Geology Reference
In-Depth Information
HILLSLOPE ENVIRONMENTS
There are two extreme cases of rock properties. The first
case is 'hard' rocks with a very high internal strength (the
strength imparted by the internal cohesive and frictional
properties of the rock). These usually fail along partings
in the rock mass - joints and fractures. The second case is
'soft' rocks of lower intact strength or intense fracturing
that behave more like soils. As a rule of thumb, bare rock
slopes form on hard rocks. However, there are circum-
stances that favour the formation of bare rock slopes on
soft rocks. For example, steep rock slopes may occur on
mudstones and shales that lie at high elevations where the
slopes are regularly undercut. Even so, such slopes denude
far more rapidly than do slopes on hard rocks, and they
are far more likely to develop a soil and vegetation cover
(Selby 1982, 152). Some rock slopes speedily come into
equilibrium with formative processes and rock proper-
ties, their form reflecting the strength of the rock units
on which they have developed. Such rock slopes occur on
massive and horizontally bedded rocks. On dipping and
folded rocks, the form of bare rock slopes conforms to
underlying geological structures.
Hillslopes are ubiquitous, forming by far the greater part
of the landscape. Currently, ice-free landscapes of the
world are 90 per cent hillslopes and 10 per cent river
channels and their floodplains. Hillslopes are an inte-
gral part of the drainage basin system, delivering water
and sediment to streams. They range from flat to steep.
Commonly, hillslopes form
catenas
- sequences of linked
slope units running from drainage divide to valley floor.
Given that climate, vegetation, lithology, and geologi-
cal structure vary so much from place to place, it is not
surprising that hillslope processes also vary in different
settings and that hillslopes have a rich diversity of forms.
Nonetheless, geomorphologists have found that many
areas have a characteristic hillslope form that determines
the general appearance of the terrain. Such characteristic
hillslopes will have evolved to a more-or-less equilib-
rium state under particular constraints of rock type and
climate.
Hillslopes may be bare rock surfaces, regolith and soil
may cover them, or they may comprise a mix of bare rock
and soil-covered areas. Hillslopes mantled with regolith
or soil, perhaps with some exposures of bare rock, are
probably the dominant type. They are usually designated
soil-mantled hillslopes
. However, hillslopes formed in
bare rock -
rock slopes
- are common. They tend to
form in three situations (Selby 1982, 152). First, rock
slopes commonly form where either uplift or deep inci-
sion means that they sit at too high an elevation for debris
to accumulate and bury them. Second, they often form
where active processes at their bases remove debris, so pre-
venting its accumulation. Third, they may form where
the terrain is too steep or the climate is too cold or too
dry for chemical weathering and vegetation to create and
sustain a regolith. More generally, bare rock faces form
in many environments where slope angles exceed about
45
◦
, which is roughly the maximum angle maintained by
rock debris. In the humid tropics, a regolith may form
on slopes as steep as 80
◦
on rocks such as mudstones and
basalts because weathering and vegetation establishment
are so speedy. Such steep regolith-covered slopes occur on
Tahiti and in Papua New Guinea where, after a landslide,
rock may remain bare for just a few years. Rock properties
and slope processes determine the form of rock slopes.
HILLSLOPE FORMS
Slope units
The term slope has two meanings. First, it refers to the
angle of inclination of the ground surface, expressed
in degrees or as a percentage. Second, it refers to the
inclined surface itself. To avoid misunderstanding, the
term
hillslope
usually applies to the inclined surface and
the term
slope angle
,
slope gradient
, or simply
slope
to its inclination. All landforms consist of one or more
slopes of variable inclination, orientation, length, and
shape (Butzer 1976, 79). Most hillslope profiles con-
sist of three slope units - an upper convex unit where
gradient increases with length, a straight middle unit of
constant gradient, and a concave lower unit where gra-
dient decreases with length (Figure 7.1) (White 1966).
The transition between these slope units may be smooth
or abrupt (Figure 7.2). The middle unit is sometimes
absent, giving a
concavo-convex slope profile
, as com-
monly found in English Chalklands (Plate 7.1; see also
p. 290). The terms used to describe slope units vary.
