Penck's Model of Cycle of Erosion

This model was brought by German Geographer Walther Penck in response to W. M. Davis' Model of Geographical Cycle, which was based on Geomorphological analyses. 

Penck heavily opposed the Davis' idea of time dependent cycle of erosion. In which Time was over emphasised and even declared it as one of the three functions of land form development. 

Penck on the other hand described landform development depends on the ratio  of upliftment  intensity and the rate of erosion. 

he said that land form development is completely independent of time scale and suggested to remove 'time' from the 'Trio of Davis'.

Also Read : Ocean Current - It's Origin and Effects

The Theory

Penck's Model of Cycle of Erosion(Graph)

He argued that rate of upliftment initially remains very slow and landform development starts with existing feature less 'Primarumpf' thereafter upliftment is accelerated and ultimately it stops after passing through the intermediate phases of uniform and decelerating development.

According to Penck the landform development go through three phase process:-

1. Aufsteigende Entwickelung (Phase of Accelerated rate of Landform Development)

This phase starts with featureless Primarumpf which started to develop in different land form. Initially the upliftment is very slow but after sometime accelerated rate of upliftment take place. 

Accelerated rate of upliftment led to rise in absolute relief. The relative relief also increases due to increase in channel gradient which increases the Kinetic Energy of the streams which results into increase in carrying capacity of streams which in turn causes valley deepening but the rate of upliftment far exceeds the rate of valley deepening. 

The radius of convexity of slopes is reduced with passage of time due to parallel retreat of the steeper slope segments.

With the passage of time and more accelerated rate of upliftment and erosion the 'Primarumpf' is surrounded by a series of benches called as'piedmont treppen'. Each of these develops as 'piedmont flat' also called as 'piedmontflache' (in German) on the slowly rising margins of the dome.

2. Gleichformige Entwickelung 

This phase is called As the phase of uniform development. It is divided into  three sub-phases on the basis of rate of upliftment and degradation:- 

Phase (A)  

In this phase accelerated rate of upliftment still continued. The rate of upliftment is still more than the rate of erosion hence both Absolute as well as relative relief continued to increase. 

Due to high rate of upliftment than the rate of erosion hence both Absolute as well as relative relief continued to increase but at relatively lower rate than in the Aufsteigende Entwickelung. 

This Sub-phase is called uniform development phase because of uniform rate of valley deepening and lowering of divide of summits.

Phase (B) 

In this Sub-phase both the rate of upliftment as well as the rate of erosion both are almost equal which leads uniform development of both absolute as well as relative relief hence it is called as uniform development phase. 

The slopes of valley sides are still straight a's in the previous Sub-phase because of parallel retreat.

Phase (C)

On this phase the rate of erosion became more than the rate of upliftment because upliftment stopped completely . Stopping of upliftment lead to decrease in absolute relief because erosion however slowed but still continues .

Relative relief remains constant because the rate of lowering of the divide summits equals the rate of valley deepening.

Thus this Sub-phase is also characterized by uniform development of Landscape.

3. Absteigende Entwickelung  

This phase is also known as the Wanning development of Landscape. 

In this phase, upliftment stopped already as well as valley deepening is also stopped but down-wasting of divide summits still continues, which lead to depositi  in lower valleys and decrease in both absolute as well as relative relief. 

In this phase both sides of slopes lead to development of two different type of land -forms. The conical and steep rising landform is called 'gravity slopes' or 'Boschungen', while the lower segment of the valley sides is known as wash slopes or "Haldenhang'.

Haldenhang composed of talus materials of lower inclination, is formed at the base of the valley sides due to rapid parallel retreat of gravity slopes and consequent elimination of much of the convex accelerating/waxing slopes.

Haldenhang continues to expand at the cost of upper gravity slopes.

After further erosion 'gravity slopes' further eroded and forms small but steep slopped conical landform called as 'inselberg'  which on further erosion forms lower, featureless 'Endrumpf', which can be compared with the Davis' 'Peneplane'.

Evaluation

1. Penck's model was very wrongly interpreted by English translator due to it's publication in obscure German language.
2. This Model was severely criticized in America.  
3. He was criticized for his concept  of 'parallel retreat' of slopes and continued crustal movement. However later interpretation of his model shows  that instead of parallel retreat, he proposed the 'replacement of slopes' in which upper slope unit of hill slopes and valley sides was considered to be replaced by lower unit of gentler slopes.
4. He was severely criticized for his long continued crustal movement theory and tectonic speculations which is not fit according modern theories .
5. His slope replacement concept and the concept of weathering process are definitely of much geographical significance. 

Also Read : Rejuvenation and Polycylic Landforms

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