Old Red Sandstone

Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It also extends northwards into Greenland and Svalbard. These areas were a part of the paleocontinent of Euramerica (Laurussia). In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. The presence of Old in the name is to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.

Sedimentology

The Old Red Sandstone describes a group of sedimentary rocks deposited in a variety of environments in the late Silurian, through the Devonian and into the earliest part of the Carboniferous. The body of rock, or facies, is dominated by terrigenous deposits and conglomerates at its base, and progresses to a combination of dunes, and sediments that may have been laid down in lakes, river, estuaries, and possibly other coastal environments. The Old Red Sandstone was long thought to have been deposited mostly in freshwater, but more recent studies have discovered marine fossils (such as brachiopods) in some locations. Its vertebrate fauna also occurs in typically marine environments, and an isotopic study also found significant marine influence indicating a brackish environment in mineralised tissues of its vertebrates, even in taxa that had long been thought to have been freshwater inhabitants.