The ‘tunnel’ valleys of East Anglia and Fenland, UK: subaerial, not subglacial

Abstract

Integrated field surveys, continuously cored boreholes and seismic-reflection surveys in Fenland and the adjacent marine embayment of The Wash have revealed a network of concealed valleys. These are interpreted as a pre-glacial, cold-climate fluvial system that entered the North Sea via a gap in the Chalk escarpment at the mouth of The Wash. The pre-glacial Fenland valley system was overridden by an ice sheet during the Anglian Stage (Marine Isotope Stage (MIS) 12) when it became infilled with glacial deposits, mostly till and varved clay. East of the Chalk escarpment, a pre-glacial network of dipslope valleys drained directly to the North Sea. This was also overridden by Anglian ice and locally over-deepened, presumably by subglacial meltwater. The over-deepened parts have been referred to as tunnel valleys by various authors, but they can be more accurately described as glacially modified fluvial valleys. The analysis of high-resolution seismic-reflection profiles in the North Sea and adjacent areas has revealed complex anastomosing networks of tunnel valleys that were formed subglacially without a fluvial precursor or superimposed on possible fluvial systems that were glacially modified to the extent that they cannot now be confidently identified as fluvial in origin. In comparison, the concealed fluvial valleys of East Anglia and Fenland have been subject to minor glacial modifications, and are morphologically distinct from the North Sea examples. They should not be referred to as tunnel valleys. Prior to the Anglian glaciation, the rivers that flowed from the Midlands to the North Sea passed through one of two gaps in the Chalk escarpment, the Goring Gap in the south and a similar gap that is now concealed beneath the Wash. The field evidence does not support the presence of an additional gap in the intervening Bury St Edmunds area. A wide embayment to the north of this in the Thetford to Swaffham area in which the crest of the Chalk escarpment has been displaced eastwards by up to 20 km is an Anglian glaciation erosion feature.