Abstract

Mr A. W. Baird writes: Roberts, Smith & Harris (1984) have produced a significant piece of structural mapping in Glen Dessary but I cannot agree with their interpretation of its regional significance. I propose an alternative which does not involve the unlikely situation of regional scale vertical extension, and is more in keeping with the structural history obtained from studies nearby (e.g. Baird 1982; Powell et al. 1981).

Roberts et al. describe a series of D3 major folds with sub-vertical axial plane traces. The D3 Glen Dessary Synform has an intensely curvilinear hinge line with a sub-vertical extension (X) direction within its fold axial plane. All the D3 major folds east of the Glen Dessary Synform have sub-horizontal fold hinge lines. They attribute the change of hinge line orientation and development of curvilinear folds to a rapid increase in the amount of D3 strain westwards, and they link this strain variation to the generation of the ‘steep belt’, with the Loch Quoich line representing the eastern limit of intense D3 strain and upright reworking.

The post-D2 Glen Dessary Syenite was intruded at 456 ± 5 Ma. (van Breemen et al. 1979). Elsewhere in the Western Moine schists, D2 is regarded as a Precambrian (Grenville) tectonic event (Brook et al. 1976; Brook et al. 1977; Brewer et al. 1979; Powell et al. 1983). Therefore Roberts et al. argue that the production of the steep belt during D3 deformation must represent the first Lower Palaeozoic structural reworking of the Moine rocks. However

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