Penck’s Model of Slope Development

Walter Penck was a German. His work appeared a little obscure. Nevertheless, the arguments that he put forward were based on logic. His work was neglected for a long. He was dissatisfied with some of his assumptions of Davis, like the process of upliftment of landform, which was too short and a prolonged period of stability where the cycle would run its full course.

He offered an alternative model where the landscape would simultaneously be eroded with the process of its upliftment. The rates of denudation would vary, and ultimately it would result in a low featureless plain called ‘Endrumpf’. He attempted to establish a relationship between the tectonic history of the region and the nature of the slope (Spark, 1986).

Penck, in his model, took a straight steep slope unit bordering a river valley. He assumed equal weathering over the entire slope. The weathered material would fall under the force of gravity and reach the lowest part of the slope. The material lying at the lowest level would not be removed further because there is no gradient below it.

In such a situation, there would be a parallel retreat of the slope (fig). The unit AB due to weathering and its removal, would retreat to position CD. In the next stage, the profile would shift further from CD to EF.

All the weathered material will not be removed, for some must remain at the foot of the unit to provide a slope of transport down to the non-eroding river (Small, 1978).

The subsequent stages would see the retreat of the main slope unit to position XY. There is a marked reduction in the length of the slope unit as it retreats from position AB to XY.

At each successive stage, it leaves behind basal fragments, which combine to give a uniform slope covered with a uniformly thick layer of detritus extending from the original slope unit down to the river.

It is observed that a steep slope retreats upslope maintains its gradient, and gives rise to a basal slope of the lesser gradient. Now, two slopes have emerged- the upper slope and the lower basal slope.

Now the basal slope undergoes weathering, and its material is reduced to finer particles. The weathered material is removed from the basal slope, but again the lowest particle is not removed as there is no gradient below it. Therefore, a new slope unit of a gentler angle is added below the basal slope. (fig)

Over a period of time, a lot of new slope units of a gentler gradient than the next unit above will be formed at the foot of the slope and undergo migration upslope. The overall result will be the development of basal concavity (Small,1978).

The original slope will eventually be destroyed, the relief will be lowered, and the slope angle will decline. Thus it is observed in his model that flattening of the slope takes place from below upwards.

Penck’s model of slope evolution is essentially deductive. He has elaborated upon the development of slopes under conditions of the accelerated and decelerated rate of erosion.

According to Penck, slope forms and how they are altered are determined by the rate of the river flowing at the base of the slope. According to him:

1. Convex slopes develop where the rivers are having an accelerated rate of downward erosion.
2. Rectilinear slopes are formed where rivers are eroding at a constant rate and
3. Concave slopes occur above the rivers, which erode at a decelerating rate.

Thus the rate of river erosion forms the overriding factor determining the form of the slope.

Analysis of Penck’s Model

The essence of Penck’s theory remains. However, the slope form and angle are primarily determined by the rate of erosion by rivers (Small, 1978). No doubt slopes may evolve in response to the river incision in the manner discussed by Penck. Other factors like structures, climate and rock type also play important roles in it.

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