An Fharaid Mhór

[NC 060 244]

Potential GCR site

Description

Sabine (1952) cut a section of a dyke-rock, from the Geological Survey's collections, that had been found cutting Lewisian gneisses near Achmelvich, WNW of Lochinver (Figure 7.13). Peach et al. (1907) thought it was equivalent to the Canisp Porphyry found at the Laird's Pool, Lochinver (Figure 7.13), but Sabine discovered that it is a nepheline-bearing rock similar to the dykes at Camas Eilean Ghlais. Sabine (1952) provides a sketch-map of the locality, and a detailed map of the Achmelvich peninsula, showing several slightly sinuous 'ledmorite' dykes striking approximately 110°, is given by Barber et al. (1978). The dykes occur on the An Fharaid Mhor-Clachtoll Lewisian GCR site and an example can be found on the shore of Loch Roe [NC 060 244]. A second dyke can be reached more easily by descending a gully in the cliffs on the western side of An Fharaid Mhór [NC 053 244], where it is about 1–2 m thick. This dyke is a fine-grained, aphanitic, pale- chocolate-brown rock that is very distinct from the enclosing gneisses. It weathers in and cannot be traced in the higher ground on An Fharaid Mhór.

Interpretation and conclusions

The An Fharaid Mhor site provides an accessible second site for this important type of nepheline-syenite ('ledmorite') dyke in the Foreland, in this case cutting Lewisian gneiss. Although rocks that may be correlated with these dykes have not been discovered between this locality and the Loch Borralan intrusion, the measured strike would extrapolate only slightly to the north of the present position of the Loch Borralan 'ledmorites'.

References