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Drainage development, thermokarst activity and periglacial landscape change in Southern England

Drainage development, thermokarst activity and periglacial landscape change in Southern England

Introduction

Thermokarst landforms have been widely recognised in southern England, and particularly Fenland by workers such as Rodney Burton & Richard West. These landforms are represented by near-circular depressions and larger flat-bottomed embayments cut into Jurassic or Cretaceous clay or silt-rich bedrock, and surrounded by low, rounded hills. It appears that these features were formed during the Devensian (the last cold stage), or in some cases earlier cold stages, by the same processes which operate in arctic thaw lakes today. Their formation has been independent of past fluvial processes which have formed separate terrace aggradations. This research theme centres on the investigation of thermokarst processes, the mode of formation of thermokarst landforms, their geological setting and role in landscape change.

What is thermokarst?

During the middle and Late Devensian, periglacial conditions with permafrost and patterned ground affected southern Britain. The erosional processes which operate under these conditions can cause considerable depression of the land surface, and include thermo-abrasion, thermo-denudation and thermokarst. The thermal and mechanical energy of water gives rise to thermo-abrasion. Similarly, the thermal energy of the air, and solar radiation on slopes exposed at the edge of a water body results in thermo-denudation. In addition, the thawing of ice-rich sediment causes thermokarst. As a contemporary example, the rate of slope recession (backwearing) due to these processes in Siberian reservoirs may be up to 10 metres a year. At more southerly latitudes there would be a higher summer heat gain which could cause even greater erosion. These effects would be especially severe in areas with a high water table and where the silt or clay-rich bedrock was unprotected by overlying Pleistocene sediment.

Formation of thermokarst features

Thermokarst landforms probably originate along pre-existing minor lines of drainage. Surface water from springs, and from the melting of ground ice may accumulate to form a thaw lake, which becomes enlarged into a roughly circular depression through the action of thermokarst and other processes. This is similar to the way 'thermocirques' form today in Siberia. Pleistocene sediments overlying bedrock, or changes in bedrock lithology may give rise to the spring-lines which feed thaw lakes, and yet may protect the surrounding clay bedrock from thermokarst processes. As the lake level rises, water may escape at the lowest point in the encircling hills forming a narrow outflow channel. The continuation of backwearing leads to an enlargement of the depression, often with preferential erosion at right angles to the prevailing wind direction creating an elongated flat-bottomed embayment. Such depressions are often called 'alases' or 'alas valleys'. Embayments containing thaw lakes have been noted from various localities in the Arctic foothills of Alaska and from the Canadian Arctic. The continued elongation of an embayment, or the joining of two embayments across a col could result in the formation of a channel. Thus thermokarst landforms may be represented by small circular depressions, larger possibly elongate embayments, and by channels formed by the extension of a single embayment, or coalescence of several such embayments.

Landscape Change

Thermokarst landforms may be of polycyclic origin, with periglacial backwearing processes (and solution) operating in each cold-stage since the late Anglian. For this reason, the role of thermokarst in modifying the East Anglian landscape should not be under-estimated. Thermokarst depressions have been identified on a wide variety of other sediments including Eocene, Cretaceous and Jurassic clays and the Lower, Middle and Upper Chalk, in most cases closely associated with overlying glacial and fluvial deposits. Such deposits mantle and protect bedrock from intense periglacial weathering and erosion, but exposed bedrock is rapidly attacked and removed by solifuction and mass wastage. This has led to a general inversion of relief across Southern England since the Anglian, leading to the maxim 'the hills of today are the valleys of the past'. The figure below illustrates the stages in the formation of the Cambridge District since the Anglian.

I Anglian ice sheet reaches Chalk scarp
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II Anglian ice sheet breaches Chalk scarp
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III Post-Anglian drainge
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IV Periglacial erosion of exposed Chalk
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V Continued erosion of Chalk
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VI Present Day valley of the River Cam
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