Patterned ground: a periglacial legacy in the landscape
Peter Hoare explains some unusual features of the Breckland landscape that are best seen from the air.
Across substantial regions of the modern world, winters are sufficiently cold to freeze the ground to a depth of several metres, but complete thawing takes place during the following summer. In periglacial regions, with their significantly harsher conditions (mean annual air temperature <−2ºC), long-term freezing may create so-called permafrost that is hundreds of metres deep; ca 60% of Russia, 50% of Canada, 23% of China and 90% of Alaska are underlain by permafrost. Here, summer temperatures are only sufficient to melt the top few metres, forming an active layer. Alternate freezing and thawing of the active layer rearranges the component sediments, and may also disturb the bedrock surface if it is sufficiently shallow. Under these conditions, patterned ground is formed.
Throughout much of the East Anglian region known as Breckland (which includes Barnham East Farm), Upper Cretaceous Chalk bedrock lies within 2.5 m of the ground surface; its upper part is frost shattered (brecciated) and this material is often overlain by windblown (aeolian) coversand and other thin sediments. Large areas of Breckland display inactive or ‘fossil’ patterned ground, consisting of stripes and cellular (often polygonal) forms, indicating that active layer processes took place in the past. Polygons are commonly ca 10 m in diameter; they grade into stripes, which are spaced ca 7.5 m apart, on slope angles of between ca 1° and 6°.
Shallow-lying patterned ground is evident at the surface because vegetation responds to variations in subsoil characteristics (particle size, pH, depth, moisture content, etc.) created by periglacial processes. Crop- or scorch-marks such as those seen in the photograph were especially clear during the long spell of hot and dry weather in the summer of 2018. Every other Breckland stripe is underlain by coversand with a pH of <4.5 (acidic) and heather thrives; Chalk bedrock lies close to the surface beneath the intervening stripes, the soil pH is >8 (alkaline) and grass flourishes.
In the most recent study of the Breckland patterns (Boreham & Rolfe 2016–17), the authors concluded that Devensian (MIS 2) frost cracks developed into what they termed ‘tiger’ stripes; solifluction played its part in drawing material downslope. Although the exact mechanisms underlying patterned-ground formation in Breckland are not fully understood, their presence may be used to say something about former climatic conditions.
Further reading
Boreham, S. & Rolfe, C.J. 2016-17. Imaging periglacial stripes using ground penetrating radar at the ‘GRIM’ training site, Grime’s Graves, Breckland, Norfolk. Bulletin of the Geological Society of Norfolk 66, 31–43.
Across substantial regions of the modern world, winters are sufficiently cold to freeze the ground to a depth of several metres, but complete thawing takes place during the following summer. In periglacial regions, with their significantly harsher conditions (mean annual air temperature <−2ºC), long-term freezing may create so-called permafrost that is hundreds of metres deep; ca 60% of Russia, 50% of Canada, 23% of China and 90% of Alaska are underlain by permafrost. Here, summer temperatures are only sufficient to melt the top few metres, forming an active layer. Alternate freezing and thawing of the active layer rearranges the component sediments, and may also disturb the bedrock surface if it is sufficiently shallow. Under these conditions, patterned ground is formed.
Throughout much of the East Anglian region known as Breckland (which includes Barnham East Farm), Upper Cretaceous Chalk bedrock lies within 2.5 m of the ground surface; its upper part is frost shattered (brecciated) and this material is often overlain by windblown (aeolian) coversand and other thin sediments. Large areas of Breckland display inactive or ‘fossil’ patterned ground, consisting of stripes and cellular (often polygonal) forms, indicating that active layer processes took place in the past. Polygons are commonly ca 10 m in diameter; they grade into stripes, which are spaced ca 7.5 m apart, on slope angles of between ca 1° and 6°.
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| Barnham East Farm brickearth pit (A); cellular patterns on flat, relatively elevated land (B); stripes on gentle slopes (C); note two sets of stripes converging to create a chevron pattern in the southwest corner of the image, indicating opposing gentle sideslopes of a minor (dry) valley (D). (Credit: Google Maps, accessed 8 June 2019.) |
In the most recent study of the Breckland patterns (Boreham & Rolfe 2016–17), the authors concluded that Devensian (MIS 2) frost cracks developed into what they termed ‘tiger’ stripes; solifluction played its part in drawing material downslope. Although the exact mechanisms underlying patterned-ground formation in Breckland are not fully understood, their presence may be used to say something about former climatic conditions.
Boreham, S. & Rolfe, C.J. 2016-17. Imaging periglacial stripes using ground penetrating radar at the ‘GRIM’ training site, Grime’s Graves, Breckland, Norfolk. Bulletin of the Geological Society of Norfolk 66, 31–43.
