Earth Hummocks in West Dartmoor, SW England: Characteristics, Age and Origin James Killingbeck and Colin K. Ballantyne* School of Geography and Geosciences, University of St Andrews, St Andrews, UK ABSTRACT Extensive fields of vegetated, dome-shaped earth hummocks 5–35 cm high and typically 80–200 cm in diameter occur on slopes of up to 15  at elevations of 320–440 m asl on Dartmoor, SW England, but are limited to terrain underlain by metasedimentary rocks and dolerite, and absent from granite areas. Hummocks occur within archaeo- logical sites, implying formation within the last ~ 3000 years and ruling out development over permafrost. The hummocks are composed of frost-susceptible silty soil (modal grain size 10–100 mm) with occasional clasts, but exhibit no evidence for cryoturbation or diapirism. Hummock size and spacing are fairly consistent at particular sites but vary between sites. Hummock age, distribution and characteristics are incompatible with non-frost action origins and most proposed frost action origins (cryostatic pressure, soil circulation, permafrost aggradation, soil injection), but regular spacing and granulometry favour initiation by differential frost heave under conditions of periodic shallow (0.2–0.4 m) seasonal ground freezing. We suggest that hummock growth may reflect migration of silt in advance of inclined freezing planes until equilibrium is achieved with soil loss down hummock sides. Our results confirm that hummock formation by frost action occurs on silty soils in humid cool temperate climates with only limited seasonal frost penetration. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS: earth hummocks; differential frost heave; frost susceptibility; seasonal ground freezing; loess; patterned ground INTRODUCTION The term earth hummock describes patterned ground comprising vegetated mounds, typically 0.1–0.6 m high and 0.5–2.0 m wide, separated by a network of troughs. Earth hummocks are widespread in arctic tundra environ- ments (Tarnocai and Zoltai, 1978; Kojima, 1994; Figure 1a), in subarctic permafrost areas south of the treeline (Kokelj et al., 2007), on subarctic, alpine and subalpine mountains (Scotter and Zoltai, 1982; Grab, 1994; Mark, 1994; Scott et al., 2008) and in maritime periglacial environments such as Iceland, where they are referred to as thúfur and occur in permafrost-free areas (Schunke, 1977; Gerrard, 1992; Van Vliet-Lanoë et al., 1998; Figure 1b). Earth hummocks usually occur as ‘hummock fields’ on flat or gently sloping terrain. Where the gradient exceeds 3–6  , some hummocks grade downslope into relief stripes (Lundqvist, 1962; Nicholson, 1976; Mark, 1994). Elsewhere hummocks occur on slopes of up to ~ 15  , and are sometimes termed slope hummocks (Lewkowicz and Gudjonsson, 1992; Lewkowicz, 2011). In Great Britain, earth hummocks occur mainly on uplands (Ball and Goodier, 1970; Tufnell, 1975; Ballantyne, 1984, 1986; Ballantyne and Harris, 1994) but extend locally below 200 m asl in northern England (Cotton, 1968; Pemberton, 1980). Hummocks in upland Britain rarely exceed 0.3 m in height or 1.5 m in diameter, and thus are generally smaller than those in arctic or subarctic environments (Schunke and Zoltai, 1988; Grab, 2005a). They also differ in form. Arctic hummocks and Icelandic thúfur tend to be knob-shaped and closely spaced (Figure 1a and b), whereas those in upland Britain typically form domes separated by broad depres- sions (Figure 1c and d). Most authors have interpreted hummocks in upland Britain as relict, lateglacial landforms (e.g. Chattopadhyay, 1982; Ballantyne, 1986), but Pemberton (1980) and Tufnell (1975) have described evidence for hummock formation in Britain within the last two centuries. Some of the most extensive hummock fields in Britain are located in west Dartmoor, SW England (Figures 1c, d and 2). The Dartmoor hummocks have been variously attributed to: (1) the injection of soil into hummocks by ear- lier or deeper freezing of inter-hummock depressions; (2) the accumulation of soil or windblown sediment around vegetation clumps; and (3) selective erosion of soil between hummocks by slopewash (Gerrard, 1988; Bennett et al., * Correspondence to: Professor C. Ballantyne, School of Geography and Geosciences, University of St Andrews, St Andrews KY16 9AL, UK. E-mail: ckb@st-andrews.ac.uk PERMAFROST AND PERIGLACIAL PROCESSES Permafrost and Periglac. Process. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ppp.1739 Copyright © 2012 John Wiley & Sons, Ltd. Received 12 October 2011 Revised 2 February 2012 Accepted 12 February 2012