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). They are 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 lampreys are very common, as are Ostracoderms and plants. In the early 1800s, the paleotology of the formation was studied intensively by Henry Thomas De la Beche, Roderick Murchison, and Adam Sedgwick -- 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 stratigraphy'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.
External Links Cawdor Castle which, despite being brown, is made of Old Red Sandstone