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Hutton's Unconformity is a name given to various notable
geological Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...
sites in
Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
identified by the 18th-century Scottish geologist
James Hutton James Hutton (; 3 June Old Style and New Style dates, O.S. 1726 – 26 March 1797) was a Scottish geologist, Agricultural science, agriculturalist, chemist, chemical manufacturer, Natural history, naturalist and physician. Often referred to a ...
as places where the junction between two types of rock formations can be seen. This geological phenomenon marks the location where rock formations created at different times and by different processes adjoin. For Hutton, such an unconformity provided evidence for his Plutonist theories of
uniformitarianism Uniformitarianism, also known as the Doctrine of Uniformity or the Uniformitarian Principle, is the assumption that the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in ...
and the
age of Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years. This age may represent the age of Earth's accretion, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of ...
. An unconformity is a break in the normal progression of sedimentary deposits, where newer deposits are laid on top of older. The example Hutton discovered is known as an angular unconformity in which a sharp change in younging direction can be seen in the orientation of bedding planes. The younging direction points ''from'' the oldest bed, to the ''youngest'' bed in the sequence of sedimentary beds. In his search, Hutton and colleagues examined rock outcrops and cliffs, both riverside and sea, and found several locations where two adjoining rock types had been laid bare, the most noted being at Siccar Point on the coast of
Berwickshire Berwickshire (; ) is a historic county, registration county and lieutenancy area in south-eastern Scotland, on the English border. The county takes its name from Berwick-upon-Tweed, its original county town, which was part of Scotland at the ...
.


Theory of rock formations

Hutton hit on a variety of ideas to explain the rock formations he saw, and, after a quarter century of work, he read his paper, '' Theory of the Earth; or an Investigation of the Laws Observable in the Composition, Dissolution and Restoration of Land upon the Globe,'' to the
Royal Society of Edinburgh The Royal Society of Edinburgh (RSE) is Scotland's national academy of science and letters. It is a registered charity that operates on a wholly independent and non-partisan basis and provides public benefit throughout Scotland. It was establis ...
on 7 March and 4 April 1785. Later that year Hutton read an abstract of ''Concerning the System of the Earth, its Duration and Stability'' to a Society meeting, and had it printed and circulated privately. In it, he outlined his theory that the "solid parts of the present land appear in general, to have been composed of the productions of the sea, and of other materials similar to those now found upon the shores." From this he deduced that the land was a composition which had been formed by the operation of second causes in an earlier world composed of sea and land, with tides, currents, and "such operations at the bottom of the sea as now take place" so that "while the present land was forming at the bottom of the ocean, the former land maintained plants and animals; at least the sea was then inhabited by animals, in a similar manner as it is at present", and that most, if not all, of the land had been produced by natural operations involving the consolidation of masses of loose materials collected at the bottom of the sea, followed by the elevation of the consolidated masses to their present position.''Concerning the System of the Earth''
abstract


Hutton's search for unconformities

Early geologists had interpreted angular unconformities in terms of Neptunism (holding that rocks had formed from the crystallisation of minerals from ocean waters after
Noah's Flood The Genesis flood narrative (chapters 6–9 of the Book of Genesis) is a Hebrew flood myth. It tells of God's decision to return the universe to its pre- creation state of watery chaos and remake it through the microcosm of Noah's ark. The B ...
) but Hutton wanted to examine such formations himself in support for his theory of
Plutonism Plutonism is the geology, geologic theory that the igneous rocks forming the Earth originated from intrusive Magma, magmatic activity, with a continuing gradual process of weathering and erosion wearing away rocks, which were then deposited on t ...
, in which rocks are formed from volcanic action. In 1785 on Hutton's first field excursion, he was invited by the Duke of Atholl to search for exposures in a remarkably straight, deeply incised valley in the Scottish Highlands known as the Glen Tilt, near the town of
Blair Atholl Blair Atholl (from the Scottish Gaelic: ''Blàr Athall'', originally ''Blàr Ath Fhodla'') is a village in Perthshire, Scotland, built about the confluence of the Rivers Tilt and Garry in one of the few areas of flat land in the midst of the G ...
. Upstream of one of the Duke's hunting lodges, known as Forest Lodge, Hutton and his friend, John Clerk of Eldin, found exposures at the Dail-an-eas Bridge that demonstrated red igneous granite veins were injected into older grey schists formations. This discovery proved that Werner's Neptunism theory was in error. Werner argued that sedimentary rocks, including what are now known as metamorphic rocks such as schists, were formed after granites. John Clerk drew these exposures with great accuracy and these drawings provide important visual evidence of Hutton's discoveries. On a trip to the
Isle of Arran The Isle of Arran (; ) or simply Arran is an island off the west coast of Scotland. It is the largest island in the Firth of Clyde and the seventh-largest Scottish island, at . Counties of Scotland, Historically part of Buteshire, it is in the ...
in 1787 he found his first example of an unconformity to the north of Newton Point near
Lochranza Lochranza () is a village located on the Isle of Arran in the Firth of Clyde, Scotland. The population, somewhat in decline, is around 200 people. Geography Lochranza is the northernmost of Arran's villages and is located in the northwestern c ...
, but the limited view did not give the information he needed. It occurs where vertically orientated
Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...
Dalradian
schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...
s are overlain by more recent cornstones in the Kinnesswood Formation of the Inverclyde Group ( Lower Carboniferous), with an obvious difference in dip between the two rock layers, but he incorrectly thought that the strata were conformable at a depth below the exposed outcrop. In 2017, the Arran Geopark Project erected a marker stone to indicate the significance of Hutton's discovery and its location (map reference: NR936521) Later in 1787 Hutton noted what is now known as the Hutton Unconformity at Inchbonny,
Jedburgh Jedburgh ( ; ; or ) is a town and former royal burgh in the Scottish Borders and the traditional county town of the Shires of Scotland, historic county of Roxburghshire. History Jedburgh began as ''Jedworð'', the "worth" or enclosed settlem ...
, in layers of
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
visible on the banks of the
Jed Water The Jed Water is a river and a tributary of the River Teviot in the Borders region of Scotland. In total the Jed Water is over long and it falls . It flows into the Teviot near Jedfoot Bridge () two miles north of Jedburgh. Jed Water rises f ...
. He later wrote of how he "rejoiced at my good fortune in stumbling upon an object so interesting in the natural history of the earth, and which I had been long looking for in vain". That year he found the same sequence in Teviotdale.


Siccar Point

Around the start of June 1788, Hutton went with John Playfair to the
Berwickshire Berwickshire (; ) is a historic county, registration county and lieutenancy area in south-eastern Scotland, on the English border. The county takes its name from Berwick-upon-Tweed, its original county town, which was part of Scotland at the ...
coast and found more examples of this sequence in the valleys of the Tour and Pease Burns near
Cockburnspath Cockburnspath ( ) is a village in Berwickshire in the Scottish Borders. It lies near the North Sea coast between Berwick-upon-Tweed and Dunbar. Cockburnspath is the eastern terminus of the Southern Upland Way as well as the northern terminus of ...
. They then took a boat trip from Dunglass Burn east along the coast with the geologist Sir James Hall of Dunglass. They found the sequence in the cliff below St. Helens, then just to the east at Siccar Point found what Hutton called "a beautiful picture of this junction washed bare by the sea". Continuing along the coast, they made more discoveries including sections of the vertical beds showing strong ripple marks which gave Hutton "great satisfaction" as a confirmation of his supposition that these beds had been laid horizontally in water. Playfair wrote:Gillen, Con (2003) ''Geology and landscapes of Scotland''. Harpenden. Terra Publishing. Page 95. At Siccar Point, during the lower
Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...
Llandovery epoch around 435 million years ago, thin beds of fine-grained
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
were laid down gradually, deep in the Iapetus Ocean, alternating with thicker layers of hard
greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...
formed when torrents swept unsorted sandstone down the
continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margi ...
. During the following 65 million years, the ocean closed and the layers of rock were buckled almost vertically, getting forced to the surface as the ocean floor was subducted under the northern continent. Erosion of the exposed edges of layers formed a characteristic shape of ribs of hard greywacke with narrow gaps where mudstone was worn away, and fragments of greywacke lay on the surface as a talus deposit. In the Famennian Late
Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...
period around 370 million years ago this was a low-lying tropical area just south of the equator, where rainy season rivers deposited sands and silts rich in
iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...
, then during the dry seasons these were blown about by winds to form easily eroded layers and dunes. Over time these consolidated to form rocks; initially mixed with the greywacke fragments to form a basal conglomerate which has been defined as the Redheugh Mudstone Formation and dated by fossils of '' Bothriolepis hicklingi''. Above this, layers of Old Red Sandstone formed the Greenheugh Sandstone Formation which is estimated to be around thick. This in turn was uplifted above the sea, tilting to a shallow slope, then eroded in places such as Siccar Point to expose the underlying layers.


See also

* Geology of Scotland


References


External links

{{Commons category, Hutton's Unconformities
VFE: Siccar Point, Scotland
— virtual excursion in ''Historical Geology'' (a free online textbook for Historical Geology courses) Isle of Arran Historical geology Stratigraphy of the United Kingdom Geology of Scotland Unconformities