Page 184 - Fundamentals of Geomorphology

HILLSLOPES 167

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