OLD RED SANDSTONE

thick sequence of Devonian rocks (formed from 408 to 360 million years ago) that are continental rather than marine in origin and occur in northwestern Europe. They have been extensively studied in Great Britain. The rocks were deposited in structural basins between the ranges of the Caledonian Mountains, which are also of the Devonian Period; great thicknesses, 11,000 m (37,000 feet) of sands and muds, slowly accumulated in these sinking basins. The sediments are poorly sorted and quite variable; sandstones that are cross-bedded, siltstones and shales that have mud cracks, and even raindrop impressions all occur. The sandstones are red, green, and gray, and the shales are gray. Plant fossils occur, and the fossil fauna is characterized by primitive, fishlike vertebrates, which are thought to have inhabited freshwater streams and rivers.