The Portsoy–Duchray Hill Lineament has been widely quoted in the literature as a major zone of discontinuities, running from Portsoy on the Banff coast to the Duchray Hill area in Perthshire. A review of the evidence indicates that there is no single structural entity running from Portsoy to Duchray Hill; the term ‘Portsoy-Duchray Hill lineament’ should be avoided. There are, however, significant along- and across-strike variations in geological history across a zone which extends from Portsoy to the Cabrach (the ‘Portsoy Line’). Certain elements can be identified also in areas further south, e.g. the Coyles of Muick shear zone and Glen Doll fault.

The Portsoy Line was initiated as a zone of syn-depositional faulting, with resulting facies variations similar to those seen across the Glen Doll fault. The Portsoy Line was reactivated as a ductile shear zone during Caledonian orogenesis, while the Glen Doll fault is a later, more brittle, structure. The shear zone from Portsoy to the Cabrach forms the western branch of a system of shears which acted to focus syn-orogenic basic magmatism, and differential movement during post- metamorphic uplift. The Coyles of Muick shear zone is the most southerly example of the system of shears, and there is probably a transfer zone along Deeside between it and the Portsoy-Cabrach shear zone, now obscured in part by the Ballater Granite. The focused shear zones serve to delineate the Buchan area, with its low pressure metamorphism and distinctive structural style, from areas to the south and west where the structural level was deeper, and deformation more pervasive.