Old Red Sandstone

The Old Red Sandstone is an assemblage of sedimentary rocks largely of Devonian age, spanning the latest Silurian through earliest Carboniferous, roughly 419 to 358 million years ago. It is predominantly sandstone but includes conglomerate, shale, mudstone, siltstone and limestone, with a characteristic red coloration from iron oxide minerals, though the sequence ranges from grey and green through to red and purple. The rocks are exposed across the North Atlantic region, including Great Britain, Ireland, Norway, Greenland and Svalbard, as well as the eastern seaboard of North America, all once part of the ancient paleocontinent Euramerica. Originally thought to be entirely freshwater deposits formed in dunes, lakes, rivers and estuaries, more recent research has found marine fossils such as brachiopods and isotopic evidence of brackish conditions even in organisms once considered strictly freshwater dwellers. The formation preserves early fishes, arthropods and plants, and while most terrestrial red-bed deposits contain few fossils, isolated fossil-rich layers within the Old Red Sandstone have made it invaluable for the study of early vertebrate evolution. This entity documents the Old Red Sandstone specifically as a geological formation, a Devonian-age continental and marginal-marine sedimentary sequence.