Composition and age of the conglomerate outliers around the Kyle of Tongue, north Sutherland, Scotland

Abstract

The Caledonian metamorphic complex around the Kyle of Tongue is intruded by the Ben Loyal igneous complex and overlain by a series of small outliers which consist largely of red conglomerate. These were mapped and described by Peach & Horne in 1914 as Old Red Sandstone, but their conclusions were largely rejected by McIntyre & others in 1956 and the sediments have subsequently been regarded as Triassic deposits. The conglomerates were largely derived from local Moinian and Lewisian rocks, but they also contain boulders of a distinctive red quartz syenite. These were derived entirely from the subsidiary intrusion of Ben Stumanadh and not from the main pluton of the Ben Loyal complex. The sediments represent alluvial deposits transported NNW along a series of minor valleys cut in the metamorphic floor. These formed tributaries to a NE trending valley—a precursor of the existing Kyle of Tongue—which may in turn have acted as tributary to a major valley trending ESE. Although complete proof is not possible, the available evidence strongly suggests that the sediments are of Old Red Sandstone age.