Origin and structure of Devensian depressions at Letton, Herefordshire

Circular to oval enclosed depressions in soft sediments of Pleistocene age are normally interpreted as being either glacial or periglacial in origin. Where these features are developed in glacial sediments, a glacial (and specifically ‘kettle hole’) genesis is considered most likely. Some have been re-interpreted as periglacial in origin and are thought to be the remains of cryogenic mounds (former pingos or palsas/lithalsas). A group of enclosed depressions in the Letton area of Herefordshire within the Last Glacial Maximum ice limit have been investigated by electrical resistivity tomography and ground probing radar. Their morphology and internal structure, and their existence in glacio-lacustrine sediments of Late Devensian age suggests that they are kettle holes resulting from ice block discharge into shallow lakes. The lack of any ramparts and the fact that they do not overlap also indicate that they are not the remains of cryogenic mounds. There are numerous groups of circular to oval enclosed depressions situated in soft sediments of Pleistocene age in northwest Europe. In formerly glaciated terrain these have often been interpreted as kettle holes, a form of proglacial feature resulting from the burial of ice blocks and their subsequent melt-out, which causes localised ground subsidence (Maizels, 1977). Since these features are relatively common and merely confirm the role of glaciation in the landscape, they have not generally been investigated in detail. Nevertheless, some Last Glacial age kettle holes in Britain are notable for the organic materials that were preserved within them, such as woolly mammoth (Mammuthus primigenius (Blumenbach), Allen et al, 2009). In the 1960s, interest in enclosed depressions increased following the publication of investigations of such features in Belgium and Wales (Pissart, 1956, 1963). The features were interpreted not as kettle holes, but as the remains of former pingos mounds or small hills that develop in permafrost through the growth of a core of ice (Mackay, 1998). Pissart referred to them as ‘relict pingos’, and their distribution appeared to provide a means of reconstructing the former distribution of permafrost (Washburn, 1983). Distinguishing kettle hole depressions from relict pingos became essential. It was determined that a key diagnostic criterion for relict pingos was the existence of a raised rim or rampart around the depressions (Watson, 1971; Watson and Watson, 1974; Sparks et al, 1972). At that time, kettle holes were not generally believed to display this attribute (cf. Maizels, 1992). The ramparts were believed to have formed by material gradually slipping off the sides of the mound and/or from a displacement of material from the interior of the mound during the growth of the ice core (Mackay, 1988). Modern arctic pingos of the hydraulic type were originally seen as the modern analogue (Gurney, 1998), although more recently a palsa (Gurney, 2001) or lithalsa (Pissart, 2002) analogue has been invoked. At some sites the rampart around the features was incomplete or even completely absent (Gurney, 2000). The rampart may have originally existed but had been lost through subsequent land use (ploughing, drainage operations etc). Such rampart-less features require information on the sub-surface to enable correct interpretation. Although research has tended to focus on either glacial or periglacial origins, it must be remembered that there are many other explanations for such Figure 1. Feature #30; this depression has been dug out so that it now forms a perennial pond; it does however indicate the typical size and shape of the depressions at Letton; see Figure 2 for its exact location. MERCIAN GEOLOGIST 2010 17 (3) 181 enclosed depressions. These include dissolution (to form dolines), nivation, subsidence, mineral workings (e.g. collapsed flint mines, marl pits etc) and bomb craters. Detailed discussions of the possible origins for the enclosed depressions of Norfolk are given by Prince (1961, 1964) and West (1987). Until recently the investigations of these features primarily involved mapping and the excavation of trenches through the features. A more sophisticated and less invasive approach could now include the use of geophysical techniques to determine the sub-surface structure of the depressions in combination with mapping. To date only a few sites in west Wales have been subject to such an approach, at Cledlyn (Harris, 2001) and at Llanio Fawr (Ross et al, 2007). The investigations at the latter site favoured a glaciofluvial origin.