THE STRUCTURE AND SEDIMENTOLOGY OF RELICT TALUS, TROTTERNISH, NORTHERN SKYE, SCOTLAND

Sections up to 3.5m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpme nt reveal stacked debris-flow deposits intercalated with occasional sl opewash horizons and buried organic soils. Radiocarbon dating of burie d soil horizons indicates that reworking of sediment by debris flows p redates 5.9-5.6 Cal ka BP, and has been intermittently active througho ut the late Holocene. Particle size analyses of 18 bulk samples from t hese units indicates that c. 27-30 per cent of the talus deposit is co mposed of fine (<2mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insig nificant in generating fine material. We conclude that the fine sedime nt within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse d ebris, implying that c. 27-30 per cent of rockwall retreat since degla ciation reflects granular weathering rather than rockfall. The abundan ce of fines within the talus deposits is inferred to have been of crit ical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upp er parts of the slope. A wider implication of these findings is that t he mechanical properties of talus slopes cannot be regarded as those o f free-draining accumulations of coarse clasts, and that models that t reat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the u pper straight slope on talus accumulations as the product of mass-tran sport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. (C) 1998 John W iley & Sons, Ltd.