Geology Reference
In-Depth Information
Much climatic geomorphology has been criticized for
using temperature and rainfall data, which provide too
gross a picture of the relationships between rainfall, soil
moisture, and runoff, and for excluding the magnitude
and frequency of storms and floods, which are impor-
tant in landform development. Some landforms are more
climatically zonal in character than are others. Arid,
nival, periglacial, and glacial landforms are quite distinct.
Other morphoclimatic zones have been distinguished,
but their constituent landforms are not clearly deter-
mined by climate. In all morphoclimatic regions, the
effects of geological structure and etching processes are
significant, even in those regions where climate exerts a
strong influence on landform development (Twidale and
Lageat 1994). It is likely that, for over half the world's
land surface, climate is not of overarching importance
in landform development. Indeed, some geomorpholo-
gists opine that landforms, and especially hillslopes, will
be the same regardless of climate in all geographical and
climatic zones (see Ruhe 1975).
The conclusion is that, mainly because of ongoing
climatic and tectonic change, the climatic factor in
landform development is not so plain and simple as cli-
matic geomorphologists have on occasions suggested.
Responses to these difficulties go in two directions -
towards complexity and towards simplicity. The com-
plexities of climate-landform relations are explored in at
least two ways. One way is to attempt a fuller character-
ization of climate. A recent study of climatic landscape
regions of the world's mountains used several pertinent
criteria: the height of timberline, the number and char-
acter of altitudinal vegetational zones, the amount and
seasonality of moisture available to vegetation, physio-
graphic processes, topographic effects of frost, and the
relative levels of the timberline and permafrost limit
(Thompson 1990). Another way of delving into the
complexity of climatic influences is to bring modern
views on fluvial system dynamics to bear on the ques-
tion. One such study has taken a fresh look at the
notion of morphogenetic regions and the response of
geomorphic systems to climatic change (Bull 1992).
A simpler model of climatic influence on landforms is
equally illuminating (Ollier 1988). It seems reasonable
to reduce climate to three fundamental classes: humid
where water dominates, arid where water is in short
supply, and glacial where water is frozen (Table 15.1).
Each of these 'climates' fosters certain weathering and
slope processes. Deep weathering occurs where water
is unfrozen. Arid and glacial landscapes bear the full
brunt of climatic influences because they lack the pro-
tection afforded by vegetation in humid landscapes.
Characteristic landforms do occur in each of these cli-
matic regions, and it is usually possible to identify past
tropical landscapes from clay minerals in relict weath-
ering profiles. It seems reasonable, therefore, by making
the assumption of actualism (p. 27), to use these present
climate-landform associations to interpret relict features
that bear the mark of particular climatic regimes. Julius
Büdel (1982, 329-38), for instance, interprets the 'etch-
plain stairways' and polja of central Dalmatia as relicts
from the late Tertiary period, when the climate was
more 'tropical', being much warmer and possibly wetter.
Such conditions would favour polje formation through
'double planation' (p. 381): chemical decomposition and
Table 15.1 A simple scheme relating geomorphic processes to climate
Climate
Weathering process
Weathering depth
Mass movement
Glacial
Frost (chemical effects
reduced by low
temperatures)
Shallow
Rock glacier
Solifluction (wet)
Scree slopes
Humid
Chemical
Deep
Creep
Landslides
Arid
Salt
Deep
Rockfalls
Source:
Adapted from Ollier (1988)
