Over the years on our 10 Tors training expeditions we have come across the bumps on the north and north west side of the moor.
A couple of weeks ago Dave Berry took these photos and sent them to me in the hope I could find someone who knew what they were. As a result I sent the pictures to the archaeologists and nature conservation advisors in the National Trust and the Dartmoor National Park Authority.
Are they ant hills? Something archaeological like pillow mounds?
Well within hours Jim Parry the NT archaeologist has consulted with Janet Lister the NT nature conservation advisor and ….. concluded these bumps were of periglacial origin i.e. on the edge of areas where the glaciers in the last ice age got to and therefore areas which were subject to seasonal freezing and thawing.
A bit of googling and Jim found the answer – thufurs
‘A special form of cryoturbation is represented by earth hummocks or thufur, they are vegetated oval mounds with heights of 30 to 50 cm. Earth hummocks develop because of local patchy freezing of pore-water in the active layer, yet no pure ice core forms but a small proportion of the soil freezes. This core of frozen ground causes moisture migration towards it, concurrently causing a small scaled displacement of soil material in the same direction. Frost heaving leads to the development of a small mound, and as the freeze/thaw processes re-occur many times, this displacement is amplified.’
‘The picture above shows the internal structure of a hummock: an accumulation of humus can be discerned. As humus has a high capacity for storing water, it leads to the development of more ice in the earth hummock. Often being frozen, the decay of the organic material is slowed down, which in turn raises the amount of humus in comparison to unfrozen inter-hummock areas. The thick layer of humus also has an insulating effect, thus the formation of ice and its preservation are positively affected.’