Pebbly versus bouldery rock glaciers: Morphology, structure and processes

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Differences in rockwall geology cause two types of rock glaciers: a bouldery rock glacier, having the active layer composed of matrix-free boulders derived from crystalline rocks and massive limestone; and a pebbly rock glacier, consisting of matrix-supported debris derived from less resistant shale and platy limestone. Such material composition controls transport processes responsible for the shape, size and internal structure of the two types of rock glaciers. This paper compares the major processes controlling the morphology and structure of the two types in the Swiss Alps, based on mapping, description of geological and morphological characteristics, direct observation of stratigraphy and geophysical soundings of internal structure. In the Swiss Alps, pebbly rock glaciers are distinguished from bouldery rock glaciers by the clast size of 15–20 cm in the mean b-axis diameter. The former are fed by small-scale rockfalls, debris flows and solifluction, whereas the latter mainly originate from large rockfalls. Pebbly rock glaciers are generally smaller (< 200 m in length) than bouldery rock glaciers, because the small exposure of the less resistant source rockwall (< 50 m in height) strongly constrains debris supply. As a result, pebbly rock glaciers usually terminate within a valley-side slope, whereas many bouldery rock glaciers extend into the valley bottom. The location of pebbly rock glaciers results in the low frontal slope (≤ 20 m) and the lack of transverse ridge-furrow topography, because of the lack of compressive flow. The pit-borehole stratigraphy and low DC resistivities (< 10 kΩm) indicate ice-cemented or slightly supersaturated permafrost in the pebbly rock glaciers, which presumably originates from groundwater freezing. In contrast, highly ice-supersaturated structure indicated by high DC resistivities (> 100 kΩm) in bouldery rock glaciers appears to originate from snow banks buried with deposits of large rockfalls from the large source rockwall.