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The nature and evolution of screes?

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Introduction

The nature and evolution of screes? This essay will take a holistic approach to the study of screes. It will aim to identify the key processes at work on scree slopes, the evolution of their form and how varying climatic conditions affect these variables. Before any detailed analysis of scree formation and its evolution can be discussed, a simple definition of a scree will be given. According to The new Penguin dictionary of geology, a scree can be described as, "A sloping accumulation of loose clasts of granule grade or larger, generally in the form of a wedge, metres to hundreds of metres in height, at the base of a steep rock face from which the clasts fall as a result of weathering and erosion." However it must be recognised that debris produced by these weathering and erosional processes are too coarse to be easily removed by streams, glaciers, or waves. Once a scree has been formed, the rock debris can be characterised by the spacing between the joints in the cliff material and the rock type, and hence the strength, and also the comminution that it experiences whilst being transported until it is deposited. ...read more.

Middle

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) ...read more.

Conclusion

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. ...read more.

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