What is Old Red Sandstone?
[[File:Siccar point SE cliff.jpg|thumb|right|250px|[[James Hutton|Hutton's]] [[Unconformity|angular unconformity]] is at [[Siccar Point]] where 345 million year old [[Devonian]] Old Red Sandstone overlies 425 million year old [[Silurian]] [[greywacke]].<ref name=FieldExcursion/>]] [[File:Siccar Point red capstone closeup.jpg|right|thumb|250px|Siccar Point: eroded and gently sloping [[Devonian]] Old Red Sandstone layers cap over [[Conglomerate (geology)|conglomerate]] layer and older vertically bedded [[Silurian]] [[greywacke]] rocks.]] [[File:St Magnus Cathedral Kirkwall.jpg|right|thumb|230px|St. Magnus Cathedral, Kirkwall, [[Orkney]], built of local sandstone]] [[File:Hutton Unconformity, Jedburgh.jpg|thumb|200px|The unconformity at [[Jedburgh]]: illustrated by John Clerk in 1787 with a recent photography by Keith Montgomery]] [[File:Euramerica en.svg|thumb|250px|The Old Red Sandstone Continent in the [[Devonian]]]] The '''Old Red Sandstone''' is a dark red [[sandstone]] laid down mainly in the [[Devonian]] period over a large part of the continent of [[Laurussia]]. The base of the ORS is now known to be in the [[Silurian]] and the top in the [[Carboniferous]].<ref name=ORS>{{cite book|editor=Barclay W.J.|editor2=Browne M.A.E.|editor3=McMillan A.A.|editor4=Pickett E.A.|editor5=Stone P.|editor6=Wilby P.R.|title=The Old Red Sandstone of Great Britain|chapter-url=http://jncc.defra.gov.uk/pdf/v31chap1.pdf|accessdate=26 February 2012|series=Geological Conservation Review Series|volume=31|publisher=Joint Nature Conservation Committee|isbn=978-1-86107-543-7|chapter=1: Introduction to the Old Red Sandstone of Great Britain|year=2005|archive-date=7 March 2012|archive-url=https://web.archive.org/web/20120307074820/http://jncc.defra.gov.uk/PDF/V31chap1.pdf|url-status=dead}}</ref> Laurussia is often called the '''Old Red Continent''', or [[Euramerica]]. It included what is now much of northern [[Europe]], [[Greenland]] and [[North America]]. It was, at the time, between 0<sup>o</sup> and 30<sup>o</sup> south of the [[equa