Cycle Of Erosion By Penck
Introduction: German scientist Walther Penck rejected the Davisian model of Geomorphological cycle based on time. He presented his own model of ‘Morphological system’ or ‘Morphological Analysis’ for the explanation of landscape development.
Penck criticised the assumption of Davis’s cycle of erosion. Davis told that erosion starts only after completing the upliftment of the land mass and within very short period time. Penck also expressed disagreement with the stage concept of Davis.
According to Penck the landscape is the result of relative intensity of the degradational process, phase and rate of upliftment. It means, the landforms are determined by the rates of upliftment and the rates of erosion. In the place of ‘stage’ penck used the term ‘Entwickelung’ means development.
Penck used the term ‘Primarumpf’ to represent the characteristic landscape before upliftment. According to penk ‘Primarumpf ‘ is an initial surface or a primary peneplain that either newly emerged surface from below sea-level or peneplain type of landscape, converted into featureless landmass by upliftment.
Penck has said that the upliftment and degradation is a continuous process. He also said that the landforms are an expression of the phase and rates of upliftment in relation to the rates of degradation.
The landscape started to develop with the upliftment of ‘primarumpf’, a unique featureless landscape. But the rate of upliftment is varying. In the beginning the uplift is characterised by exceedingly slow upheaval of long duration. Then the rate of upliftment is accelerated and ultimately it stops after passing through the intermediate phases of uniform and decelerating rates and upheaval.
Basic premises of penckian model:
Following are the basic premises of penckian model of landscape development.
- The morphological characteristics on the earth surface is a result of competition between crustal movement and denudational process.
- Landscape development is also time dependent.
- Upliftment and erosion are always co-existent.
There are three phases through which landscape development passes.
Aufsteigende Entwickelung: This phase is characterised by accelerating rate of landform development. Initially the rate of upliftment of land is very slow but after sometime rate of upliftment increases. Due to increase of upliftment, channel gradient also increases that leads to increase the flow velocity and kinetic energy of the river. Thus river started to degrade their valley with accelerated rate of down cutting. But the rate of upliftment is more than the valley deepening, resulted to formation of deep and narrow V shaped valley. As the rate of upliftment is more than the valley deepening by river thus, increasing both maximum altitude and maximum relief.the valley side slope become convex, but this convexity is reduced due parallel retreat of the slope. With the passage of time and accelerated upliftment and degradation ‘Primarumpf’ is surrounded by series of benches, called as ‘Piedmont trappen’.
Gleichformige Entwickelung: it means uniform development of landscape. This phase is divided into three phases.
Phase a: This phase is also characterised by accelerating rate of upliftment . Absolute height is also increasing, because the rate of erosion is still lower than the upliftment. But the relative height is increasing in lower rate. Because, the rate of valley deepening and lowering of summit due to weathering became equal. The valley sides attained straight slope. Thus this phase is called the phase of uniform development.
Phase b: Absolute relief neither increases nor decreases. Though the rate of upliftment is still continuies, but the lowering of divide summits due to denudation become equal to the rate of valley deepening by river. Thus, relative relief also remain constant.
Phase c: This phase is characterised by totally absence of upliftment of landscape. Now landscape started to decreasing in height due to rapid rate of erosion. Relative relief also remain constant. Because, the lowering of land became equal to valley deepening by the river.
Absteigende Entwickelung: it means wanning development of the landscape. This phase is characterised by progressively decline of landscape. Absolute relief decreases rapidly because absence of upliftment of the land. This phase is dominated by lateral erosion instead of down cutting by the river,due to decrease in channel gradient and kinetic energy of the river. Thus the relative relief also decreases.
In this phase the valley side slope consists of two segments. The upper most part of the valley with its steep angle, termed as Gravity Slope or Boschungen. Where as the lower segment of the valley with gentle slope is termed as Wash slope or Haldenhang.
In the advance stage of this phase the gravity slope are reduced and formed a steep side conical hill, called inselberg. Eventually this inselberg also consumed and total area dominated by a series of concave wash slope. Such extensive surface produced at the end of this phase is called Endrumpf.
Cycle of Erosion By W.M.Davis –