Post-Caledonian Geological History
The account of the Old Red Sandstone (ORS) given by Wally Mykura (1991) in the third edition of this book remains an authoritative account containing many details which will not be repeated here. Wally Mykura wrote at a time of active controversy concerning the relation of the Grampian Block to Devonian successions south of the Highland Boundary Fault. His untimely death robbed him of the opportunity to discuss problems which have been reconciled to some extent by recent work and revision of absolute ages for the Devonian (Tucker et al. 1998).
The chapter title follows tradition, recognizing that both the base and the top of the Devonian cannot generally be defined in Scotland. The Stonehaven Group of ORS facies is now thought to be as old as the Mid-Silurian (Marshall 1991) and the top of the Upper ORS passes conformably up into Carboniferous strata. The term Old Red Sandstone (discussion in Waterston 1965) was originally derived from the ‘Oelter rother Sandstein’ of Werner, and was applied to the Permian red sandstones of Germany. Phillips (1818) used ORS in its present sense, and Murchison (1839) considered it should be regarded as a system. However, it was Sedgwick & Murchison (1839) who established the Devonian system for marine strata in SW England and included the ORS within the Devonian. Old Red Sandstone is now used as a facies term and, whilst the rocks are mostly of Devonian age, there are notable exceptions.
The distribution of ORS strata (Fig. 8.1) shows