GLACIOLOGICAL CONSTRAINTS ON PROTALUS RAMPART DEVELOPMENT

The growth of protalus ramparts is self-limiting, as progressive thick ening of a firn field will increase basal shear stress, encouraging cr eep of ice and basal sliding, and thus cause the transformation of sta tionary firn into a small glacier with consequent destruction or modif ication of the rampart. Modelling df the threshold conditions under wh ich significant movement of the firn field begins indicates that the m aximum distance between the rampart crest and the talus foot upslope c annot exceed c. 30-70m, the limiting value being dependent on the grad ient of the firn field. This finding implies that ridges or ramps of d ebris lying at greater distances from the talus foot cannot be 'true' protalus ramparts formed by debris descending an essentially stationar y firn field, and hence that many landforms previously described as pr otalus ramparts have been misinterpreted. Modelling of the limiting co nditions for gravitational transport of debris suggests that clasts ma y 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 rea ches a position 85-125m from the talus foot. This implies the possible existence of 'protalus moraines' whose form is influenced by moving i ce, yet which continue to receive a supply of debris by direct gravita tional transport across the ice surface.