Old Red Sandstone

Old Red Sandstone
Cross-section showing quartz and chert pebbles in a sample from central England (scale bar is 10 mm).
Type	Geological formation
Age	Late Silurian to earliest Carboniferous
Lithology
Primary	sandstone
Other	conglomerate, shale, mudstone, siltstone, limestone
Location
Country	United Kingdom
Extent	700 km[1]

The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.

Hutton's angular unconformity at Siccar Point where 345 million year old Devonian Old Red Sandstone overlies 425 million year old Silurian greywacke.^[2]

Sedimentology

Bedding plane of Old Red Sandstone with quartz and chert pebbles; central England;
(scale bar is 10 mm.)

The Old Red Sandstone describes a suite of sandstone rocks deposited in a variety of environments during the Devonian period but extending back into the late Silurian period and forward into the earliest part of the Carboniferous period. 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 East Avalonia, Baltica and Laurentia to form the Old Red Sandstone Continent- an event known as the Caledonian Orogeny.

The majority of the Old Red Sandstone rocks of Great Britain are found in four major basinal areas — the Orcadian Basin (which extends from Shetland to the Moray Firth), the various basins of the Midland Valley of Scotland, the Scottish Border Basin and the Anglo-Welsh basin of south Wales and the Welsh Marches.Many fossils are found within the rocks, early fishes,arthropods and plants. The rocks may appear barren to many amateur geologists but careful study particularly with a an accomplished fossil hunter, pockets of fossils can be found. Rocks of this age were also laid down in south-west England (hence the name 'Devonian') though these are of true marine origin and are not included within the Old Red Sandstone. ^[1]

History of study

In 1787 James Hutton noted what is now known as Hutton's Unconformity at Inchbonny, Jedburgh, and in the Spring of 1788 he set off with John Playfair to the Berwickshire coast and found more examples of this sequence in the valleys of the Tour and Pease Burns near Cockburnspath.^[3] 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",^[4] where 345 million year old Devonian Old Red Sandstone overlies 425 million year old Silurian greywacke.^[2][5]

In the early 1800s, 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, and in fact it was he that coined that word. Many of the science of stratigraphy's early debates were about the Old Red Sandstone. Old Red Sandstone often occur in conjunction with conglomerate formations, one such noteworthy cliffside exposure being the Fowlsheugh Nature Reserve, Kincardineshire.

Common Building Stone

St. Helen's Chapel at Siccar Point has walls faced in Old Red Sandstone, with greywacke used on the inner face and surrounding drystane dykes.

In regions where the formation is near the surface, many stone houses are built of the rocks quarried from it. Notable examples can be found in the area surrounding Stirling [1], Stonehaven [2], Perth [3], and Tayside. The inhabitants of Caithness [4] at the northeastern tip of Scotland also used the stone to a considerable extent. Old Red Sandstone has also frequently been used in buildings in Herefordshire, Monmouthshire and the former Brecknockshire (now south Powys) of south Wales.

Note that in older geological works predating theories of plate tectonics, the United States' Catskill Delta formation is sometimes referred to as part of the Old Red Sandstone. In the modern day, however, 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.

Notable buildings constructed of Old Red Sandstone

The nave of Sweetheart Abbey, constructed of Old Red Sandstone.

• Muchalls Castle, Scotland [5]
• Stonehaven Tolbooth, Scotland [6]
• Sweetheart Abbey, Scotland [7] & [8]
• Tintern Abbey, South Wales [9] & [10]
• Ross-on-Wye market hall, Herefordshire [11]
• Shrewsbury Castle, Shropshire [12]
• New York Life Insurance Building, Montreal

See also

• New Red Sandstone
• Geology of the British Isles

References

External links

• Cawdor Castle which, despite being brown, is made of Old Red Sandstone
• Sweetheart Abbey