Internal Structure and Geological Context of Ramparted Depressions, Llanpumsaint, Wales

In Europe, ramparted depressions have traditionally been interpreted as the relict forms of periglacial ground‐ice mounds. In many cases, however, such interpretations have been based on limited subsurface evidence. We present detailed sedimentological and geophysical investigations of ramparted depressions from Llanpumsaint, Wales. These data are used to establish internal structure and to evaluate possible mechanisms for landform formation. Borehole and geophysical data have revealed a thick (∼30 m) sequence of glaciolacustrine sediments beneath the study site. The geological context (drainage of a large proglacial lake) would have been conducive to the formation of: (i) permafrost‐related ground‐ice mounds, at times when exposed frost‐susceptible glaciolacustrine sediments were subject to permafrost aggradation; and (ii) craters associated with the in‐situ meltout of blocks of glacier ice grounded in the lake during periods of falling water levels. Rampart deformation structures are consistent with both models, but units of sand and gravel within the ramparts favour a hypothesis that these landforms represent the collapsed remains of ground‐ice mounds. This study highlights the importance of recognising and evaluating all possible (periglacial and non‐periglacial) models for the development of ramparted depressions. We recommend that future studies carefully consider all possible mechanisms of formation, particularly where subsurface information is limited. Copyright © 2011 John Wiley & Sons, Ltd.