Walther Penck (30 August 1888 – 29 September 1923) was an Austrian geographer,
born in Vienna as the son of geographer Albrecht Penck.
Walther Penck worked 1912–1914 in Argentina as
a geographer, and is best known
for his contributions to the field of geomorphology. In particular, he opposed key elements of the Davisian cycle of erosion, concluding that the process of uplift and denudation
occur simultaneously, at a gradual and continuous rate.[1] His book, Morphological Analysis of
Landforms, was published posthumously in 1924 by his father.
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 dependent 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.
Phases
Penck's hypothesis envisages the initial uplift begins
with regional up-doming and the landscape development goes through the
following phases:
(a) Phase 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.
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