Glaciological constraints on protalus rampart development

The growth of protalus ramparts is self‐limiting, as progressive thickening of a firn field will increase basal shear stress, encouraging creep of ice and basal sliding, and thus cause the transformation of stationary firn into a small glacier with consequent destruction or modification of the rampart. Modelling of the threshold conditions under which significant movement of the firn field begins indicates that the maximum distance between the rampart crest and the talus foot upslope cannot exceed c. 30–70 m, the limiting value being dependent on the gradient of the firn field. This finding implies that ridges or ramps of debris lying at greater distances from the talus foot cannot be [true] protalus ramparts formed by debris descending an essentially stationary firn field, and hence that many landforms previously described as protalus ramparts have been misinterpreted. Modelling of the limiting conditions for gravitational transport of debris suggests that clasts may continue to reach the front of a steep incipient glacier even after ice movement has been initiated, at least until the toe of the ice reaches a position 85–125 m from the talus foot. This implies the possible existence of [protalus moraines] whose form is influenced by moving ice, yet which continue to receive a supply of debris by direct gravitational transport across the ice surface.