Old red sandstone

Old Red Sandstone Stratigraphic range: late Silurian to earliest Carboniferous
Cross-section showing quartz and chert pebbles in a sample from central England (scale bar is 10 mm).
Type	Supergroup
Lithology
Primary	Sandstone
Other	Conglomerate, shale, mudstone, siltstone, limestone
Location
Region	North Atlantic
Extent	700 km

The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Britain, Ireland and Norway and in the west along the northeastern seaboard of North America. It also extends northwards into Greenland and Svalbard. In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status and which is of considerable importance to early paleontology. For convenience the short version of the term, ORS is often used in literature on the subject. The term was coined to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.

The Old Red Sandstone describes a suite of sedimentary rocks deposited in a variety of environments during the Devonian but extending back into the late Silurian and on into the earliest part of the Carboniferous. The body of rock, or facies, is dominated by alluvial sediments and conglomerates at its base, and progresses to a combination of dunes, lakes and river sediments.

The familiar red colour of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone — the sequence also includes conglomerates, mudstones, siltstones and thin limestones and colours can range from grey and green through red to purple. These deposits are closely associated with the erosion of the Caledonian Mountain chain which was thrown up by the collision of the former continents of Avalonia, Baltica and Laurentia to form the Old Red Sandstone Continent- an event known as the Caledonian Orogeny.