Snow-push processes in pronival (protalus) rampart formation: Geomorphological evidence from Smorbotn, Romsdalsalpane, southern Norway

It is demonstrated that pronival (protalus) ramparts can be formed by a sno w-push mechanism and need not accumulate in the conventional manner as a re sult of supranival processes. Ridges in pronival positions up to 1.2 m high and of unequivocal snow-push origin are described from two sites in Smorbo tn cirque, Romsdalsalpane, southern Norway. The seven lines of evidence are : (1) parallel abrasion tracks on large boulders embedded in the substrate; (2) displaced surface and embedded clasts with proximal furrows; (3) corru gated (flute-like) substrate surfaces; (4) the sickle-shaped plan-form of t he ridges; (5) generally asymmetrical ridge cross-profiles (shallow, concav e proximal; steep, convex distal); (6) strong preferred orientations and di ps of surface-embedded clasts on ridge proximal slopes; and (7) a subnival/ pronival ridge comprising loosely packed diamicton forming along the contac t zone between the snowbed and substrate. This evidence indicates ridge for mation by snow sliding involving bulldozing of the substrate. Factors consi dered important in favouring snow push producing distinct pronival ramparts at the sites include: a maritime periglacial climate with heavy winter sno wfall and rapid snow-firn conversion producing snow densities of up to 900 kg m(-3); a deformable substrate with relatively small inputs of rockfall o r avalanche debris; and a steep headwall susceptible to snow avalanching an d hence enhanced snow supply. Consideration is given to the prospect that l arger pronival ramparts can form incrementally by a snow-push mechanism.