The periglacial environment of Great Britain during the Devensian

Abstract

The main question considered is the extent to which extra-glacial Britain had a permafrost environment in the Devensian, so the three features used in identifying contemporary permafrost are discussed. These are ice wedge polygons, pingos and thaw lakes. As their environments have been studied estimates of temperatures may be made when they are found in a Devensian context. A theoretical case can be made for regarding some involutions as representing the former active layer of permafrost but they have not been related to a contemporary environment. They cannot therefore give a measurement of Devensian conditions. Fossil ice wedges are very widespread in Great Britain, being found from the English Channel to the Scottish Highlands, and associated with late Early, Middle and Late Devensian deposits. Today ice wedges grow in continuous permafrost where the mean annual air temperature is — 6 to — 8 °C or lower. Remains of pingos of the open system type which today develop in discontinuous permafrost with a mean annual air temperature of — 3 to — 6 °C, are found in Wales and East Anglia. These continued to form until the end of the Younger Dryas, and indicate that discontinuous permafrost in England and Wales thawed completely only with the beginning of the Flandrian. The pingos must have begun to form after the transition from continuous to discontinuous permafrost, which probably took place with the rise in temperatures ahead of the Allerød. The area of continuous permafrost and ice wedge formation in Alaska is almost limited to the tundra. Apart from the coastal zone, discontinuous permafrost carries boreal forest with mean July air temperatures of around 15 °C, so that periods of tree growth in the Devensian do not necessarily imply the temporary disappearance of permafrost. Continuous permafrost in southern England implies a minimum fall in mean annual air temperatures of 16-17 °C, and a fall in the July mean of 4-5 °C. During the maximum ice advance the fall in mean annual air temperatures is estimated to be as much as 25 °C, with a fall in the July mean of 10 °C.