Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age.

In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup status[4] and which is of considerable importance to early paleontology.

The Old Red Sandstone describes a group of sedimentary rocks deposited in a variety of environments in the late Silurian, through the Devonian and into the earliest part of the Carboniferous.

The body of rock, or facies, is dominated by terrigenous deposits and conglomerates at its base, and progresses to a combination of dunes, and sediments that may have been laid down in lakes, river, estuaries, and possibly other coastal environments.

Rocks of this age were also laid down in South West England (hence the name 'Devonian'; from Devon) though these are of true marine origin and are not included within the Old Red Sandstone.

[1] The Orcadian Basin extends over a wide area of North East Scotland and the neighbouring seas.

The unit is up to 128m thick in its type area and consists of green and red sandstones and conglomerates, typically containing large (10–30 cm or 4–12 in across) elliptical well rounded clasts, accompanied by siltstones, mudstones and limestones.

There is a continuous outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh and beyond to Arran.

Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire.

In the Brecon Beacons, the Plateau Beds Formation is unconformably overlain by the Grey Grits Formation though further east these divisions are replaced by the Quartz Conglomerate Group which is itself subdivided into a variety of different formations.The sequence in Pembrokeshire differs from that of the main part of the basin to the east, and falls into two parts.

[15] In North Pembrokeshire to the north of the Ritec Fault, both the middle and upper ORS are missing with only the lower ORS present; this is divided into an earlier Milford Haven Group comprising in ascending order, the Red Cliff, Sandy Haven and Gelliswick Bay formations and a later Cosheston Group with, again in ascending order, its constituent Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff and New Shipping formations.

These respectively equate with the Temeside, Raglan Mudstone and St Maughans formations of the central and eastern part of the basin.

The present day outcrop occupies a narrow zone from Dulas Bay on Anglesey's northeast coast, southwards to the town of Llangefni.

[2][20] In the early 19th century, the paleontology of the formation was studied intensively by Hugh Miller, Henry Thomas De la Beche, Roderick Murchison, and Adam Sedgwick—Sedgwick's interpretation was the one that placed it in the Devonian: he coined the name of that period.

[citation needed] In the modern day it is recognized that the two are not stratigraphically continuous but are very similar due to being formed at approximately the same time by the same processes.