Development of a ‘soft deforming bed’ within a subglacial shear zone : an example from Bacton Green.

The concept of ‘soft deforming beds’, which has developed progressively over the past 25‐30 years, has revolutionised our understanding of glacier‐bed interactions (Boulton, 1986; Boulton and Hindmarsh, 1987; Murray, 1997). Many studies demonstrate the complex role played by ‘softdeforming beds’ in controlling ice‐mass behaviour from both modern and geological examples. However, despite a wealth of literature examining ‘soft deforming beds’ a number of questions and controversies are still to be resolved. In particular, the precise role played by porewater and its spatial and temporal distribution within the ‘soft deformable bed’ is generally poorly understood. This has implications for the amount and distribution of porewater and deformation within the bed, how it varies in time and space, and its role in controlling glacier behaviour. In northern East Anglia, ‘soft deformable beds’ of various scales have been recognised at a number of sites (Banham, 1977; Hart, 1987; Hart et al., 1990; Hart and Boulton, 1991a,b; Hart and Roberts, 1994; Roberts, 1995; Fish et al., 2000; Lee, 2001, 2003, 2009; Roberts and Hart, 2005; Pawley, 2006; Hart, 2007; Lee and Phillips, 2008; Phillips et al., 2008; Waller et al., 2009). In this chapter, we examine a 14 m thick ‘soft deforming bed’ at Bacton Green, north Norfolk described previously by Lee and Phillips (2008). The site isn’t visited during the course of the workshop, although the deformation does occur at the same stratigraphic level, and forms part of the same shearing event, that formed the subglacial shear zone at West Runton described later (see Chapter 6; Phillips et al., 2008).