cliffs and rock slopes Many of the most spectacular landforms on Earth are associated with high mountain chains, towering cliffs, and extensive, steep rock outcrops. Although the slopes that develop in lithified, jointed rock masses vary in form, they include a number of regularly occurring components (see
hill slopes). At the crest of the slope a free face is often present, where rock
in situ forms a steep, near-vertical break between ground at the top and base of the cliff. The free face is usually skirted by debris or
talus, consisting of blocks of rock that have become detached from above. Talus will vary in size and shape, according to the properties of the rock and the joint patterns. As a rule, loose material transported from the upper to lower parts of the talus slope decreases in size. Talus can take the form of a sheet of debris where rockfall activity has been more or less uniform along a cliff. Talus cones can develop if falling debris is funnelled or local activity rates increase above mean conditions. At the toe of the slope a rock
pediment can be found. Pediments are low-gradient ground surfaces, seldom of more than 2° to 3°, which extend from a cliff front away into the surrounding landscape, in some instances over many kilometres (see
pediments). The presence of each of the three rock-slope components depends on the environmental setting. At the coast, where the processes of basal erosion are active, rock debris is likely to be removed by the sea. In these situations the free face can extend to sea level and talus slopes will be absent. Along scarp fronts in tectonically stable, arid environments, all three components are usually found along a slope profile.
The rate at which rock slopes change is varied because of the many controls on development that operate at different rates. The most important natural controls on cliffs and rock slopes can be considered in three groups: climate, weathering, and geology. Climate influences the failure mechanism of a jointed rock mass, rates of rock disintegration, changes in form, and rates of slope development. In wet environments, high-intensity rainstorms may cause a sudden increase in water moving through discontinuities in a rock mass, destabilizing the slope and generating failure. In cold, polar regions, frost and ice may widen joints and promote disintegration of blocks of rock. The weight of snow and ice at the top of a slope may add a load to the rock mass which creates a change in stress and leads to failure. Weathering promotes the breakdown of rock, either as the detachment of material from outcrops
in situ or through the disintegration of blocks previously separated from a rock outcrop. Weathering may be physical, chemical, or biological, depending on the environment, climate, and rock properties. Physical breakdown of rocks results in progressively smaller fragments without change to the mineral matrix. Chemical disintegration alters the original mineral composition. Biological change includes both physical and chemical alteration and is usually confined to the uppermost few metres of rock. In tropical areas, chemical weathering will be significant because of the high temperature and humidity, particularly in rock materials that have a susceptible mineral matrix. In the arid zone, physical weathering is usually the main agent of rock disintegration. Thermal expansion and contraction of rock occurs between day and night time as temperatures fluctuate, generating sheeting of the outer layers of blocks and the sudden splitting of boulders.
Geological factors have great control on the form and development of rock slopes. At the largest scale, regional tectonics and structures, such as fault systems, will influence cliff patterns. Earthquakes and relative movements across plate margins have probably triggered more large rockfalls and slope changes than any other single cause. A good comparison can be seen in the juxtaposed countries of Australia and New Zealand. Australia has an ancient landscape, far removed from plate margins and with little tectonic activity. Combined with an arid climate and slow weathering rates, the slopes of areas such as the Kimberley Plateau in Western Australia are smooth, curvaceous, and reflect development over tens of thousands of years. In contrast, the South Island of New Zealand sits astride a collision zone between the Pacific and Indian-Australian plates. The product is the Southern Alps. Uplift rates exceed 10 mm a year, precipitation is intense, weathering rapid, and denudation rates high. The topography is rugged and complex, prone to rockfalls and avalanches. The slopes and cliffs reflect a highly unstable and dynamic landscape. At a local level, particularly at individual sites, the main geological controls are the properties of intact rocks and structural discontinuities. The stronger the rock, the greater its competence and the steeper the slopes that are likely to develop. The significance of inherent rock strength is, however, commonly eclipsed by the properties of eclipsed by joints and bedding planes. The orientation, width, spacing, and continuity of joints play an important role in slope form and evolution. Widely spaced joints, which dip out of a slope and have a clay fill, promote the movement of one block over another. The consequence will be an increased likelihood of failure, a reduced slope angle, more talus, and enhanced slope retreat rates. The dip (inclination) of the bedding planes frequently controls rock-slope form and stability. If the bedding is horizontal, slope failure will be rare, the rock slope will be almost vertical, and cliffs may develop. Inclined bedding will reduce the angle of hillslope stability and slope gradient. The likelihood of failure will also increase; bedding that dips out of the slope will lead to the least favourable situation. The interaction of intact rock properties and discontinuity characteristics in controlling the form and development of slopes is often examined using rock-mass strength classification techniques. Properties of site rock materials are synthesized in a semi-quantitative format to explain differences in slopes and the evolution of features such as glaciated valleys, bornhardts (large inselbergs) and scarp fronts.
It is the failure of rock that generates the most rapid change in slope profiles and determines the relative dominance of the main rock-slope components of free-face, talus slope, and pediment. The type of failure is greatly influenced by the intensity and inclination of discontinuities. In the most simple case, failure entails detachment of individual blocks of rock. The precise failure mechanism relates to the orientation of joint sets, the dimensions of the block that becomes detached, and the shape of the resulting space or rock scar (Fig. 1). For slides and wedge failures, orientation of joints is the determining factor. The failure of a number of blocks is usually termed a
fall, with debris accumulating as a talus slope below the zone of rock detachment. More complex movements develop where failure generates a number of displaced blocks. Topples are a good example: a mode of failure in which blocks overturn and a column of rocks can shift. The precise nature of the displacement depends on local conditions, including the size and shape of individual boulders.
Large movements involve masses of material and the controls on failure are somewhat different (see
landslides). Not only are the properties of individual joints or blocks significant, but factors such as the location and dynamics of the water-table and relative relief become important. The proportions of solid rock material, water, and air will vary, affecting the velocity of movement and destructive force of a rockfall. Rock debris becomes fluidized by water and air which becomes trapped in the moving mass. If a cushion of air forms at the base of the moving material, velocities can be great. An extreme form of movement is a rock avalanche, where large volumes of debris move considerable distances, over short time periods. Rock avalanches are associated with steep slopes, well-jointed rock, a weathering environment which reduces material strength, and travel distances which allow moving rock to descend at high velocities. The result is rapid change in slope form, the displacement of many thousands of tonnes of material, and a major natural hazard. Examples include the rock slide on the flanks of Mount Cook, New Zealand, in 1991, and the failure on Mount Huascaran, Peru, in 1970. The Mount Cook event reduced the 3764 m peak of New Zealand's highest summit by 10 m. An estimated 2.5 million m3 of rock travelled 699 m horizontally and involved a vertical descent of 2700 m. During the Huascaran event, a mass of rock and ice moved at velocities exceeding 97m s
−1, destroying the town of Yungay and killing at least 17 000 people. As the two examples confirm, the largest failures and most rapid rock-slope development rates are associated with steep, mountain environments and with active tectonics. Rock avalanches with particularly large volumes have been called
stursztroms and are major processes of erosion and rock-slope development in high mountains. In contrast, individual block detachment can occur from any rock slope and may reflect no more than the passage of time and the natural course of events, with weathering gradually widening a joint and a block suddenly falling away from the cliff as a consequence.
As rock-slope failures occur and form changes, there may be a gradual process of slope retreat. Two alternative suggestions have been proposed for rock-slope development. The first, parallel retreat, is a mechanism by which overall slope shape is maintained but the location of the cliff front recedes through time. Cliffs forming the margins of rifts are thought to have developed in this way, with retreat occurring since the Late Cretaceous in Precambrian to Palaeozoic rocks. An example is Fish River Canyon, Namibia, where a complex sequence of rock slopes and cliffs has retreated up to 10 km from the line of the river. The alternative model involves down-wearing of the landscape, with a gradual decline in mean slope angle and the relaxation or lowering of the terrain. Profiles eventually reach equilibrium with a thin talus cover, termed a
Richter slope. Thereafter the slope weathers uniformly, maintaining its angle but reducing in size. Good examples are found in cold environments such as the Transantarctic Mountains and the Koettlitz Valley, Antarctica. Recent developments in computing and the application of advanced mathematical modelling now permit the use of rock-mass property quantitative data to examine rock-slope evolution and link controls on development, mechanisms of failure, and changes in cliff form.
Robert J. Allison
Bibliography
Duff, D. (ed.) (1994) Holmes' principles of physical geology (4th edn). Chapman and Hall, London.
Selby, M. J. (1993) Hillslope materials and processes (2nd edn). Oxford University Press.
Summerfield, M. A. (1991) Global geomorphology. Longman, Harlow.
