Engineering works carried out in central London have unearthed drift-filled hollow features which exhibit curious characteristics.
They extend deep into the bedrock geology and are infilled with disturbed superficial deposits and highly weathered bedrock.
Up to 500 m wide and 60 m in depth these features are a significant consideration for developers not only during construction but for the long-term use of the site.
In 1980, Hutchinson, a geologist at Imperial College, investigated the occurrence of these features in central and southern England and suggested that they may be relic 'pingo' features developed in former periglacial and glacial areas.
Active pingos such as those found in the Tuktoyaktuk Peninsula area of Canada, comprise ice-cored hills that are typically conical in shape. They form as a consequence of the freezing of water, which moves under a pressure gradient to the site and are therefore associated with groundwater discharge zones.
Along the Golmud to Lhasa railway route (Tibetan plateau, China) active, migrating pingos, associated with fault zones, cause structural damage.
With the more recent discovery of similar hollow features near the Blackwall Tunnel and in the Lea Valley the BGS is undertaking a project to look at the occurrence and genesis of these geological features in central London.
The association of these features with faulting and the hydro-geological environment in which they are found will be investigated in an attempt to define their likely geographical extent.
Royse, K R, De Freitas, M, Burgess, W, Cosgrove, J, Ghail, R, Gibbard, P, King, C, Lawrence, U, Mortimore, R, Owen, H and Skipper, J. In press. Geology of London, UK. Proceedings of the Geologist Association
Royse, K R. 2008. Unlocking the potential of 3D geology. Geoconnexion, 1, 11–13.
Ellison, R A, Woods, M A, Allen, D J, Forster, A, Pharaoh, T C and King, C. 2004. Geology of London. British Geological Survey.