Old Red Sandstone, series of red and brown sandstones, conglomerates, and shales deposited in Wales and Scotland and in England near the Welsh and Scottish borders in the Devonian period of geologic time. The Old Red Sandstone, in contrast to the typical formations of the Devonian, is largely a continental formation, laid down in freshwater and on land as a result of the erosion of the highlands of the Silurian period. It is very thick in Scotland and contains a large assemblage of well-preserved fossils, particularly of the Devonian fishes. The Old Red Sandstone was correlated with the marine Devonian by the British geologists Rodney Murchison and Adam Sedgwick. The New Red Sandstone is a thick red sandstone of Permian and Triassic age found in the British Isles.