Abstract 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

Abstract The Kačák Event is a late Eifelian (Mid-Devonian) episode of marine dysoxia/anoxia with associated extinctions. It has been widely recognized in the shelf seas that surrounded the Old Red Sandstone continent. It was contemporary with the lacustrine Orcadian Basin in Scotland. This basin contains the distinctive Achanarras lake horizon that contains a rich and diverse fish fauna. The Achanarras lake was wide and deep and would have been filled by rainfall from a monsoon system at an insolation maximum. Faunal elements within the lake are in common with the Kernavė Member in Estonia and this level can be conodont dated as late Eifelian eiflius or ensensis Zone. Therefore the group of lacustrine flooding climatic events that occur at and above the Achanarras level can be correlated with the marine Kačák Event ( sensu lato ) and both can be regarded as having a common climatic cause and driven by an insolation maximum. A reconstruction of the Orcadian Basin drainage system and a water balance model based on the calcium flux within the lake shows that a very significant volume of water would have been seasonally discharged to the Rheic Ocean and would have caused an additional environmental effect.