The Old Red Sandstone and the Highland Boundary in Arran, Scotland

Abstract
The Old Red Sandstone of Arran is described and divided into a number of formations. South of the Highland Boundary line its thickness averages 1800 m.; north of it only 36 m. is preserved. Its age is discussed, the conglomerates, breccias, sandstone and siltstones are described, and a modified sandstone classification is applied. Cornstones, volcanic rocks and epidotised rocks are also described. The source of the rocks is discussed. Gunn's original mapping has been modified, especially in the reinterpretation of the Old Red Sandstone-Dalradian boundary as a step-faulted unconformity. We confirm that the deformation of the Old Red Sandstone is largely the result of the intrusion and uplift of the Tertiary Granites. The history of ideas on the Highland Boundary in Arran is reviewed. There is stratigraphical evidence of Old Red Sandstone movement along this line in North-East Arran, and the North Sannox Fault may have originated then. The fault can no longer be regarded as passing south of the granite. A review of the geological history includes consideration of the environment and fluviatile mode of sedimentation of the Old Red Sandstone and the tectonic sequence of its Lower Tertiary deformation.

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