Pronival rampart formation in relation to snow-avalanche activity and Schmidt-hammer exposure-age dating (SHD): Three case studies from southern Norway

Abstract

Pronival (protalus) ramparts at three sites in southern Norway—Smørbotn, Alnesreset and Nystølsnovi—are assessed in relation to evidence of snow-avalanche activity, and dated using Schmidt-hammer exposure-age dating (SHD). The position of each rampart suggests frequent snow avalanches played a major role in rampart formation. The pronival ramparts at Smørbotn and Nystølsnovi are characterized by a high proportion of unweathered, very angular clasts, many clean boulders (without any lichen or moss cover), steep distal slope angles, and a disturbed ground cover of vegetation with characteristic alpine pioneer plant species on crests and proximal slopes. High Schmidt-hammer R-values (mean 56.2–58.5) yielded SHD ages of <2900years and <1550years, respectively. These ramparts are currently active and have probably developed since the early Holocene under environmental conditions similar to those at present. Pronival ramparts at Alnesreset are characterized by highly weathered, mainly angular boulders (virtually none of which is clean), a mature, undisturbed vegetation cover, and lower distal slope angles. Mean R-values of 39.9–40.3 from these ramparts yielded SHD ages within the range 9450 to 14,650years (with 95% certainty), indicating formation during the Younger Dryas Stadial and relict status for most of the Holocene. Our results: (1) show that rockfall is not the only primary process involved in the formation of pronival ramparts; (2) broaden the range of landforms included in snow-avalanche geomorphology; (3) demonstrate the feasibility of dating pronival ramparts; and (4) indicate their potential in palaeoenvironmental reconstruction.