Theme lecture: Periglacial and slope processes

Abstract

Following a palaeoclimatic outline of the Late Quaternary, the paper reviews the periglacial and slope processes which have most effect on engineering works, particulary with regard to relic forms of such features in Britain. The first topics covered are; frost heave and thaw consolidation, thermokarst and periglacial mass movements, with particular attention to periglacial solifluction and slope development. Ground water discharge features, comprising pingos, anomalous depressions in the London Basin and perforated clay feather edges, are then discussed, as are superficial valley disturbances in various geological settings. The paper concludes by exploring theoretical and geological approaches to the determination of the former depths of permafrost in Britain.

Anomalous buried hollows in London: development of a hazard susceptibility map

“…groundwater head exceeds ground elevation) with the potential to supply groundwater to the surface via a pathway such as a pipe or root. The association of the buried hollows with the feather edge of the London Clay Formation where the chalk confining layer is thinner further suggested to Hutchinson (1980Hutchinson ( , 1991 that these features are associated with zones of former artesian groundwater conditions. The occurrence of bedrock diapirs in several of the hollows (Berry 1979) is also indicative of excess pore water pressures.…”

“…Summary information for each of the identified features is provided in the Appendix. Based on the work of Hutchinson (1980Hutchinson ( , 1991 a series of 'contributory factors' relating to the geological and hydrogeological setting of the hollows were developed to constrain their likely distribution, with the intersection of these rules providing areas where these features are most likely to be encountered. The rules for the component layers were developed within a GIS (Table 3) and were defined using the original set of 31 buried hollows.…”

“…However, as noted by Hutchinson (1991) the actual uplift McMillan et al 2011) pressures required to generate uplift of the Lambeth Group were considerably higher than the Historic Maximum Value (HMV). Higher artesian pressures could result from both melt water recharge and the relative lowering of base level.…”

“…This interpretation assumes that they formed during the growth and decay of permafrost. More recently, they have been interpreted as the scars of hydraulic pingos that were fed by artesian water (Hutchinson 1980(Hutchinson , 1991. Pingos are periglacial ice mounds developed in areas of permafrost or discontinuous permafrost (Gurney 1998).…”

“…They have two main genetic forms: (1) hydrostatic (closed) pingos where an isolated lens of unfrozen water-laden sediments, such as a sub-channel talik, provides the source of water for the growth of the pingos' ice-core; (2) hydraulic (open) pingos where sub-permafrost or intra-permafrost groundwater flow, which is recharged from areas of discontinuous permafrost, sustains the pingo ice growth (Mackay 1985). Hutchinson (1980Hutchinson ( , 1991 identified a number of common geological and hydrogeological characteristics associated with the distribution of the hollows. These can be summarized as follows.…”