DAVIES,PENCK & KING

Geographical Cycle of Davies

William Morris Davies was an american geographer who gave first general theory on landform development..Davis' most influential scientific contribution was the "geographical cycle",  which he defined in an article,’The Rivers and Valleys of Pennsylvania,’ published at the end of 19th century.

According to him, uplifted landmass undergoes sequential development or erosion till base level in various stages.This sequential development referred as cycle of erosion.

FUNDAMENTAL PRINCIPLES

1.Cyclic nature of landform evolution.

2 Uniformitarianism:The same physical processes and laws that operate today, operated throughout geologic time, although not necessarily always with the same intensity as now

BASIC POSTULATES

1. Upliftment takes place on featureless plain which he modified 10 yrs later to accept it can occur from geosyncline.
2. Upliftment on geological timescale is sudden.In later works, he accepted it to be episodic.
3. Endogenetic and exogenetic forces act in isolation.Thus, erosion starts only when upliftment stops.
4. Vertical erosion takes place until base level is reached.
5. End product is a peneplain which is a featureless plain with residual hill.
6. DAVIES TRIO: He suggests landform development depends primarily upon three factors.These are referred as Davies Trio:

1. Structure:Structure refers to physical,chemical and geological properties of rock which determine rate of erosion
2. Process:Process refers to geomorphic process of weathering and gradation(aggradation + degradation) carried out by various agents like wind,air,ground water,sea waves,etc

                        Davies studied mostly temperate latitudes where  river are prominent agent of erosion, thus fluvial cycle has been referred as normal cycle of erosion by him

3. .Stage: Geomorphic process and nature of landforms populating a landscape depends on the existing stage classified as youth,mature and old.

THE GRAPHICAL MODEL

Initially, upliftment of mountains occurs.After it stops, erosion starts and continues in following stages:

Youth Stage

• No submittal erosion occurs thus, absolute relief remains constant.
• Valley deepening occurs due downward cutting by streams which increases relative relief.Valleys are steep with convex surface.
• Headward erosion result in development and channel lengthening of various streams.
• Presence of gorges,steep v shaped valleys,waterfalls,rapids,potholes,etc.
• At the end of this stage, maximum relative relief is reached.

Mature Stage

• Valley deepening decelerates and valley widening becomes prominent.
• V-shaped valley changes to U-shaped.
• Erosion of summits starts resulting in decrease of absolute relief.Thus relative relief decreases too.
• Velocity of streams decreases due to increased sediments.Meandering starts.

Old stage

• Valley deepening stops while widening continues.
• Summit gets eroded rapidly.Thus, absolute relief starts converging into relative relief curve.
• Characterized by formation of distributaries,delta, intense meandering, oxbow lake, sand bars, etc.
• Stage ends in featureless plains with rounded hills called monadnocks. End product is also referred as Peneplain.

CRITICISM

1. Upliftment instead of sudden, is a very long process
2. Endogenetic and exogenetic forces don’t act in isolation.
3. Too much emphasis on stage, role of structure and process less discussed.
4. L.C.King objected that fluvial cycle is normal cycle as ⅔ of landscape is either arid,semi arid or Savanna type.
5. Cyclicity criticised by J.T.Hack and Strahler in there non-cyclic models.
6. Landform development is polycyclic.i.e before a cycle completes another cycle starts and gets superimposed.
7. Role Isostatic adjustments ignored.

Penck’s Model of cycle of erosion

Penck provided his theory in his work  ‘Morphological analysis and its perspective’.This theory was a reaction to Davies theory on cycle of erosion. Contrary, to Davies he emphasized on role of endogenetic forces in landform development and also, replaced stage with a time-independent concept of Phase.

BASIC POSTULATES

1. Upliftment starts on a featureless plain dotted with residual hardrocks, similar to peneplain of Davies.Penck called it Primarumf.
2. Upliftment is very long process and too protracted in time.
3. Endogenetic and exogenetic works in tandem.Thus, upliftment and erosion goes side by side.
4. Landforms are the expression of ratio of work done by both endogenetic and exogenetic forces.
5. Landform development takes place in Phases rather stages.
6. End product of landform development is called Endrum

GRAPHICAL MODEL

1. Waxing phase

• Upliftment is greater than summit erosion.Thus, absolute relief increases.
• Valley deepening is faster than upliftment of valley.Thus,relative relief increases.
• Mountain slopes are undergoing parallel retreat at various section forming treppen.
• Various channels are developed due to channel lengthening by headward erosion.

2. Phase of uniform development

• Summit erosion and valley deepening rate almost similar.Thus, absolute relief almost constant.
• Valley deepening and valley upliftment also almost similar. Thus, valley height and relative relief almost similar.
• This phase is further divided in 3 sub-phase A,B,C based on developments in absolute relief. In A, absolute relief rises with a decelerating phase. In sub-phase B, it becomes constant and in C, it absolute relief starts to fall slightly.

3. Waning Phase

• Upliftment stops completely.Summit erosion is fast.Absolute relief decline rapidly.
• Relative relief also decline rapidly as valley deepening has stopped.
• Lateral erosion results in valley widening.

4 Endrumf

• Landscape development ends with a featureless low plain dotted by inselberg.It is called endrumf.

L.C.King theory

His theory on geomorphic cycle is based on landscapes of semiarid and arid.He has criticised many elements of Davies theory, though there are some similarities too.

Davies considered fluvial cycle as normal cycle which was severely criticized by King.According to him ⅔ of landscapes in world are either arid,semi-arid or savanna type.Thus, arid cycle should be the normal cycle.

BASIC POSTULATES

• Upliftment of mountains are sudden process.
• Erosion process starts once upliftment stops.
• Landforms are in equilibrium with climate and thus it does not affect its cyclicity.
• Slopes of mountains are constantly retreating or apparently shifting which is called parallel retreat.
• Mountain slope can be divided into broadly 4 sections.Slope in each sections are changing through different mechanism as explained in the figure.

• Landscape development is cyclic process and passes through stages like an organic life - youth,mature and Old.

GRAPHICAL MODEL

Upliftment of mountains is a sudden process.After upliftment stops,cycle of erosion starts which follows following stages:

1.Youth Stage:

• Inselbergs rounded summit erodes through soil creep mechanism.
• Parallel face or scarp face below the summit section goes parallel retreat.
• At the end of this stage, inselbergs are transformed into rounded hillocks calle Koppes.
• Pedimentation does not occur in this stage.
• Only valley deepening occurs and no lateral erosion takes place.

2.Mature Stage:

• Valley deepening stops.Lateral erosions results in valley widening.
• Pedimentation starts where rectilinear slope below parallel scarp section develops.
• Pedimentation include cavitation at the foot and sheet erosion by seasonal streams.
• Parallel retreat continues and they are combinedly called pediplanation.
• At the end of this stage, Koppes are transformed in debris rocks.

3.Old Stage

• Pedimentation continues at large scale and valley sides develop vast pediments.
• Extensive pediments of valley sides starts to coalesce together to form extensive area of pediments.
• Sediments in valley are deposited in fan shape known as Bajada.
• In the mid valley, salty marshes or lakes are formed which are known as playa.
• Thus, in the end of cycle the landscape formed is referred as pedeplain characterized by rock debris, extensive pediments,bajada and playa.