THE ACHANARRAS LIMESTONE OF THE MIDDLE OLD RED SANDSTONE, CAITHNESS, SCOTLAND

Abstract

Achanarras quarry, in central Caithness, exposes about twenty-five feet of the Caithness Flagstone Series, here consisting of calcareous flags and siltstones; the quarry has long been famous for its rich vertebrate fauna. The sequence has been measured, the approximate ranges of the genera noted, and lithological varieties of the flagstone facies examined. These varieties show laminae varying in thickness from 0.5 to 2.0 millimetres, comprising layers rich in detrital quartz, carbonate and organic matter. Analogies are sought in past and present sediments, especially in Eocene and modern lake deposits; the most striking of the latter exhibit annual laminations, and a similar regime is deduced for the Achanarras flagstones. The environment so pictured reinforces previous views on the Orcadian basin. The eleven feet of flagstone facies of the quarry are tentatively considered to represent about 4,000 years. The abnormally rich fauna probably reflects exceptionally good conditions of preservation.