Diagram 1- illustrates talus slope, debris cones, alluvial fans and rock avalanches.
Sellby (1982)
Mountain environments are often dominated by fluvial systems so it is hard to determine whether the cones are produced by rockfalls or other non fluvial processes. This is recognised in Sellby (1982) “…streams may also be active in mountain slopes there is no simple distinction between cones formed by rockfalls, landslides, snow avalanches, or debris flows and alluvial fans produced by stream deposition.” At this point there must be a distinction made between cones and norlamsl slopes/alluvial fans. Cones usually have a gradient ranging from greater than 11 degrees and up to 46 degrees. Whilst slopes have a much gentler gradient.
As stated above there are many differing ideas within the study of scree slope development which make development of models for the forms, development, and deposition of talus sheets and talus cones some what difficult. Selby (1982) identifies several factors which make the development of models difficult. He argues that talus that many talus deposits were created in the 10ky of Holocene time, in which there were several deviations in climatic conditions, influencing the supply of debris and the processes acting to modify the surfaces of the talus deposit. Linked in with this idea he also argues that process acting on a talus in one climatic situation may be very different to the process at work in a contrasting climatic area. He states that considerable microclimatic variations between different talus slopes, cause considerable variations in rates of supply to talus surfaces and “…and in the importance of such processes as snow and slush avalanches, nivation, interstitial ice and creep created by that ice.” Selby (1982). He also comments on the size of material on talus slopes, comparing tabular and cubic fragments, and the different processes of how the different types of debris reaches the talus slope, whether it be by falls of individual clasts as blocks or by catastrophic events, such as dry and wet avalanches. He also raises the issues of how differing talus slopes, acted upon by different climatic conditions, experience different processes on the talus surface, “…the processes acting to modify the talus surface include…creep and rolling of particles caused by collisions; creep caused by needle ice; subsidence caused by melting caused by melting of buried snow and ice.” Selby (1982).
It has been identified that the formation of talus slopes have differing geomorphic processes acting on them, in contrasting climatic conditions. Attention will now be given to three general models of talus accumulation and redistribution. The talus creep model devised by Thornes (1971) sighted in Gerrard (1990), suggests that talus behaves like a conveyor belt with material moving down as more material is added to the top, however he argues that this process has little empirical data to support it. The rock fall model, developed especially by Kirkby and Statham (1975) is commonly recognised as the dominant process and control on talus form. It emphasis the accumulation of individual particles that roll and slide downwards until they reach a zone where particles of a similar size to their own form the surface. Selby (1982) suggests that consequently small particles will come to dominate the upper slopes while large particles will dominate the lower slopes, hence producing a gradation from top to the bottom of the slope. However Caine’s (1982) work, studying the coarse, blocky detritus below the alpine cliffs in the higher mountains of Tasmania showed that particle size analysis of the talus indicated large clasts which were moderately well sorted. However he found no evidence to suggest that there was any gradation of clasts down the talus slope, “…there is no evidence of the increase in clast size with downslope distance which has frequently been reported from other field studies.” Caine (1982). At this point it must be re-emphasised that different talus slopes display contrasting features in conjunction with varying topographical, geomorphological and climatic processes.
Kirkby and Statham’s model uses this gradation of clast size down slope, to explain the concavity of the lower section of talus slopes. They argue that large blocks will have higher kinetic energy and hence will travel further away from the base of the scree. They also postulates that some particles will travel further causing a thinning tail of the talus slope. This hypothesis is supported in Caine (1982), “…on long talus slopes, the profile is locally complex and frequently has a general convex form, with a bulging lower section and relatively gentle gradients near its top.” Caine (1982) adds further to this idea of a convex profile, by describing the stepping of the Markham Heights Talus, which he explains by the action of rock glaciers.
Further evidence of the creation of a convex talus slope profile is illustrated in the third model of talus accumulation and redistribution, slush avalanching. Selby (1982) states the importance of slush avalanches in redistributing debris from the head of the talus slope to the base, so the depositional layer increases with thickness with distance away from the source of slush flow, thus producing a concave form.
Although all three models provide useful insight into the processes acting on talus slopes, their anomalies must be considered. Gerrard (1990), argues that the processes of rapid mass movement would destroy any sorting, which contradicts the ideas of ordered sorting put forward in the models. He also argues that observations do not support talus accumulation models that assume the addition of wedges or sheets that cover sequentially all the surface, “…the presence of an older basal fringe of larger boulders developed by rock fall and snow avalanching conflicts with models predicting the greatest accumulation at the talus base.” Gerrard (1990).
Talus slopes have four main kinds of fabric which make up their deposits. Gerrard (1990) identifies that talus slopes can have an open work fabric, in which there are several small calsts which usually results from the fall of individual pieces of small rock or minor rockfall events. Gerrard (1990) identifies three more types of fabric. The first, a partly open work fabric which is caused by the in filling of voids of an open work fabric and also by the washing down of small grains. Secondly a closed clast support fabric which has all its voids filled with fine grain material which is a result of washing down of small fragments. Thirdly, he identifies a matrix supported fabric which are most commonly created by debris flows, solifluction, or by a wash.
In conclusion it is evident that the study of talus slope production and theis evolution has many grey areas. The contrasting geomorphic and topographic processes, the time of deposition or the varying climatic/microclimatic conditions, make applying simple and universal models some what inadequate. This point is raised in Gerrard’s (1990) work “…it is unlikely that the evolution of any scree slope is dominated by the operation of a single process…”. It should also be recognised that in many area talus slopes are inactive. Selby (1982), explains how many slopes have reached a state of equilibrium. He also points out that many slopes have become essentially fossilised by the formation of soil on the talus slope, leading to the growth of vegetation.
Bibliography
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Caine,N. 1982 Toppling failures from alpine cliffs on Ben Lomand, Tasmania, Earth Surface Processes and Landforms.
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Gerrard, A.J. 1988, Mountain environments
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Kearey, P. 1990 The Dictionary of Geology, The New Penguin
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Sellby, M.J. 1982 Hillslopes materials and processes, Oxford
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Young, A. 1970 Slopes, Oliver and Boyd
