Paraglacial rock slope failure as an agent of glacial trough widening

Abstract

Abstract Rock slope failure (RSF) generates the largest single erosional events in the glacial–paraglacial land system, leaving numerous obvious cavities and less obviously weakened valley walls. Its contribution to trough widening in a mountain range has not previously been systematically quantified. Map-based measures of RSF ‘depth of bite’ are applied to five sample areas in the Scottish Highlands, and a comparator area in north Norway, all in metasediments structurally conducive to mass deformation and block sliding. Problems in applying map-based measures include bedrock cavities remaining partially occupied by failed debris or subsequent infill, and multiple planes of reference. The most practical measure is of maximum recess depth on any single contour ( D MAX ). This is a standardizable single-point indicator of visible impact, not a measure of actual cavity depth, nor an average applying to the whole RSF. In four of the five areas, average D MAX is consistent at 40–45 m. RSF breadth averages 270–600 m over the five areas. RSF affects 9% and 14% of total valley wall length in the two densest RSF areas, rising to 47% and 52% on two specific valley sides. The depth:breadth ratio in areas dominated by slope deformation can be twice that in areas of translational sliding. An evolutionary model of glacial–paraglacial cycling proposes a ‘zone of paraglacial relaxation’ in which RSF is intense in early cycles as fluvial profiles adjust to ice discharge, diminishing with maturity as trough walls become stress-hardened, and reviving in response to neotectonic and glaciological perturbations, notably ice piracy via transfluent breaching. However, a major unknown is the efficacy of glacial exploitation of RSFs: if it takes several cycles to evacuate debris and pare back cavity angles, cumulative RSF impact is lessened. Glacial–paraglacial cycling is a classic positive feedback loop, promoting valley widening beyond the parabolic norm. Preferential exploitation of structure by RSF promotes asymmetrical trough profiles. RSF acts both as a scarp retreat process, and as a slope reduction counterpoint to glacial slope steepening. In landscape evolution, it is a powerful agent in destruction of paleic relief, notably around watersheds that are undergoing breaching by transfluent ice, where trough development and widening is still vigorous.