The geology of an area of about 27sq. km, lying within Glen Orchy is described. The area consists of rocks belonging to the upper part of the Grampian Division of the Moine succession and parts of the Appin and Argyll Groups of the Dalradian Supergroup. A lithostratigraphic sequence has been established using way-up evidence from cross-lamination and graded bedding. Moine psammite and semipelite occur at the base and pass up through a passage zone into Dalradian quartzite followed by pelite (with thin calcareous beds) and pebbly quartzite. A tectonic break, the Iltay Boundary Slide occurs between the pelite and the pebbly quartzite. Field and microscopic examination of the rocks in the area, have revealed the existence of a polyphase deformational history and has enabled four stages of folding (F1 to F4) to be recognised. The earliest deformation (D1) led to the formation of two major isoclinal recumbent folds, the Beinn Chuirn Anticline and Beinn Udlaidh Syncline. Both folds face to the south-east and plunge at a low to moderate angles to the south-west, and have curvilinear hinges. This interpretation of the Beinn Chuirn Anticline as a D1 structure disagrees with its current interpretation by other workers as a D2 synform. The Iltay Boundary Slide probably developed during D1 and was reactivated locally during the D2 deformation. No F2 major fold has been recognised in the area but F3 minor folding is associated with the development of a major late fold, the Glen Orchy Antiform. This antiform plunges to the SW-SSW or NE-NNE, and folds the Beinn Chuirn Anticline and Beinn Udlaidh Syncline. F4 folding is only locally developed and no major fold related to this phase of folding is seen. The rocks of the area lie within the garnet zone of the epidote amphibolite facies. No other higher grade index minerals were found. Periods of mineral growth have been related to the major deformational episodes on micro-textural evidence. The peak metamorphism of the area occurred during the D1 deformation, with the development of the garnet porphyroblasts. No MS2 garnet has been recognised in the area. Later retrogression resulted in the widespread alteration of biotite and garnet to chlorite
