The supposed Lewisian inlier Of Scardroy, Central Ross-Shire, And its relations with the surrounding Moine rocks

Abstract

Summary The Scardroy area lies east of the Moine thrust in central Ross-shire and is composed predominantly of metamorphosed sediments. Over considerable areas the rocks are gneissose in appearance, and these gneisses, together with associated striped hornblende-granulites and basic and ultrabasic minor intrusions, were regarded by the Geological Survey as representatives of the basement on which the sediments of the Moine Series were laid down, and were correlated with the Lewisian gneisses west of the Moine thrust. The present paper brings forward evidence to show that the gneissose rocks were formed by recrystallization and felspathization of siliceous, semi-pelitic, calcareous and hornblendic rocks which are integral parts of the Moine Series. Detailed mapping shows that certain horizons can be traced continuously across the supposed line of unconformity. Movements accompanying the metamorphism of the Moine rocks led to the production of an anticline and syncline, overturned towards the east and pitching east of south. The minor structures and the axes of quartz girdles determined by fabric analysis pitch in the same direction. On every scale, the same structural pattern characterizes both the Moine and the supposed Lewisian rocks, and no traces of older structures have been detected in the latter. A tentative stratigraphical succession, established with the aid of sedimentary structures, suggests that the rocks hitherto regarded as Lewisian are younger than a belt of undoubtedly Moinian semi-pelitic granulites. In view of the facts given above, the writers conclude that the rocks of the Scardroy area belong to a single series and that no rocks of Lewisian age are present.