Subsidence and associated ground movements on the Pennines, northern England

The Pennines form a range of hills in northern England. The elevated parts are known locally as ‘the moors’, and these run from the Scottish border in the north, southwards to the Midlands. The area is dominated by elevated moorland plateaux, which have been incised by glacial and alluvial valleys. The plateaux are capped by strong, massive, cross-bedded Kinderscout Grit of Late Carboniferous (Namurian) age (known also as the Millstone Grit). Interbedded weak shales, mudstones and siltstones crop out on the valley floor, lower slopes and middle slopes, which have been eroded by landslides. The moorland is covered by a veneer of head and peat deposits that may be up to 4 m thick. Two types of subsidence, with associated ground movements, have been observed: (1) the large-scale regional tilting, landsliding and apparent lateral spreading of periglacial moorland plateaux with associated fault scarps and fissures; (2) ‘sinkholes’ in peat, which occur as distinct subsidence depressions up to 2 m in diameter. As these ‘sinkholes’ are not generated by mining subsidence or by the dissolution of (for example, karst or gypsiferous) bedrock, the term pseudo-sinkhole has been introduced, for the purpose of this paper. Pseudo-sinkholes may be occasionally associated with peat slides, bog bursts, subsidence depressions and concentric ring fissures on peat scars. The extent and magnitude of the subsidence and ground movements vary considerably. These may affect small, localized peat-covered slopes, no more than a few metres wide, or influence the morphology of entire moorland slopes. Because of the relative remoteness of ‘the moors’ these types of ground movements do not influence structures, or affect people, and therefore tend to have not been previously investigated, documented or reported. Similar features have been reported in the South Wales Coalfield, where there is a long, complex mining legacy. Since the Kinderscout (Millstone) Grit and associated interbedded sedimentary rocks do not contain any minerals of economic significance, the observed ground movements cannot be attributed to mining subsidence. The Pennine moors therefore provide a unique opportunity to investigate subsidence (tilt), scarps and fissures in the absence of mining (or other human) influences. Subsidence in peat is likely to have been generated by the subsurface fluvial erosion of layered fibrous and amorphous peat deposits. This leads to the generation ofsubsurface voids (pipes), which subsequently undergo collapse, followed by the migration of the collapsed zone towards the ground surface. This results in the generation of crescent-shaped, concentric fissures, subsidence depressions, graben and pseudo-sinkholes. The mechanism for the large-scale regional tilting and subsidence of moorland plateaux is more difficult to determine and still not fully understood. These ground movements are complex and are associated with deep-seated landslides, complex fissure networks and, in the study area, a reactivated fault-scarp that is up to 4 m high and over 700 m long. This paper suggests that these movements were possibly generated under conditions of periglacial erosion and weathering, during glacier retreat, deglaciation and gravitational stress relief of valley sides. This is most likely to have occurred during the closing stages of the Pleistocene ice age. This may have initiated the lateral spreading of moorland plateaux, subsequently resulting in fissuring, fault reactivation, tilting and subsidence. The aim of this paper is to document and draw attention to the different types of subsidence and associated ground movements on Pennine moorland, and to suggest causal mechanisms.