Aird Torrisdale

This site is the coastal outcrop of the Naver zone of ductile thrusting between unmigmatised psammites of the A'Mhoine Nappe, with infolded Lewisian basement, and the overthrust Naver Nappe of migmatitic Bettyhill assemblage gneisses. On the north coast [NC671635] the Naver Thrust brings sheared migmatitic psammitic gneisses of the Bettyhill assemblage of the Naver Nappe over strongly deformed feldspathic and hornblendic gneisses of the Torrisdale Lewisian body. Here the actual line of the ductile thrust is exposed and is seen as a thin zone of ultra high strain. The Torrisdale Lewisian body is one of a series of Lewisian inliers occupying (Caledonian) fold cores in the footwall to the Naver Thrust. Near Achtoty [NC674625] typical A'Mhoine Nappe psammites, albeit in a state of quite high strain, are seen to underlie the lower boundary of the Torrisdale Lewisian. This boundary is probably a sheared unconformity and can be traced south-east to Torrisdale Bay [NC681622]. The Bettyhill assemblage gneisses above the thrust contain large, relatively late pegmatite bodies, and are variably migmatitised, but the migmatisation is structurally very early (D1), and clearly pre- dates the thrust movement. The importance of this site is that it represents the northern extremity of the Naver Thrust, which separates the early regional migmatites of east Sutherland from the unmigmatised Moine to the west. The nature of this boundary is rather controversial, many writers have assumed it is the lateral extension of the Sgurr Beag Thrust (slide), with the overlying rocks being the northern equivalents of the Glenfinnan division. However, recent chemostratigraphic work suggests the Bettyhill assembage is equivalent to high grade Morar division. Thus the Naver Thrust is not equivalent to the Sgurr Beag Thrust, rather it occupies a position within the Morar division, similar to the Knoydart thrust.

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