Periglacial creep folds 223 LARGE-SCALE PERIGLACIAL CREEP FOLDS IN JURASSIC MUDSTONES ON THE DORSET COAST,UK R.W. GALLOIS INTRODUCTION There is no evidence to suggest that the SW region was ever covered by a continental ice sheet, but intense erosion in periglacial climates during the cold phases of the Pleistocene gave rise to many of its erosional and depositonal features (Te Punga, 1957). Freeze-thaw action in the form of frost- shattering and cryoturbation produced large amounts of weathered material that was widely redistributed by solifluction processes, meltwater torrents and strong arctic winds. These processes were repeated in each of the cold phases, and on numerous occasions within the cold phases, but the denudational features that can now be observed were probably largely formed during the most recent cold phase (Marine Oxygen Isotope Stage 2 (MIS 2)). The deposits themselves may be the result of one or many erosional cycles, each of which has added new material to that which had already accumulated. The effects of frost shattering are well displayed in many of the hard-rock areas of SW England, for example the rock faces and screes in the Valley of the Rocks in north Devon and the granite tors and the streams of rock debris that descend from them (Palmer and Neilson, 1962). Cryoturbation features such as frost-patterned ground and ice-wedge casts have been recorded on Dartmoor (Palmer and Neilson, 1962) and at coastal localities including Croyde Bay and in east Devon (Keene, 1990). Deposits that have been attributed to the mass-movement or in-situ disturbance of mudstones in periglacial environments under the action of freeze-thaw processes are common in the UK. They are especially numerous in the Midlands and southern England where strata weakened during earlier cold- climate phases were not removed by glacial erosion during the Gallois, R.W. 2010. Large-scale periglacial creep folds in Jurassic mudstones on the Dorset coast, UK. Geoscience in South-West England, 12, 223-232. The unglaciated part of southern England was subject to periglacial climates that lasted for more than 20 ka on at least eight occasions during the last 750 ka. There are large outcrops of Jurassic mudstones in south-west England, principally the Lias Group and the Oxford Clay and Kimmeridge Clay formations, and extensive exposures of these beds in the cliffs on the Dorset coast. Notwithstanding the susceptibility of this type of mudstone to permafrost damage and deformation, there is no published record of large-scale folding in the region that has been attributed to periglacial disturbance. Three examples of folding are described here, in the Lias Group at Charmouth and Seatown in west Dorset, and in the Kimmeridge Clay on the Isle of Portland that are attributed to intermittent downhill creep of surface layers up to 20 m thick when in a partially frozen condition. The style of folding in the mudstones and the geometry of the disturbed deposits indicates that they are not tectonic in origin, nor were they formed by valley bulging or landsliding. These are the first large-scale structures of their kind to be recorded in southern England: similar folds elsewhere have been interpreted as valley bulges or tectonic in origin. At the Seatown and the Isle of Portland localities, the deformed mudstones have been preserved beneath younger landslides. The absence of similar structures elsewhere on the Dorset coast is attributed to the rapid removal of similarly weakened materials by marine erosion at times of high sea level during the last c. 6000 years. 92StokeValleyRoad,Exeter,EX45ER,U.K. (E-mail: gallois@geologist.co.uk). Keywords: Jurassic, Dorset, Lias Group, Kimmeridge Clay Formation, periglacial, creep, folds. Devensian cold phases (MIS 2-4). Large-scale examples of in-situ deformation include valley bulging, first recognised by Hollingworth et al. (1944) in Lias Group mudstones in the Midlands, and the thermokarst ‘hills and holes’ topography of Fenland where springs that emerged from adjacent high ground enabled pingos to form (Sparks etal., 1972). Some of the large- scale folds in the glaciated parts of the UK, such as those described by Sainty (1949) in the Chalk of the NE Norfolk coast, were formed by subglacial tectonics beneath a thick ice sheet. The absence of glacial deposits in southern England excludes this mechanism as a possible cause of Pleistocene folding in southern England. Small-scale (mostly <3 m deep) in-situ periglacial features in mudstones in southern England include diapiric involutions in mudstones overlain by permeable deposits in valley floodplains (e.g. Gallois and Worssam, 1993, plate 14). Large-scale mass-movement deposits include the landslide complexes of the east Devon and Dorset coasts, many of which were initiated in the late Pleistocene (Brunsden and Jones, 1976). Four examples of folding in mudstones exposed on the Dorset coast (Figure 1) that can be attributed to the action of repeated freezing and thawing in a periglacial climate are described below. One of these has been interpreted as due to landsliding, and one as a possible valley bulge. The other two examples are not explicable in terms of tectonics, valley bulging, subglacial disturbance or landsliding. The term periglacial creep folds is proposed for these previously undescribed structures and for similar large-scale folds in mudstones elsewhere in southern England.