Geology Reference
In-Depth Information
According to Penck's arguments, slopes may either
recede at the original gradient or else flatten, accord-
ing to circumstances. Many textbooks claim that Penck
advocated 'parallel retreat of slopes', but this is a false
belief (see Simons 1962). Penck (1953, 135-6) argued
that a steep rock face would move upslope, maintain-
ing its original gradient, but would soon be eliminated
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
starts growing from the bottom of the basal slope, replaces
the basal slope. Continued slope replacement then leads
to a flattening of slopes, with steeper sections formed
during earlier stages of development sometimes surviv-
ing in summit areas (Penck 1953, 136-41). In short,
Penck's complicated analysis predicted both
slope reces-
sion
and
slope decline
, a result that extends Davis's
simple idea of
slope decline
(Figure 1.3). Field stud-
ies have confirmed that slope retreat is common in a
wide range of situations. However, a slope that is actively
eroded at its base (by a river or by the sea) may decline if
the basal erosion should stop. Moreover, a tableland scarp
retains its angle through parallel retreat until the erosion
removes the protective cap rock, when slope decline sets
in (Ollier and Tuddenham 1962).
Eduard Brückner
and
Albrecht Penck
's (Walther's
father) work on glacial effects on the Bavarian Alps and
their forelands provided the first insights into the effects
of the Pleistocene ice ages on relief (Penck and Brückner
1901-9). Their classic river-terrace sequence gave names
to the main glacial stages - Donau, Gunz, Mindel, Riss,
and Würm - and sired Quaternary geomorphology.
Modern historical geomorphology
Historical geomorphology has developed since Davis's
time, and the interpretation of long-term changes of
landscape no longer relies on the straitjacket of the geo-
graphical cycle. It relies now on various chronological
analyses, particularly those based on stratigraphical stud-
ies of Quaternary sediments, and upon a much fuller
appreciation of geomorphic and tectonic processes (e.g.
Brown 1980). Observed stratigraphical relationships fur-
nish relative chronologies, whilst absolute chronologies
derive from sequences dated using historical records,
radiocarbon analysis, dendrochronology, luminescence,
palaeomagnetism, and so forth (p. 354). Such quantita-
tive chronologies offer a means for calculating long-term
rates of change in the landscape.
It is perhaps easiest to explain modern historical geo-
morphology by way of an example. Take the case of
the river alluvium and colluvium that fills many valleys
in countries bordering the Mediterranean Sea. Claudio
Vita-Finzi (1969) pioneered research into the origin
of the valley fills, concluding that almost all alluvium
Eduard Brückner and Albrecht Penck
Other early historical geomorphologists used geologi-
cally young sediments to interpret Pleistocene events.
Slope recession or backwearing
(Penck)
Slope decline or downwearing
(Davis)
Time
6
5
4
3
2
1
1
2
3
4
5
6
Pediplain
Peneplain
Figure 1.3
Slope recession, which produces a pediplain (p. 381) and slope decline, which produces a peneplain.
Source:
Adapted from Gossman (1970)
