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Found in Wales and Scotland, in England where it borders those two, and in the area around Omagh, Northern Ireland, the Old Red Sandstone is a huge set of sedimentary rocks (primarily brown and red, giving it its name) dating to the Devonian. It is a marine formation, having been laid down from 408 to 370 million years ago as earlier Silurian rocks uplifted by the formation of Pangaea eroded and slid into a body of fresh water (probably a series of large river deltas). The formation is extremely thick, up to 11,000 meters in places, and can be subdivided into three categories "Lower", "Middle", and "Upper" from oldest to youngest.

As a result, the Old Red Sandstone is a rich source of marine fossils. Agnathan fishes, all extinct in the present day except for hagfishes and lampreyGeotriinae Mordaciinae Petromyzontinae A lamprey is a type of fish with a toothed, funnel-like, jawless sucking mouth with which it bores into the flesh of other fishes to suck their blood. In Zoology, Lampreys are not reckoned to be true fish because ofs are very common, as are ostracodermOstracodermi : Animalia : Chordata : Vertebrata Super-: Agnatha Sub- Ostracodermi Sub-: Cyclostomata Orders Ostracoderms ("shell-skinned") are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates. They bs and plantGreen algae land plants (embryophytes non-vascular embryophytes Hepatophyta liverworts Anthocerophyta hornworts Bryophyta mosses vascular plants (tracheophytes seedless vascular plants Lycopodiophyta clubmosses Equisetophyta horsetails Pteridophyta "true"s. In the early 1800s, the paleotology of the formation was studied intensively by Henry Thomas De la BecheSir Henry Thomas De la Beche ( 1796 April 13, 1855) was an English geologist. His father, an officer in the army, possessed landed property in Jamaica, but died while his son was still young. De la Beche spent his early life living with his mother in Lyme, Roderick MurchisonSir Roderick Impey Murchison ( February 19, 1792 October 22, 1871), was a powerful British geologist who first described and investigated the Silurian era. He was born at Tarradale, in Ross-shire, Scotland, the son of Kenneth Murchison (died 1796). He att, and Adam SedgwickAdam Sedgwick ( March 22nd, 1785 January 27, 1873) was one of the founders of modern geology. He proposed the Devonian period of the geological timescale and later the Cambrian period. The latter proposal was based on work which he did on Welsh rock strat -- Sedgwick's interpretation was the one that placed it in the Devonian, and in fact it was he that coined that word. Many of the science of stratigraphyStratigraphy a branch of geology, is basically the study of rock layers and layering. It is primarily used in the study of sedimentary and layered volcanic rocks. The subject was essentially invented and first rigorously applied by William Strata Smith in's early debates were about the Old Red Sandstone.

In regions where the formation is near the surface, many stone houses are built of the rocks mined from the formation. Notable examples can be found in the area surrounding Stirling, Perth, and Tayside. The inhabitants Caithness at the north-eastern tip of Scotland also used the stone to a considerable extent.

Note that in older geological works predating theories of plate tectonics, the United States' Catskill Delta formation is sometimes referred to as part of the Old Red Sandstone. In the modern day, however, it is recognized that the two are unconnected and are merely very similar due to being formed at approximately the same time by the same processes in a similar situation.