1. Global sea level / Mean sea level (MSL) - for a considerable length of time, the global sea level was considered unfluctuating. We assume a constant and constant climatic condition.
2. Regional sea level - Sea level at countries and regions. In India, it is mean sea level at Chennai
3. Local base level - average levels of water in the lakes into which smaller rivers drain into. The local base level is more amenable to fluctuation than the regional and global base level.
4. Changing base level - may be related to tectonic processes or because of climatic processes.

Davisian model of Landform Evolution / cycle of erosion / Fluvial cycle of erosion

according to Davis landforms are product of structure process and stage.

structure

mineral composition presence of faults types of folds joints and cracks slope analysis relief and elevation.

• relative rock hardness - differential erosion is due to relative rock hardness. Harder rocks higher elevation
• Steeper slopes - can intensify erosion processes
• high and steep elevation - Gorges
• Soft sedimentary rocks - canyon
• Rifting zone - preferable zones occupied by river valleys.

Process.

Stage

The following were the assumptions in Davisian Model

Comments

much of Davis work was from the observations in the US with its humid temperate cool conditions where reverse dominate as the agents of denudation.

he considers these conditions as normal and generalizes it for the whole world. This technique of Davis is deductive reasoning which is scientific but there are always risks of oversimplification and over generalisation.

The following are the stage based changes due to cycle of erosion of the rivers

Davis is implying that every high mountain and elevated feature will eventually become flat featureless plain as it evolves from youth towards senile stage.

The evaluation of davision theory.

1. it is a new concept and first comprehensive model
2. His historical approach is very innovative because he was able to bring in darwinian evolution
3. The concept of stage and time as a factor of davision approach
4. Slow development according to Davis is due to slope decline or down wasting. Where the slopes are seen to be the consequence of their age and stage of evolution.

Limitations

1. this concept of Davis is not as refined as Pencks slope replacement model or king's slope retreat model.
2. Detective reasoning concept is of gross generalisation
3. He does not explain the role of other agents of denudation.
4. He ignored endogenetic processes and only reference to internal process is rapid upliftment for a very short time and there after he assumes prolonged crystal stability namely the base level for Davis is unchanging. He considered to the possibility of other cycles and calls those as interruptions are partial cycles that disrupt the normal cycle of erosion

The base level changes may be positive toward the completion of cycle when the base level gets uplifted or negative when the base level positive change will shorten the cycle and the negative change will prolong the cycle. 

Negative changes in the base level that prolong the cycle mein start the second cycle of erosion. The river which was becoming old again becomes young. This is called river rejuvenation. Young youthful features will now get superimposed on the peneplain. 

Features of river rejuvenation. 
1. Entrenched valleys - incised valleys - valley within a valley.
2. River terraces - may be paired terraces or unpaired terraces. 
3. Meanders.
4. Knick point.
5. Delta. 

PENCK.

landforms are the manifestations of the exogenetic and the endogenetic processes.

Prima Rumph - To begin with, penck assumed there exist a flat featureless plain at or near the base level called prima rumph. He explains that prima rumph maybe tectonic or erosional surface.

prima rumph can go through the following 3 phases / entwicklung. But entwicklung need not follow one another sequentially through time.

Entwicklung

• Aufsteigende (waxing) entwicklung
• Gleimorphige (constant growth)
• Absteigende (waning)

Aufsteigende (waxing) entwicklung.

• landform undergoing very rapid accelerating upliftment
• The rate of erosion is less than the upliftment rate
• Summit erosion and valley deepening occurs
• The rate of summit erosion is less than rate of valley deepening
• Absolute and relative relief increasing at accelerated rate.

Gleimorphige.

• summit erosion = valley deepening hence the relative relief remains constant but there are three sub possibilities
  • Rate of upliftment more than erosion
  • Upliftment equals rate of erosion - absolute or relative relief constant
  • Upliftment it is less than erosion - relative relief constant absolute relief decreases

Absteigende.

• upliftment is less than erosion and almost stops
• Summit erosion is more than valley deepening
• Relative and absolute relief reducing
• The land is flattening in this phase
• The final feature in this phase is a gently sloping relative Lee flat with convexo concave profile - Boschens and Haldenhangs.

End rumph - The end rumph may have loose scattered resistant rocks called as in Inselbergs . Inselberg are not same as monadnocks. Monadnocks are senile stage feature with gentle sloping angles where are inselberg are isolated hills with vertical scarp faces. The process of insulin berg formation with its perpendicular faces can be explained according to pencks model. But it can be better explain by kings models sleep retreat and pediplanation.

graphical representation of pencks model.

Evaluation.

1. time independent model
2. Process theorist
3. Accounts better for process and structures as control
4. Rejects the idea of stage based changes
5. Thanks model of slope development is called slope replacement which was earlier confused to be model of slope retreat. Slope retreat is a concept of l c king and not that of penck.

evaluation should be in the context of comparing with Davis.

Previous years questions

• 1993:
  • Explain the factors causing rejuvenation in the landscape and describe the resultant landforms.
• 1994:
  • Discuss the concept of the cycle of erosion and bring out clearly the difference between the views of Davis and Penck.
• 2002:
  • Provide a critique of the 'geographical cycle' model, propounded by Davis.
• 2005:
  • " Structure is a dominant control factor in the evolution of Landforms" Discuss with suitable examples.
• 2008:
  • Critically examine the concept of the geomorphic cycle and discuss the views of W.M.Davis and W.Penck.
• 2012:
  • Explain how Bosche and Haldenhang lead to the Theory of Slope Replacement.
• 2013:
  • Differences between Normal cycle and Arid cycle of Davis.
  • What is 'Base level'? Explain the types of base level.
• 2016:
  • "In explaining the concept of 'Pediplanation', King combined the ideas of Davis, Penck and Wood with his own." Elaborate.
• 2016:
  • "Geological structure has a dominant control on landforms and is reflected on them."Discuss.
• 2017:
  • "Climate, slope gradient and rock structure influence the avulsion of channels". Explain.
• 2018:
  • "Landscape is a function of structure, process and stage." Critique the statement.