Talus structure and evolution on sandstone mountains in NW Scotland

Abstract

Twelve gully-wall sections excavated in the upper rectilinear slope of three talus accumulations in NW Scotland reveal a common stratigraphy, similar to that previously described elsewhere in upland Britain: a lower unit of in situ rockfall debris and an upper unit of debris-flow and/or slopewash deposits, often overlying or intercalated with buried organic soils. The upper unit indicates downslope reworking of sediments derived from the top of the talus. Radiocarbon dating of buried organic soils implies that the onset of talus reworking was asynchronous, and occurred before 6.6—7.0 cal. kyr BP at one site (roughly 5.0 kyr after deglaciation) but after c. 2.7 cal. kyr BP at another. A general model for talus evolution in upland Britain is proposed, based on the geomorphological, stratigraphic and dating evidence of this and earlier studies. This comprises: (1) initial accumulation of rockfall debris within a few millennia following deglaciation; (2) near-cessation of rockfall accumulation and development of gullies near the talus crest, with concomitant redistribution of eroded talus sediments on the upper slope by debris flows and slopewash; (3) downslope extension of gullies with associated erosion of both talus and overlying debris-flow and slopewash deposits, and redistribution of these sediments as slope-foot debris cones. This sequence can be explained as an intrinsic response to diminution of paraglacial rockfall inputs, and does not necessarily imply any change in climate, frequency of extreme rainstorm events or other extrinsic effects.