Page 52 - Fundamentals of Geomorphology

THE GEOMORPHIC SYSTEM 35

The land-surface debris cascade produces landforms. The circulations are called biogeochemical cycles. The
It does so partly by selectively weathering and eroding land phase of these cycles is intimately linked with water
weaker rocks (Box 2.2). and debris movements.

Biogeochemical cycles Interacting cycles
The biosphere powers a global cycle of carbon, oxy- The water cycle and the rock cycle interact (Figure 2.2).
gen, hydrogen, nitrogen, and other mineral elements. John Playfair was perhaps the first person to recog-
These minerals circulate with the ecosphere and are nize this crucial interaction in the Earth system, and
exchanged between the ecosphere and its environment. he was perhaps the great-grandfather of Earth System

Box 2.2

ROCKS AND RELIEF

The ability of rocks to resist the agents of denudation England is in places is a prominent topographic feature
depends upon such factors as particle size, hardness, (p. 119). Basalt may cap plateaux and other sedimen-
porosity, permeability, the degree to which particles are tary hill features. Slate is a moderately strong rock,
cemented, and mineralogy. Particle size determines while schist is weak.
the surface area exposed to chemical attack: gravels Sedimentary rocks vary greatly in their ability to
and sands weather slowly compared with silts and resist weathering and erosion. The weakest of them are
clays. The hardness, mineralogy, and degree of rock chalk and rock salt. However, the permeability of chalk
cementation influences the rate at which weathering compensates for its weakness and chalk resists denuda-
decomposes and disintegrates them: a siliceous sand- tion, sometimes with the help of more resistant bands
stone is more resistant to weathering than a calcareous within it, to form cuestas (p. 133), as in the North and
sandstone. Permeability is an important property in South Downs of south-east England. Coal, claystone,
shaping weathering because it determines the rate at and siltstone are weak rocks that offer little resistance
which water seeps into a rock body and dictates the to erosion and tend to form vales. An example from
internal surface area exposed to weathering (Table 2.1). south-east England is the lowland developed on the
As a rule, igneous and metamorphic rocks are resis- thick Weald Clay. Sandstone is a moderately strong
tant to weathering and erosion. They tend to form the rock that may form scarps and cliffs. Whether or not it
basements of cratons, but where they are exposed at does so depends upon the nature of the sandstone and
the surface or are thrust through the overlying sedi- the environment in which it is found (e.g. Robinson
mentary cover by tectonic movements they often give and Williams 1994). Clay-rich or silty sandstones are
rise to resistant hills. English examples are the Malvern often cemented weakly, and the clay reduces their per-
Hills in Hereford and Worcester, which have a long meability. In temperate European environments, they
and narrow core of gneisses, and Charnwood Forest weather and are eroded readily and form low relief,
in the Midlands, which is formed of Precambrian vol- as is the case with the Sandgate Beds of the Lower
canic and plutonic rocks. The strongest igneous and Greenland, south-east England. In arid regions, they
metamorphic rocks are quartzite, dolerite, gabbro, may produce prominent cuestas. Weakly cemented
and basalt, followed by marble, granite, and gneiss. sands and sandstones that contain larger amounts of
These resistant rocks tend to form relief features in quartz often form higher ground in temperate Europe,
landscapes. The quartz-dolerite Whin Sill of northern probably because their greater porosity reduces runoff