Snow‐push processes in pronival (protalus) rampart formation: geomorphological evidence from smørbotn, romsdalsalpane, southern norway

Abstract

It is demonstrated that pronival (protalus) ramparts can be formed by a snow‐push mechanism and need not accumulate in the conventional manner as a result 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 Smørbotn 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) corrugated (flute‐like) substrate surfaces; (4) the sickle‐shaped plan‐form of the ridges; (5) generally asymmetrical ridge cross‐profiles (shallow, concave proximal; steep, convex distal); (6) strong preferred orientations and dips of surface‐embedded clasts on ridge proximal slopes; and (7) a subnival/pronival ridge comprising loosely packed diamicton forming along the contact zone between the snowbed and substrate. This evidence indicates ridge formation by snow sliding involving bulldozing of the substrate. Factors considered important in favouring snow push producing distinct pronival ramparts at the sites include: a maritime periglacial climate with heavy winter snowfall and rapid snow‐firn conversion producing snow densities of up to 900 kg m^–3; a deformable substrate with relatively small inputs of rockfall or avalanche debris; and a steep headwall susceptible to snow avalanching and hence enhanced snow supply. Consideration is given to the prospect that larger pronival ramparts can form incrementally by a snow‐push mechanism.

Key words:

• snowslide
• pronival rampart
• protalus rampart
• nivation