Page 25 - Fundamentals of Geomorphology

8 INTRODUCING LANDFORMS AND LANDSCAPES

According to Penck’s arguments, slopes may either Eduard Brückner and Albrecht Penck’s (Walther’s
recede at the original gradient or else flatten, accord- father) work on glacial effects on the Bavarian Alps and
ing to circumstances. Many textbooks claim that Penck their forelands provided the first insights into the effects
advocated ‘parallel retreat of slopes’, but this is a false of the Pleistocene ice ages on relief (Penck and Brückner
belief (see Simons 1962). Penck (1953, 135–6) argued 1901–9). Their classic river-terrace sequence gave names
that a steep rock face would move upslope, maintain- to the main glacial stages – Donau, Gunz, Mindel, Riss,
ing its original gradient, but would soon be eliminated and Würm – and sired Quaternary geomorphology.
by a growing basal slope. If the cliff face was the scarp
of a tableland, however, it would take a long time to
disappear. He reasoned that a lower-angle slope, which Modern historical geomorphology
startsgrowingfromthebottomofthebasalslope,replaces Historical geomorphology has developed since Davis’s
the basal slope. Continued slope replacement then leads time, and the interpretation of long-term changes of
to a flattening of slopes, with steeper sections formed landscape no longer relies on the straitjacket of the geo-
during earlier stages of development sometimes surviv- graphical cycle. It relies now on various chronological
ing in summit areas (Penck 1953, 136–41). In short, analyses, particularly those based on stratigraphical stud-
Penck’s complicated analysis predicted both slope reces- ies of Quaternary sediments, and upon a much fuller
sion and slope decline, a result that extends Davis’s appreciation of geomorphic and tectonic processes (e.g.
simple idea of slope decline (Figure 1.3). Field stud- Brown 1980). Observed stratigraphical relationships fur-
ies have confirmed that slope retreat is common in a nish relative chronologies, whilst absolute chronologies
wide range of situations. However, a slope that is actively derive from sequences dated using historical records,
eroded at its base (by a river or by the sea) may decline if radiocarbon analysis, dendrochronology, luminescence,
the basal erosion should stop. Moreover, a tableland scarp palaeomagnetism, and so forth (p. 354). Such quantita-
retains its angle through parallel retreat until the erosion tive chronologies offer a means for calculating long-term
removes the protective cap rock, when slope decline sets rates of change in the landscape.
in (Ollier and Tuddenham 1962).
It is perhaps easiest to explain modern historical geo-
morphology by way of an example. Take the case of
Eduard Brückner and Albrecht Penck the river alluvium and colluvium that fills many valleys
in countries bordering the Mediterranean Sea. Claudio
Other early historical geomorphologists used geologi- Vita-Finzi (1969) pioneered research into the origin
cally young sediments to interpret Pleistocene events. of the valley fills, concluding that almost all alluvium

Slope recession or backwearing Slope decline or downwearing
(Penck) (Davis)

Time
6 5 4 3 2 1 1 2 3
4
5
6

Pediplain
eneplain
P ediplain P Peneplain
Figure 1.3 Slope recession, which produces a pediplain (p. 381) and slope decline, which produces a peneplain.
Source: Adapted from Gossman (1970)