ABSTRACT

The islands of Raasay and Scalpay lie off the Northwest coast of Scotland. In this paper the Torridonian (Precambrian) sedimentary rocks of these islands are described and their possible mode of deposition discussed. Three major facies are distinguished on grounds of lithology and sedimentary structures. These facies appear to indicate three different sedimentary environments. A Basal facies of screes and fanglomerates, probably subaerial in their lower part, is followed by, and passes laterally into Grey facies sandstones and shales of lacustrine or shallow water marine origin. These are in turn overlain by Red facies sandstones and shales believed, in the light of research into recent river alluvium, to be fluviatile deposits. The coarse grade, great thickness, and unidirectional cr ss-stratification of this last facies are used to argue that these beds were laid down by braided, rather than meandering, rivers. Sub-facies indicative of race, shoal, and abandoned channel subenvironments are also recognizable.

An analysis of sedimentary structures indicative of paleocurrent directions shows that most of the sediment was derived from the North and West. Local deviations from this direction near the base of the succession probably reflect control by the irregular topography of the Lewisian Gneiss beneath.