Periglacial discontinuities observed in Jurassic mudstones between Buckinghamshire and Warwickshire

Evidence for periglacial weathering in the shallow subsurface is widespread throughout the lowland southern British Isles. However, the geological indicators can be quite subtle and not always apparent during ground investigation. Narrow (0.3 – 0.6 m wide) trial pit excavations are a cost-effective way to investigate periglacial discontinuities, but they can mechanically disturb the ground and obscure the geometry of small-scale features. The aim of this study was to identify and record discontinuities, including relict shear surfaces in periglacially affected ground along the route of the HS2 railway between Wendover in Buckinghamshire and Southam in Warwickshire. Five case studies in full-scale earthwork excavations show that observed sheared surfaces occurred at a range of scales between millimetres to single discontinuities with a persistence of 10 or more metres. All individual discontinuities occurred within fields of disturbed ground extending up to hundreds of metres. The observations show that the Spink (1991) schema developed for Eocene clays is broadly applicable to Jurassic-aged mudstone strata examined across an extensive length of excavations (∼60 km).