Protalus rampart development and the limits of former glaciers in the vicinity of Baosbheinn, Wester Ross

Abstract

Synopsis J. B. Sissons’ interpretation of a massive debris accumulation at the NW end of the Baosbheinn ridge in terms of two stages of protalus rampart development is shown to be inconsistent with the field evidence, and palaeoclimatic inferences and rockwall retreat rate calculations based on this interpretation are consequently invalid. Part of the debris accumulation is reinterpreted as two lateral moraine fragments that were deposited at the southern limit of westwards-moving ice from the Loch Maree trough at the maximum of the Wester Ross Readvance, some time before c. 13,000 B.P. During the Loch Lomond Stadial of c. 11,000–10,000 B.P. two valley glaciers advanced northwards on either side of Baosbheinn into the area formerly occupied by Wester Ross Readvance ice and truncated the earlier readvance limit, thereby isolating the lateral moraine fragments on the flanks of Baosbheinn. The upper part of the debris accumulation is apparently a giant protalus rampart of Loch Lomond Stadial age, but various characteristics of this feature suggest that unlike other Lateglacial ramparts it was not formed by slow rockfall accumulation. Instead, it probably formed when a massive rockslide or series of rockslides involving failure of c. 200,000 m 3 of rock over-rode a former snowbed and accumulated as a boulder ridge that partly buried the higher lateral moraine fragment.

Winter-talus ridges, nivation ridges, and pro-talus ramparts

“…Similar definitions have been proposed by several authors in the past, including the present writer (Ballantyne, 1986;Ballantyne and Kirkbride, 1986). As noted by 10hnson (1983), however, the validity of this mode of accumulation tends to have been assumed rather than demonstrated, as most accounts of protalus ramparts concern relict (usually late Pleistocene) examples.…”

Exposure dating and reinterpretation of coarse debris accumulations (‘rock glaciers’) in the Cairngorm Mountains, Scotland

“…Sissons, 1975Sissons, , 1979aSissons, , 1980Dawson, 1977;Chattopadhyay, 1984;Ballantyne, 1986a;Wilson, 1990aWilson, ,b, 1993Maclean, 1991;Sandeman and Ballantyne, 1996;Anderson et al, 1998;Harrison et al, 2008). These features have usually been attributed to the Loch Lomond (Younger Dryas) Stade of ca.…”

The schmidt hammer, weathering and rock surface roughness

“…It records the distance of rebound of a spring-loaded mass (Day and Goudie, 1977). It is commonly used as a field measure of the hardness of natural rock surfaces (Deere and Miller, 1966;Barton and Choubey, 1978;Day, 1980;Selby, 1980;Summerfield and Goudie, 1980;Sheorey et al, 1984;Whitlow and Shakesby, 1988) and as an index of degree of rock surface weathering (Barton and Choubey, 1978;Day, 1980;Matthews and Shakesby, 1984;Ballantyne, 1986;Dawson et al, 1986;Matthews et al, 1986;Shakesby et al, 1987;Sjoberg, 1987aSjoberg, , 1987bSjoberg, , 1987cCampbell et al, 1987;McCarroll, 1987McCarroll, , 1989aMcCarroll, , 1989bBallantyne et al, 1989). Although it is logical to expect rock weathering to result in a loss of surface strength and therefore a decline in Schmidt hammer rebound (R-) values, a clear correlation of R-values and an independent measure of degree of weathering has not been demonstrated.…”