Earth hummocks in west Dartmoor, SW England:characteristics, age and origin

• Published in: Permafrost and Periglacial Processes
• Main Authors: Killingbeck, James, Ballantyne, Colin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: permafrost; Permafrost and Periglacial Processes
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/earth-hummocks-in-west-dartmoor-sw-england(fe3901dd-861c-48ef-9fb9-ea616d01adf8).html; https://doi.org/10.1002/ppp.1739

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 altitudes of 320-440 m on Dartmoor, SW England, but are limited to terrain underlain by metasedimentary rocks and dolerite, and absent from granite areas. Hummocks occur within archaeological 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 µm) 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.