Geology Reference
In-Depth Information
12
Zonality, azonality, and the coastal context
12.1
INTRODUCTION
In geomorphology, zonality and azonality are terms used to denote the distribution of landforms,
particularly with respect to global climate, but also in relation to lithology and structure. Zonal
forms are those which are restricted to a particular climatic region (or to a particular bedrock),
whereas azonal features are not so limited in their distribution.
Several references have been made, in the preceding chapters to forms which are typical of
granite landscapes but which are also well-developed in other rock types, and in a range of climatic
conditions. Some of these, however, are, apparently, more widely, or are better developed, in some
climatic and geological settings than others. No typical granite form is unique, or peculiar, to that rock
type and, few, if any, of them are restricted to a single climatic zone (Campbell and Twidale, 1995).
Thus, it was at one time widely held that inselbergs are arid or semi-arid tropical and subtropi-
cal forms, and that their occurrence in high latitude regions and in the humid tropics argues
climatic change. This trend of thought loses conviction with the realisation (Chapter 6) that many
such inselbergs are two-stage forms initiated at the weathering front by the exploitation by shal-
low groundwaters of structural weaknesses in the country rock, weaknesses which are in many
instances of ancient derivation. The regolith is widely developed, and shallow groundwaters are
ubiquitous. Thus, such developments are likely to have been widespread, and although proceeding
at different rates according to the character of the country rock, and to climatic, and hence shallow
regolithic, conditions, the end results are likely to be similar. Differential weathering of the coun-
try rock rendered the weaker zones and compartments more susceptible to erosion, mostly by
rivers but, according to environment, also by waves, and by glaciers, even by mass movements
(solifluction) and by the wind.
12.2
LITHOLOGICAL ZONALITY AND AZONALITY
Some structural features of granite are also developed in other lithological settings and have an
overriding influence. Just as regional stress finds expression in fracture patterns and hence land-
forms in granite, so lineaments have been related to fracture patterns and landform development,
for example in the Palaeozoic sandstones of the Grampian Ranges of western Victoria. Sheet frac-
tures and structures are commonplace (Fig. 12.1) , as are Kluftkarren, and not only in various
climatic settings, but also in coastal as well as terrestrial contexts. In addition, rocks of similar
physical qualities tend to produce similar forms. Climate and mineralogy influence the rate of
development, but not the end results.
As indicated in Table 12.1 t here are no landforms which are exclusively developed on granite.
Most forms familiar in granite terrains are also found on various igneous, metamorphic and sedimen-
tary rocks provided the host rock is physically hard and massive, i.e. possessing widely-spaced,
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