The glacial origins of relict ‘pingos’, Wales, UK

Abstract

Ramparted depressions (doughnut-shaped debris-cored ridges with peat- and/or sediment-filled central basins) are commonly perceived to represent the relict collapsed forms of permafrost ground-ice mounds (i.e. pingos or lithalsas). In Wales, UK, ramparted depressions of Late Pleistocene age have been widely attributed to permafrost-related processes. However, a variety of alternative glacial origins for these enigmatic landforms are also consistent with the available geological and geomorphological evidence, although previous studies have barely considered such alternative processes of formation. From detailed geophysical, sedimentological and remote-sensing studies at two field sites, we demonstrate that: (i) the wastage of stagnating glacier ice is a viable alternative explanation for the formation of ramparted depressions in Wales; (ii) the glacial geomorphology and geology of these landforms is analogous to supraglacial and subglacial landforms from the last Laurentide and Fennoscandian ice sheets; (iii) these landforms have significant potential for characterising the nature of deglaciation around the margins of the Irish Sea during the last glacial cycle, and may record evidence for the overextension and stagnation of the south-eastern margin of the Irish Sea Ice Stream; and (iv) investigations of ramparted depressions within formerly glaciated terrains must consider both glacial and periglacial mechanisms of formation.

Keywords

• glacial geology
• permafrost
• pingo
• near surface geophysics
• stagnant ice
• Quaternary