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Monday, 23 January 2023

Periglacial Britain

I have mentioned this article briefly in the past, but it now seems more relevant than ever as we try to reconstruct the conditions that might have been experienced in Great Britain before, during and after the LGM.  This is a vast paper -- almost 100 pages long -- and it contains a wealth of information as well as a comprehensive reference list.  I intend to take a more detailed look at it as I try and work out what happened during the growth phase of the Preseli Ice Cap and the Welsh Ice cap and when they were transformed from being independent ice caps into ice domes surrounded by, and maybe partly overwhelmed by, the fast streaming ice associated with the Irish Sea Ice Stream.....

Chapter 5 Periglacial and permafrost ground models for Great Britain 
J. B. Murton & C. K. Ballantyne

From: GRIFFITHS, J. S. & MARTIN, C. J. (eds) 2017. Engineering Geology and Geomorphology of Glaciated and Periglaciated Terrains – Engineering Group Working Party Report. Geological Society, London, Engineering Geology Special Publications, 28, 501–597


Periglacial environments are characterized by cold-climate non-glacial conditions and ground freezing. The coldest periglacial environments in Pleistocene Britain were underlain by permafrost (ground that remains at or below 0°C for two years or more), while many glaciated areas experienced paraglacial modification as the landscape adjusted to non-glacial conditions. The growth and melt of ground ice, supplemented by temperature-induced ground deformation, leads to periglacial disturbance and drives the periglacial debris system. Ice segregation can fracture porous bedrock and sediment, and produce an ice-rich brecciated layer in the upper metres of permafrost. This layer is vulnerable to melting and thaw consolidation, which can release debris into the active layer and, in undrained conditions, result in elevated porewater pressures and sediment deformation. Thus, an important difference arises between ground that is frost-susceptible, and hence prone to ice segregation, and ground that is not. Mass-movement, fluvial and aeolian processes operating under periglacial conditions have also contributed to reworking sediment under cold-climate conditions and the evolution of periglacial landscapes. A fundamental distinction exists between lowland landscapes, which have evolved under periglacial conditions throughout much of the Quaternary, and upland periglacial landscapes, which have largely evolved over the past c. 19 ka following retreat and downwastage of the last British–Irish Ice Sheet.

Periglacial landsystems provide a conceptual framework to interpret the imprint of periglacial processes on the British landscape, and to predict the engineering properties of the ground. Landsystems are distinguished according to topography, relief and the presence or absence of a sediment mantle. Four landsystems characterize both lowland and upland periglacial terrains: plateau landsystems, sediment-mantled hillslope landsystems, rock-slope landsystems, and slope-foot landsystems. Two additional landsystems are also identified in lowland terrains, where thick sequences of periglacial deposits are common: valley landsystems and buried landsystems. Finally, submerged landsystems (which may contain more than one of the above) exist on the continental shelf offshore of Great Britain. Individual landsystems contain a rich variety of periglacial, permafrost and paraglacial landforms, sediments and sedimentary structures. Key periglacial lowland land-systems are summarized using ground models for limestone plateau-clay-vale terrain and caprock-mudstone valley terrain. Upland periglacial landsystems are synthesized through ground models of relict and active periglacial landforms, supplemented by maps of upland periglacial features developed on bedrock of differing lithology.

This diagram has some relevance for our understanding of what went on 
on Salisbury Plain.....

1 comment:

Tony Hinchliffe said...

You write about the Salisbury Plain " Clay - With - Flints Puzzle" on pages 181 - 182 of 'The Stonehenge Bluestones', June. 2018. I have walked near them, north of the so - called 'Codford Circle', where they occur on Clay Pit Hill which is due south of the village of Chitterne.