A 35-year record of solifluction in a maritime periglacial environment

Eight segmented PVC columns (Rudberg pillars) inserted vertically in the treads of three vegetation-covered (‘turf-banked’) solifluction lobes at altitudes of 912–1031 m in the Fannich Mountains of NW Scotland were exhumed 35 years after insertion, and downslope displacement of each segment was measured to derive velocity profiles for each site. Data from these profiles yielded average surface velocities of 7.8–10.6 mm a-1 (mean 8.8 mm a-1), average volumetric velocities of 8.3–13.3 cm3 cm-1 a-1 (mean 10.5 cm3 cm-1 a-1) and maximum displacement depths of 290–445 mm (mean 390 mm). Measured volumetric velocities for these maritime periglacial sites are fairly similar to those recorded in high alpine environments, but markedly less than most reported rates for solifluction in areas of warm permafrost or deep seasonal freezing. Movement affects only the uppermost parts of individual lobes, and the measured volumetric velocities imply either very slow advance of lobe fronts (~ 0.7 mm a-1) or slow thickening and steepening of stationary lobe risers. Velocity profiles decline approximately exponentially with depth over the depth range 50–400 mm, consistent with movement by frost creep alone or frost creep plus gelifluction. Comparison with measured rates of periglacial mass transport elsewhere on British mountains suggests (1) that, contrary to traditional views, surface velocities are similar to (and may exceed) those of ploughing boulders in the same area, and (2) that both surface velocities and volumetric velocities are markedly less than at unvegetated sites where needle ice creep is the dominant component of solifluction.