Brief notes on Penck's Model of Landform Evolution

Walther Penck, the German geologist, put forward his thesis of landform evolution in 1924. His main work was in the German language which was translated into English (Czech and Boswell 1953).

Penck criticised and opposed the cycle concept of Davis. In fact, Davis and Penck saw geomorphology through different eyes.

Davis attempted the explanatory description of landscape, adopted another standpoint that the main purpose of geomorphological research is to obtain information that might contribute to the understanding of crustal earth movements.

Penck did not agree with Davis' assumption of very rapid uplift of a land surface followed by a long period of crustal stability. Penck, contrary to Davis' concept, built his theory on the alternative assumption of uplift so prolonged that the landscape would be eroded at the same time as it was being elevated.

The characteristic feature of Penck's hypothesis is that landform evolution of a given region is dependant on the tectonic activity of the region concerned.

In the opinion of Davis, 'landscape is a function of structure, process and stage', whereas in the opinion of Penck, 'landscape is the result of the relative intensity of the degradational processes and the phases and rates of uplift'.

Opposed to the view of Davis, he was convinced that landforms were shaped and developed by the rate of uplift and that of erosion.

In his morphological system model Penck hypothesized that 'geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation'.

The interaction, according to him, between these factors is continuous. In other words, according to Penck's notion, the shapes of slopes, and thus the shape of the landscape in general, were determined primarily by the rate of river erosion, and that this in turn was determined primarily by the rate of uplift of the land.

Penck visualized three situations. First, a constant rate of uplift would produce a constant rate of downcutting by streams, and the result would be straight valley-side slopes which together made up a landscape of Mittel relief (medium relief).

Steepness of slope, according to him, would depend on rate of incision. Secondly, a rapid rate of uplift would produce convex valley- side slopes and strong Steal relief (strong relief).

Thirdly, slower rate of uplift would give concave or waning slopes. After uplift ceases, a flat or gently undulating surface called an Endrumpf (terminal surface) would remain.

As regards slopes, he made several assumptions : (i) Any slope, even it was curved, is made up of a number of straight slope segments, called slope units, (ii) All slope units undergo parallel retreat, (iii) The rate of retreat are supposed to depend on gradient; steep slopes retreating quickly.

Penck's hypothesis envisages the initial uplift begins with regional up-doming and the landscape development goes through the following phases:

(a) Fase of waxing rate of landform development (Aufsteigende Entwickelung)

Endogenetic forces cause the slow rise of the initial land surface (Primarumpf) but later on the upliftment is rapid.

In this phase, because of upliftment and the increase in the channel gradient and stream velocity rivers continue to degrade their valleys with accelerated rate of valley deepening.

The rate of upliftment is faster than the rate of down-cutting. It results in the formation of gorges and narrow V-shaped valleys. Since the upliftment of landmass far exceeds the valley deepening, the absolute height goes on increasing.

Altitude of the summit of interfluves and valley bottom continues to increase due to the faster rate of upliftment than that of the vertical erosion.

This phase is characterized by the maximum altitude and the maximum relief (relative heights of the valley floors).

(b) Phase of uniform development of land form (Gleichformige Entwickelung)

This phase may be divided into three sub-phases on the basis of upliftment and consequent degradation

(i) The first sub-phase is characterised by the continuance of accelerated rate of uplift. The absolute height continues to increase because the rate of upliftment is still greater than the rate of down-cutting.

The maximum altitude or the absolute relief is achieved, but relative relief remains unaffected because the rate of valley deepening is almost equal to the rate of lowering of the summits of stream interfluves.

The valley walls are steep. This is known as the phase of uniform development because of uniformity in the rate of valley deepening and lowering of divide summits.

(ii) In the second sub-phase the absolute relief neither increases nor decreases. This is due to the fact that rate of upliftment and the rate of erosion are the same. However, in this phase the absolute height and the relative relief's are unchanged. So this may be called the phase of uniform development of landforms.

(iii) In this sub-phase there is no more upliftment of land.

(c) Phase of Wanning development of landscape (Absteigende Entwickelung)

The erosional processes dominate in this phase. The lateral erosion rather than vertical erosion is more important. There is progressive decrease in the height of the landforms. In other words, the absolute and the relative relief decline.

The valley side slope consists of two parts, the upper and the lower. The upper segment continues to have steep angle which is called as gravity slope.

The lower segment of the slope is called wash slope. The wash slope is composed of talus materials of lower inclination which is formed at the base of valley sides.

The later part of this phase is marked by the presence of inselbergs and a series of concave wash slopes.

This type of extensive surface produced at the fag end of absteigende entwickelung has been labelled is endrumpf which may be equivalent to peneplain as envisaged by Davis in his cycle concept. Thus, the cycle of landscape development as envisaged by Penck ends in endrumpf.