Chapter 1 Introduction

Abstract

The Lewisian outcrops at Gairloch and Loch Maree have long been regarded as key areas in the Lewisian complex of NW Scotland because of the presence of the supracrustal rocks of the Palaeoproterozoic Loch Maree Group enclosed within the Archaean gneisses. Long before the age of this Group was known, attention was focused on these areas because the existence of metasedimentary rocks offered the possibility of establishing a stratigraphy within the Lewisian complex. Until relatively recently, Lewisian 'stratigraphy' was based entirely on the relative ages of igneous and metamorphic events. The early Geological Survey workers debated whether the acid gneisses represented deformed granites which intruded the metase-diments or whether they represented an older basement on which the sediments had been laid down. During the discussion that followed the reading of the classic paper on the Lewisian by Sutton & Watson (1951), C. E. Tilley proposed that the critical region for interpreting Lewisian stratigraphy was neither Loch Laxford nor Loch Torridon, but Loch Maree and Gairloch. The pioneering work by Sutton & Watson on the Lewisian complex of NW Scotland stimulated the study of Precambrian basement complexes worldwide. Their methods offered the expectation that dyke swarms and other igneous suites could be used as stratigraphic markers to construct a chronology of tectono-thermal events in what seemed hopelessly confusing gneiss terrains. The Lewisian complex of NW Scotland is part of a widespread system of Palaeoproterozoic orogenic belts in the North Atlantic region. In a recent palaeomagnetic reconstruction of Laurentia and Baltica during