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Old Red Sandstone

This article, as it stands, focuses rather narrowly. It concerns itself primarily with the United Kingdom, a perspective that, frankly, leaves much of the world’s geological narrative conspicuously absent. One could argue it’s a rather provincial viewpoint, don’t you think? If you’re feeling particularly ambitious, or perhaps just bored, you could always endeavor to improve it, engage in a spirited debate on the talk page, or even summon the courage to forge an entirely new article. The wizard is, I’m told, quite helpful. Or not. Who am I to say?

Old Red Sandstone

Stratigraphic range: Late Silurian – earliest Carboniferous, spanning approximately 419 to 358 million years ago (Ma).

A rather dramatic formation, the Old Red Sandstone, abbreviated as ORS, is a collection of rocks found predominantly in the North Atlantic region, largely dating back to the Devonian period. Its presence stretches across Great Britain and Ireland into Norway in the east, and along the eastern seaboard of North America in the west. Further north, it extends into Greenland and Svalbard. These lands, you see, were once part of the ancient supercontinent known as Euramerica, or Laurussia. Within Britain, it’s classified as a lithostratigraphic unit, a layered sequence of rock strata that geologists have seen fit to grant supergroup status. It holds considerable weight in the early study of paleontology, which is hardly surprising given its age. The "Old" in its name serves a crucial purpose: to distinguish it from the younger New Red Sandstone, another widespread rock formation found throughout Britain.

Sedimentology

Observe the bedding plane of the Old Red Sandstone, adorned with pebbles of quartz and chert, as seen in this specimen from central England. The scale bar, a mere 10 mm, offers a rather humbling perspective on the vastness of time.

The Old Red Sandstone is not a singular entity but rather a grouping of sedimentary rocks that accumulated in a variety of environments. This deposition spanned from the late Silurian, through the entire Devonian period, and into the very beginnings of the Carboniferous. Initially, the rock strata were dominated by terrigenous deposits and conglomerates. As time progressed, the depositional environments shifted, incorporating elements like dunes, and sediments that likely settled in lakes, river systems, estuaries, and other coastal settings. For a considerable period, it was believed that the Old Red Sandstone was deposited primarily in freshwater environments. However, more recent investigations have unearthed marine fossils, such as brachiopods, in certain locations. Even the vertebrate fauna found within these rocks are often associated with typically marine habitats. Furthermore, an isotopic study revealed significant marine influence, indicating a brackish environment in the mineralized tissues of its vertebrates, even in species long presumed to be freshwater inhabitants. This suggests that, at least in some strata, deposition occurred along coastlines, likely in marginal marine settings.

The characteristic red hue of these rocks is attributed to the presence of iron oxide. However, it’s important to note that not all Old Red Sandstone is red, nor is it exclusively sandstone. The sequence also encompasses conglomerates, mudstones, siltstones, and thin layers of limestone. The colors can vary dramatically, ranging from shades of grey and green to deep reds and purples. These deposits are intimately linked to the erosion of the Caledonian Mountain chain, a colossal mountain range formed by the collision of the ancient continents of Avalonia, Baltica, and Laurentia. This tectonic event, known as the Caledonian Orogeny, essentially created the "Old Red Sandstone Continent."

Fossils are not uncommon within these ancient rocks, offering glimpses of early fishes, arthropods, and plants. Yet, as is often the case with terrestrial red beds, the vast majority of the rock is devoid of fossils. However, localized beds within the formation do contain these precious remnants of past life. It’s worth noting that rocks of a similar age were also laid down in South West England – hence the name of the Devonian period, derived from the county of Devon. These particular deposits, however, are of purely marine origin and are not considered part of the Old Red Sandstone proper.

Stratigraphy

Given that the Old Red Sandstone is predominantly composed of terrestrial rocks, it generally lacks the marine fossils that would typically aid in correlating different occurrences of the rock, both between and within distinct sedimentary basins. Consequently, local stage names were developed and are still in use to some extent today, although international stage names are becoming increasingly common. In the Anglo-Welsh Basin, for instance, the literature frequently refers to the Downtonian, Dittonian, Breconian, and Farlovian stages. The existence of numerous distinct sedimentary basins across Britain has been well-established.

Orcadian Basin

The Orcadian Basin sprawls across a significant portion of North East Scotland and the adjacent seas, encompassing the Moray Firth and its surrounding land areas, Caithness, Orkney, and parts of Shetland. South of the Moray Firth, two distinct sub-basins have been identified: one at Turriff and another at Rhynie. In certain areas of Shetland, the rock sequence exceeds 4 kilometers (13,000 feet) in thickness. The primary basin is understood to be an intramontane basin, a result of crustal rifting linked to post-Caledonian extension, potentially accompanied by strike-slip faulting along the Great Glen Fault system.

Argyll

Exposures of the Old Red Sandstone are scattered around Oban and the Isle of Kerrera on the West Highland coast. This particular unit is sometimes referred to as the Kerrera Sandstone Formation. In its type locality, the unit can reach a thickness of up to 128 meters. It is characterized by green and red sandstones and conglomerates, often containing large (10–30 cm or 4–12 inches across), well-rounded, elliptical clasts. These are accompanied by siltstones, mudstones, and limestones. On Kerrera, a conglomerate composed of andesite boulders rests unconformably upon the black, pyritic slates of the Easdale Subgroup, belonging to the Dalradian sequence. Near Oban, the contact is merely an erosional one, with debris from the slate incorporated into a basal conglomerate. The ORS deposits in the Oban area are dated from the latest Silurian (Pridoli) to the earliest Devonian. They are interpreted as alluvial fans that filled a depositional basin originating from the east and northeast. Small outliers are also found near Taynuilt and on either side of Loch Avich. The deposits are particularly prominent on Kerrera, forming the bedrock across half of the island. These are conformably overlain by peperite and the basaltic and andesitic Lorne plateau lavas. The ORS found on Kerrera and in isolated spots around Oban are renowned for their fossil content, particularly fish.

The Old Red Sandstone at Gardenstown, Aberdeenshire, showcases its distinctive layering.

Midland Valley of Scotland

The Midland Valley graben, demarcated by the Highland Boundary Fault to the north and the Southern Uplands Fault to the south, is host to a substantial volume of Old Red Sandstone sedimentary rocks, as well as igneous rocks from the same period, associated with extensive volcanism. A continuous outcrop of these rocks runs along the Highland Boundary Fault from Stonehaven on the North Sea coast, extending to Helensburgh and beyond to the Isle of Arran. A more fragmented series of outcrops follows the line of the Southern Uplands Fault, stretching from Edinburgh to Girvan. Old Red Sandstone frequently occurs alongside conglomerate formations; one particularly striking cliffside exposure is the Fowlsheugh Nature Reserve in Kincardineshire.

Scottish Borders

A series of outcrops are present, extending from East Lothian southward through Berwickshire. It is within this basin that Hutton's famous unconformity at Siccar Point is located, a pivotal discovery in the history of geological study.

Anglo-Welsh Basin

This rather extensive basin covers a significant portion of South Wales, stretching from southern Pembrokeshire in the west, through Carmarthenshire, into Powys and Monmouthshire. It continues across the southern Welsh Marches into Herefordshire, Worcestershire, and Gloucestershire. Outliers in Somerset and north Devon mark its furthest limits.

With the exception of the south Pembrokeshire region, all parts of this basin exhibit a range of lithologies assigned to the Lower Devonian and Upper Devonian periods. The contact between these two divisions is marked by an unconformity, indicating a complete absence of any Middle Devonian sequence.

South Powys/Brecon Beacons

The lowest formations here are of Upper Silurian age. These include the Downton Castle Sandstone Formation and the overlying Moor Cliffs Formation (previously known as the Raglan Mudstone Formation). The upper boundary of this formation is defined by a well-developed calcrete, known as the Chapel Point Limestone. The lowermost Devonian formation is the Freshwater West Formation (formerly the St Maughans Formation), which is overlain by the Senni Formation (formerly the Senni Beds), itself succeeded by the Brownstones Formation. In the eastern part of the basin, another calcrete, the Ffynnon Limestone (sometimes referred to in the plural), is found at the boundary between the Freshwater West and Senni formations. The Senni Formation is not present further east.

The Upper Devonian sequence is notably thinner and comprises a series of formations that are more restricted in their lateral extent. Within the Brecon Beacons, the Plateau Beds Formation is unconformably overlain by the Grey Grits Formation. However, further east, these divisions are replaced by the Quartz Conglomerate Group, which is further subdivided into various formations.

The Lower Old Red Sandstone at Yesnaby, Orkney, displays cross-bedded aeolian sandstone, a testament to ancient wind-blown deposition.

Pembrokeshire

The stratigraphic sequence in Pembrokeshire deviates from that found in the main part of the basin to the east and is divided into two distinct parts.

In North Pembrokeshire, north of the Ritec Fault, both the Middle and Upper ORS are absent, with only the Lower ORS present. This Lower ORS is further subdivided into an earlier Milford Haven Group, comprising, in ascending order, the Red Cliff, Sandy Haven, and Gelliswick Bay formations, and a later Cosheston Group. The Cosheston Group, again in ascending order, includes the Llanstadwell, Burton Cliff, Mill Bay, Lawrenny Cliff, and New Shipping formations. These formations correspond, respectively, to the Temeside, Raglan Mudstone, and St Maughans formations found in the central and eastern parts 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, 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 absent, while 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 its counterpart the Red Cliff Formation in North Pembrokeshire, both date to the late Silurian period.

Anglesey

A small, isolated basin exists on Anglesey, where both alluvial and lacustrine deposits have been recorded. Similar to Pembrokeshire, both the Middle and Upper ORS are absent. The Lower ORS is represented by the Bodafon, Traeth Bach, Porth y Mor, and Traeth Lligwy formations, in ascending order. Calcretes are also documented, indicating the presence of carbonate-rich soils that developed between periods of sediment deposition. The present-day outcrop occupies a narrow zone extending from Dulas Bay on Anglesey's northeast coast southward to the town of Llangefni.

History of Study

In 1787, James Hutton observed what is now famously known as Hutton's Unconformity at Inchbonny, near Jedburgh. In early 1788, accompanied by John Playfair, he journeyed to the Berwickshire coast, discovering further examples of this geological sequence in the valleys of the Tower and Pease Burns, close to Cockburnspath. They then embarked on a boat trip eastward along the coast from Dunglass Burn, accompanied by the geologist Sir James Hall of Dunglass. It was at Siccar Point that they encountered what Hutton described as "a beautiful picture of this junction washed bare by the sea," a place where 345-million-year-old Old Red Sandstone lies atop 425-million-year-old Silurian greywacke.

During the early 19th century, the paleontology of this formation was the subject of intensive study by prominent figures such as Hugh Miller, Henry Thomas De la Beche, Roderick Murchison, and Adam Sedgwick. It was Sedgwick who was instrumental in classifying these rocks within the Devonian period, even coining the name for that geological era. The term 'Old Red Sandstone' itself was first introduced in 1821 by the Scottish naturalist and mineralogist Robert Jameson. He used it to describe the red rocks that lay beneath the 'Mountain Limestone,' which is synonymous with the Carboniferous Limestone. At that time, these rocks were mistakenly thought to be the British equivalent of Germany's Rotliegendes, which, in reality, is of Permian age. Many of the foundational debates in the science of stratigraphy revolved around the Old Red Sandstone.

In older geological texts, predating the advent of plate tectonics theories, the Catskill Delta formation in the United States was occasionally referenced as part of the Old Red Sandstone. Modern understanding, however, recognizes that while the two formations are not stratigraphically continuous, they share remarkable similarities due to being formed by analogous processes during approximately the same geological timeframe.

Use as a Building Stone

The walls of St. Helen's Chapel at Siccar Point are faced with Old Red Sandstone. Notice how the greywacke is used for the inner face and surrounding dry stone dykes.

Across the regions where it outcrops, the Old Red Sandstone has been extensively employed as a building material. Some notable examples of its use can be found in the vicinities of Stirling, Stonehaven, Perth, and the Tayside region. The inhabitants of Caithness, situated at the northeastern tip of Scotland, also utilized this stone to a considerable extent. Furthermore, Old Red Sandstone has frequently been incorporated into buildings in Herefordshire, Monmouthshire, and the former Brecknockshire (now part of south Powys) in South Wales.

Notable Buildings

See also