Old Red Sandstone, abbreviated ORS, 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 eastern seaboard of North America. It also extends northwards into Greenland and Svalbard.Friend, P.F. and Williams, B.P.J. (eds) 2000 New Perspectives on the Old Red Sandstone. Geological Society, London, Special publications 180 p1,35 These areas were a part of the paleocontinent of Euramerica (Laurussia). In Britain it is a lithostratigraphic unit (a sequence of rock strata) to which stratigraphers accord supergroup statushttp://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=ORS entry in BGS on-line rock lexicon and which is of considerable importance to early paleontology. The presence of Old in the name is to distinguish the sequence from the younger New Red Sandstone which also occurs widely throughout Britain.
Sedimentology
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. The Old Red Sandstone was long thought to have been deposited mostly in freshwater, but more recent studies have discovered marine fossils (such as brachiopods) in some locations. Its vertebrate fauna also occurs in typically marine environments, and an isotopic study also found significant marine influence indicating a brackish environment in mineralised tissues of its vertebrates, even in taxa that had long been thought to have been freshwater inhabitants. Thus, at least some strata appear to have been deposited on the coast, probably in marginal marine environments.
The familiar red colour of these rocks arises from the presence of iron oxide, but not all the Old Red Sandstone is red or sandstonethe 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 Continentan 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 not fossil-bearing; 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 recognized 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 128 m 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. At Oban there is merely an erosional contact incorporating debris of the slate in a basal conglomerate. The ORS deposits 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.British Geological Survey 1:50,000 scale geological map sheet 45W (Scotland) Connel 1991 Small outliers occur near Taynuilt and either side of Loch Avich.British Geological Survey 1:50,000 scale geological map sheet 37W (Scotland) Furnace 2008 The deposits are especially obvious on Kerrera where they form the bedrock across half of the island.British Geological Survey 1:50,000 scale geological map sheet 44E (Scotland) Lismore 1992 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 basinsee 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, into Herefordshire, Worcestershire and Gloucestershire. Outliers in Somerset and north Devon complete its extent.
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.
South Powys/Brecon Beacons The lowermost formations are of upper Silurian age, these being the Downton Castle Sandstone Formation and the overlying Moor Cliffs Formation (formerly the Raglan Mudstone Formation). The top of this formation is marked by a well-developed calcrete, the Chapel Point Limestone. The lowermost Devonian formation is the Freshwater West Formation (formerly the St Maughans Formation), itself overlain by the Senni Formation (formerly the Senni Beds) which is in turn overlain by the Brownstones Formation. In the east, a further calcrete, the Ffynnon Limestone (sometimes pluralised) is developed at the interface between the Freshwater West and Senni formations. The Senni Formation is not recorded further east.
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 to the east, and falls into two parts.Barclay, W. J. (2005) in The Old Red Sandstone of Great Britain (W. J. Barclay, M. A. E. Browne, A. A. McMillan, E. A. Pickett, P. Stone and P. R. Wilby) Geological Conservation Review series, No 31 JNCC, Peterborough pp. 212–213
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 (including 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 West Formation (including Rat Island Mudstone Member and Conigar Pit Sandstone Member) Moor Cliffs Formation (including Chapel Point Limestone Member and Townsend Tuff Bed) Freshwater East Formation
The Freshwater East Formation, and corresponding Red Cliff Formation of north Pembrokeshire, are both late Silurian in age.Howells, M. F. 2007 British Regional Geology: Wales (Keyworth, Nottingham. British Geological Survey) pp. 100–101
Anglesey A small and separate basin exists here where both alluvial and lacustrine deposits are recorded. Both the middle and upper ORS are missing but the lower ORS is represented, in ascending order, by the Bodafon, Traeth Bach, Porth y Mor and Traeth Lligwy formations. Calcretes are also recorded representing carbonate-rich soils developed between periods of sediment deposition. The present day outcrop occupies a narrow zone from Dulas Bay on Anglesey's northeast coast, southwards to the town of Llangefni.British Geological Survey 1:50K special map sheet Anglesey
History of study
In 1787 James Hutton noted what is now known as Hutton's Unconformity at Inchbonny, Jedburgh, and in early 1788 he set off with John Playfair to the Berwickshire coast and found more examples of this sequence in the valleys of the Tower and Pease Burns near Cockburnspath. They then took a boat trip from Dunglass Burn east along the coast with the geologist Sir James Hall of Dunglass and at Siccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea", where 345-million-year-old Old Red Sandstone overlies 425-million-year-old Silurian greywacke.