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Discussion on indications of glaciation at the base of the Proterozoic Stoer Group (Torridonian), NW Scotland

S. Davison & M. J. Hambrey reply: We thank A.D. Stewart for his stimulating comments regarding our recent paper on Torridonian glacial deposits. However, there are several differences of observations and interpretations which need to be resolved.

(i) Roches moutonnées and stratigraphic relationships. The gneiss ridges shown in Stewart's fig. 1 are not, as he correctly observes, parallel and are not aligned in the direction of the proposed ice-flow. However, roches moutonnées do not necessarily have to be parallel to flow direction; indeed many are transverse. The critical observation is the contrast between up-glacier facing slopes, which are relatively smooth, and down-glacier slopes which are fractured. There is also a marked difference in clast size between the two slopes, those on the up-glacier side being generally smaller than those on the down-glacier side. We are unsure of the purpose of Stewart's fig. 2. It certainly shows the irregular nature of the Stoer Group/Lewisian contact, but this is a reflection of differential weathering/erosion of a foliated gneiss, whereas our photograph (fig. 3a) illustrates the form more clearly.

The suggestion that the ice smoothed surface in our fig. 3a is ‘far from smooth’ is, we feel, misleading. Whilst it is not flat and polished, it is rounded and, in a geomorphological sense, is smooth. It is certainly of a much smoother and lower profile than the surface on the SW side of this feature, which is brecciated with a matrix of Torridonian sediments.

The interpretation of these features as roches moutonnées is

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This article has been cited by other articles:

				R.C. Selley STEWART, A. D. 2002. The Later Proterozoic Torridonian Rocks of Scotland: their Sedimentology, Geochemistry and Origin.: Geological Society Memoir no. 24. vi + 130 pp. London, Bath: Geological Society of London. Price {pound}70.00, US $117.00; members' price {pound}35.00, US $58.00; AAPG members' price {pound}42.00, US $70.00 (paperback). ISBN 1 86239 103 3. Geological Magazine, May 1, 2003; 140(3): 358 - 359. [Full Text] [PDF]

				References Geological Society, London, Memoirs, January 1, 2002; 24(1): 119 - 125. [PDF]

				G. M. YOUNG Some aspects of the geochemistry, provenance and palaeoclimatology of the Torridonian of NW Scotland Journal of the Geological Society, December 1, 1999; 156(6): 1097 - 1111. [Abstract] [PDF]