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PERIGLACIAL CYCLE OF EROSION


Published on: 12/14/2020 5:53:42 AM

Periglacial regions are very active geomorphological areas. The Mechanical splitting of rocks by ice (gelifraction), frost heaving of the ground (geliturbation), solifluction and nivation are all important processes. In addition, each spring, large quantities of water from melting snow and ice rapidly erode the debris scattered and moved down the slopes. Wind action is also a significant force. In 1950 L.C Peltier put forward the concept of a periglacial cycle of erosion. This is similar to the Davisian concept of the normal cycle of erosion, and the periglacial landscape attains old age after passing through the stages of youth and maturity.

During this entire period there is erosion of the higher parts and deposition in low-lying parts, resulting in overall reduction in relief, and the slope profile gradually becomes smooth and flat. The process by which there is gradual flattering of the surface in the periglacial regions is called 'cryoplanation'. In this process there is parallel retreat of the scarp face by frost shattering and the development and gradual extension of gentle slope at its base by deposition and transport of debris. Cryoplanation is thus achieved primarily by the processes of intense frost action or congelifraction and solifluction or congeliturbation. The penultimate landform is a surface of low local relief, not controlled by any base level, and has been called a 'cryoplain' or an 'altiplain'.

Evolution of The Periglacial Erosion Cycle

The evolution of the periglacial erosion cycles its different stages have been as:

Stage 1 (Initial surface): The cycle begins with the onset of periglacial conditions. In this initial stage congelifraction is especially active as a result of which there is shattering of the bare rocks on the upper slopes, and block-fields are formed by the accumulation of angular rock fragments.

Stage 2 (Early or young stage): The rock fragments move downhill along the slope by gravity, and talus or congeliturbate mantled slope is formed at the base of the slope. During this period nivation hollows as well as cryoplanation terraces also begin to be formed on the higher slopes. Till the end of the young stage, the scarp of the terraces starts retreating by frost action, and the terrace benches are extended by scarp retreat.

Stage 3 (Mature Stage): With further passage of time, the scarps are destroyed by continual recession and in their place only residual or for features are left on the summits or slopes. This is beginning of maturity when the original landscape is lost and the valleys and lower slopes are covered with a mantle of frost shattered and soliflucted materials.

Stage 4 (Old Stage of cryoplanation): Towards the end of maturity and in old age the higher land is further reduced and levelled on account of solifluction, and the accumulation of debris goes on increasing in the neighbouring valleys and low-lying lands, and the periglacial surface is converted into an almost level plain.  The debris become fine on account of continual frost weathering and solifluction (gelifluction) by further disintegration and abrasion. The finer debris are transported by the wind, and loess and sand dunes are formed at places, and areas of ventifacts and lag deposits develop by the blowing away of finer materials by the wind.

Fig: Stages of Periglacial cycle of erosion by peltier

Taking the different slope forms found in periglacial regions as the basis, pelttier has tried to synthesize them and arrange them in a sequence. His hypothesis thus provides an overall framework for the understanding of the slope evolution in periglacial regions, but it does not provide an analysis of how exactly slope form is influenced by frost shattering and solifluction. Further, adequate attention has not been paid to other processes, especially the influence of running water. But the concept of scarp recession by frost shattering and the resultant extension of the terraces is by itself an important and useful concept which helps us to understand the formation of cryoplanation terraces, summit tors, nivation hollows & cryopedimerts.

Also Read | THE WILSON CYCLE AND SUPERCONTINENTS