Old Red Sandstone

The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also extends northwards into Greenland and Svalbard. These areas were a part of the ancient continent of Euramerica/Laurussia. 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.

Sedimentology

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 to red and 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.

Many fossils are found within the rocks, including early fishes, arthropods and plants. As is typical with terrestrial red beds, the vast majority of the rock is unfossiliferous; however there are isolated, localized beds within the rock that do contain fossils. 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.

Stratigraphy

Since the Old Red Sandstone consists predominantly of rocks of terrestrial origin, it does not generally contain marine fossils which would otherwise prove useful in correlating one occurrence of the rock with another, both between and within individual sedimentary basins. Accordingly, local stage names were devised and these remain in use to some extent today though there is an increasing use of international stage names. Thus in the Anglo-Welsh Basin, there are frequent references to the Downtonian, Dittonian, Breconian and Farlovian stages in the literature. The existence of a number of distinct sedimentary basins throughout Britain has been established.

Orcadian Basin

The Orcadian Basin extends over a wide area of North East Scotland and the neighbouring seas. It encompasses the Moray Firth and adjoining land areas, Caithness, Orkney and parts of Shetland. South of the Moray Firth, two distinct sub-basins are recognised at Turriff and at Rhynie. The sequence is more than thick in parts of Shetland. The main basin is considered to be an intramontane basin resulting from crustal rifting associated with post-Caledonian extension, possibly accompanied by strike-slip faulting along the Great Glen Fault system.

Argyll

There are a scatter of exposures of the Old Red Sandstone around Oban and the Isle of Kerrera on the West Highland coast, this unit is sometimes referred to as the Kerrera Sandstone Formation. The unit is up to 128m thick in its type area and consists of green and red sandstones and conglomerates, typically containing large ( across) elliptical well rounded clasts, accompanied by siltstones, mudstones and limestones. On Kerrera a conglomerate of andesite boulders rests unconformably on Dalradian black, pyritic slates (Easdale Slate) of the Easdale Subgroup. On Oban there is merely an erosional contact incorporating debris of the slate in a basal conglomerate. The ORS around Oban are considered latest Silurian (Pridoli) to earliest Devonian in age. They are interpreted as alluvial fans which filled a depositional basin from the east and northeast. Small outliers occur near Taynuilt and either side of Loch Avich. The deposits are especially obvious on Kerrera where they form the bedrock across half of the island. These are conformably overlain by peperite and the basaltic and andesitic Lorne plateau lavas. The ORS on Kerrera and isolated localities around Oban are known for their fossils, particularly fish.

Midland Valley of Scotland

The Midland Valley graben defined by the Highland Boundary Fault in the north and the Southern Uplands Fault in the south harbours not only a considerable amount of Old Red Sandstone sedimentary rocks but also igneous rocks of this age associated with extensive volcanism. There is a continuous outcrop along the Highland Boundary Fault from Stonehaven on the North Sea coast to Helensburgh and beyond to Arran. A more disconnected series of outcrops occur along the line of the Southern Uplands Fault from Edinburgh to Girvan. Old Red Sandstone often occurs in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire.

Scottish borders

A series of outcrops occur from East Lothian southwards through Berwickshire. Hutton's famous unconformity at Siccar Point occurs within this basin - see History of study below.

Anglo-Welsh Basin

This relatively large basin extends across much of South Wales from southern Pembrokeshire in the west through Carmarthenshire into Powys and Monmouthshire and through the southern Welsh Marches, notably into Herefordshire, Worcestershire and Gloucestershire. Outliers in Somerset and north Devon complete the extent of this basin.

With the exception of south Pembrokeshire, all parts of the basin are represented by a range of lithologies assigned to the Lower Devonian and to the Upper Devonian, the contact between the two being unconformable and representing the complete omission of any Middle Devonian sequence. The lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Raglan Mudstone Formation except in Pembrokeshire where a more complex series of formations is recognised. In the east of the basin, the top of the Raglan Mudstone is marked by a well-developed calcrete, the Bishop's Frome Limestone. The lowermost Devonian formation is the St Maughans Formation, itself overlain by the Brownstones Formation though with an intervening Senni Formation over much of the area. The Upper Devonian sequence is rather thinner and comprises a series of formations which are more laterally restricted. 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.

Pembrokeshire

The sequence in Pembrokeshire differs from that of the main part of the basin and falls into two parts.

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.
*''- middle and upper ORS missing -''
* Cosheston Group
** New Shipping Formation
** Lawrenny Cliff Formation
** Mill Bay Formation
** Burton Cliff Formation
** Llanstadwell Formation
* Milford Haven Group
** Gelliswick Bay Formation
** Sandy Haven Formation (inc. Townsend Tuff Beds)
** Albion Sands Formation / Lindsway Bay Formation
** Red Cliff Formation

In south Pembrokeshire to the south of the Ritec Fault, the lower ORS is represented by, in ascending order, the Freshwater East, Moors Cliff and Freshwater West formations. These are unconformably overlain by the Ridgeway Conglomerate Formation. The middle ORS is missing whilst the Upper ORS is represented by the Gupton and West Angle formations.
* Skrinkle Sandstone Group
** West Angle Formation
** Gupton Formation
* ''- part of middle ORS missing -''
* Ridgeway Conglomerate Formation
* ''- part of middle ORS missing -''
* Milford Haven Group
** Freshwater Wes