Conglomerate () is a

clastic Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

that is composed of a substantial fraction of rounded to subangular

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

-size

clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

. A conglomerate typically contains a

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

of finer-grained

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

s, such as

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel when ...

, or

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

, which fills the

interstices An interstitial space or interstice is a space between structures or objects. In particular, interstitial may refer to: Biology * Interstitial cell tumor * Interstitial cell, any cell that lies between other cells * Interstitial collagenase, ...

between the clasts. The clasts and matrix are typically cemented by calcium carbonate,

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

, or hardened clay. Conglomerates form by the consolidation and

lithification Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificati ...

of gravel. They can be found in sedimentary rock sequences of all ages but probably make up less than 1 percent by weight of all sedimentary rocks. In terms of origin and depositional mechanisms, they are closely related to sandstones and exhibit many of the same types of

sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...

, e.g., tabular and trough

cross-bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

and

graded bedding In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...

.Boggs, S. (2006) ''Principles of Sedimentology and Stratigraphy.'', 2nd ed. Prentice Hall, New York. 662 pp. Friedman, G.M. (2003) ''Classification of sediments and sedimentary rocks.'' In Gerard V. Middleton, ed., pp. 127-135, ''Encyclopedia of Sediments & Sedimentary Rocks, Encyclopedia of Earth Science Series.'' Kluwer Academic Publishers, Boston, Massachusetts. 821 pp. Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. Fanglomerates are poorly sorted, matrix-rich conglomerates that originated as

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally ...

s on

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

s and likely contain the largest accumulations of gravel in the geologic record.

Classification of conglomerates

Conglomerates may be named and classified by the: * Amount and type of matrix present * Composition of gravel-size clasts they contain * Size range of gravel-size clasts present The classification method depends on the type and detail of research being conducted.Nichols, G. (2009) ''Sedimentology and Stratigraphy,'' 2nd ed. John Wiley & Sons Ltd, Chichester, West Sussex, United Kingdom. 419 pp. A sedimentary rock composed largely of gravel is first named according to the roundness of the gravel. If the gravel clasts that comprise it are largely well-rounded to subrounded, it is a ''conglomerate.'' If the gravel clasts that comprise it are largely angular, it is a

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

. Such breccias can be called sedimentary breccias to differentiate them from other types of breccia, e.g. volcanic and fault breccias. Sedimentary rocks that contain a mixture of rounded and angular gravel clasts are sometimes called breccio-conglomerate.

Texture

Conglomerates contain at least 30% of rounded to subangular clasts larger than in diameter, e.g., granules,

pebble A pebble is a clast of rock with a particle size of based on the Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than granules ( in diameter) and smaller than cobbles ( in diameter). A rock made predominant ...

cobble Cobble may refer to: * Cobble (geology), a designation of particle size for sediment or clastic rock * Cobblestone, partially rounded rocks used for road paving * Hammerstone, a prehistoric stone tool * Tyringham Cobble, a nature reserve in Tyr ...

s, and

boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...

s. However, conglomerates are rarely composed entirely of gravel-size clasts. Typically, the space between the gravel-size clasts is filled by a mixture composed of varying amounts of silt, sand, and clay, known as ''matrix''. If the individual gravel clasts in a conglomerate are separated from each other by an abundance of matrix such that they are not in contact with each other and ''float'' within the matrix, it is called a paraconglomerate. Paraconglomerates are also often unstratified and can contain more matrix than gravel clasts. If the gravel clasts of a conglomerate are in contact with each other, it is called an orthoconglomerate. Unlike paraconglomerates, orthoconglomerates are typically cross-bedded and often well-cemented and lithified by either

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

, or clay. The differences between paraconglomerates and orthoconglomerates reflect differences in how they are deposited. Paraconglomerates are commonly either glacial

till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

s or

deposits. Orthoconglomerates are typically associated with aqueous currents.

Clast composition

Conglomerates are also classified according to the composition of their clasts. A conglomerate or any clastic sedimentary rock that consists of a single rock or mineral is known as either a monomict, monomictic, oligomict, or oligomictic conglomerate. If the conglomerate consists of two or more different types of rocks, minerals, or combination of both, it is known as either a polymict or polymictic conglomerate. If a polymictic conglomerate contains an assortment of the clasts of metastable and unstable rocks and minerals, it is called either a petromict or petromictic conglomerate. Tucker, M. E. (2003) ''Sedimentary Rocks in the Field'', 3rd ed. John Wiley & Sons Ltd,West Sussex, England. In addition, conglomerates are classified by source as indicated by the lithology of the gravel-size clasts If these clasts consist of rocks and minerals that are significantly different in lithology from the enclosing matrix and, thus, older and derived from outside the basin of deposition, the conglomerate is known as an extraformational conglomerate. If these clasts consist of rocks and minerals that are identical to or consistent with the lithology of the enclosing matrix and, thus, penecontemporaneous and derived from within the basin of deposition, the conglomerate is known as an intraformational conglomerate. Two recognized types of intraformational conglomerates are shale-pebble and flat-pebble conglomerates. A shale-pebble conglomerate is a conglomerate that is composed largely of clasts of rounded mud chips and pebbles held together by clay minerals and created by erosion within environments such as within a river channel or along a lake margin.Williams, G. D. (1966) ''Origin of Shale-Pebble Conglomerate.'' American Association of Petroleum Geologist Bulletin. vol. 50, no. 3, pp. 573–577. Flat-pebble conglomerates (edgewise conglomerates) are conglomerates that consist of relatively flat clasts of lime mud created by either storms or tsunami eroding a shallow sea bottom or tidal currents eroding tidal flats along a shoreline.Flugel, E. (2010) ''Microfacies of Carbonate Rocks: Analysis, Interpretation and Application'', 2nd ed. Springer-Verlag, Berlin, Germany.

Clast size

Finally, conglomerates are often differentiated and named according to the dominant clast size comprising them. In this classification, a conglomerate composed largely of granule-size clasts would be called a granule conglomerate; a conglomerate composed largely of pebble-size clasts would be called a pebble conglomerate; and a conglomerate composed largely of cobble-size clasts would be called a cobble conglomerate.

Sedimentary environments

Conglomerates are deposited in a variety of sedimentary environments.

Deepwater marine

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites were ...

s, the basal part of a bed is typically coarse-grained and sometimes conglomeratic. In this setting, conglomerates are normally very well sorted, well-rounded and often with a strong A-axis type imbrication of the clasts.Walker, R. G. 1979. Facies Models. Reprinted with revisions from a series of papers in Geoscience Canada, 1976–1979, Geological Association of Canada

Shallow marine

Conglomerates are normally present at the base of sequences laid down during

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

s above an

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

, and are known as basal conglomerates. They represent the position of the

shoreline A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...

at a particular time and are

diachronous In geology, a diachronism (Greek language, Greek ''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where i ...

Fluvial

Conglomerates deposited in fluvial environments are typically well rounded and poorly sorted. Clasts of this size are carried as

bedload The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load. Bed load moves by rolling, sliding, and/or saltating (hopp ...

and only at times of high flow-rate. The maximum clast size decreases as the clasts are transported further due to

attrition Attrition may refer to *Attrition warfare, the military strategy of wearing down the enemy by continual losses in personnel and material **War of Attrition, fought between Egypt and Israel from 1968 to 1970 **War of attrition (game), a model of agg ...

, so conglomerates are more characteristic of immature river systems. In the sediments deposited by mature rivers, conglomerates are generally confined to the basal part of a channel fill where they are known as ''pebble lags''. Tucker, M. E.br>''Sedimentary petrology''
3rd edition, 2001, WileyBlackwell Conglomerates deposited in a fluvial environment often have an AB-plane type imbrication.

Alluvial

Alluvial deposits form in areas of high relief and are typically coarse-grained. At mountain fronts individual

s merge to form braidplains and these two environments are associated with the thickest deposits of conglomerates. The bulk of conglomerates deposited in this setting are clast-supported with a strong AB-plane imbrication. Matrix-supported conglomerates, as a result of debris-flow deposition, are quite commonly associated with many alluvial fans. When such conglomerates accumulate within an alluvial fan, in rapidly eroding (e.g.,

desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...

) environments, the resulting rock unit is often called a ''fanglomerate''.

Glacial

Glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...

s carry a lot of coarse-grained material and many glacial deposits are conglomeratic.

tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

s, the sediments deposited directly by a glacier, are typically poorly sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely milled rock fragments. Waterlaid deposits associated with glaciers are often conglomeratic, forming structures such as

esker An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Amer ...

Examples

An example of conglomerate can be seen at

Montserrat Montserrat ( ) is a British Overseas Territories, British Overseas Territory in the Caribbean. It is part of the Leeward Islands, the northern portion of the Lesser Antilles chain of the West Indies. Montserrat is about long and wide, with r ...

, near

Barcelona Barcelona ( , , ) is a city on the coast of northeastern Spain. It is the capital and largest city of the autonomous community of Catalonia, as well as the second most populous municipality of Spain. With a population of 1.6 million within ci ...

. Here, erosion has created vertical channels that give the characteristic jagged shapes the mountain is named for (Montserrat literally means "jagged mountain"). The rock is strong enough to use as a building material, as in the

Santa Maria de Montserrat Abbey Santa Maria de Montserrat () is an abbey of the Order of Saint Benedict located on the mountain of Montserrat in Monistrol de Montserrat, Catalonia, Spain. It is notable for enshrining the image of the Virgin of Montserrat. The monastery was fou ...

. Another example, the Crestone Conglomerate, occurs in and near the town of Crestone, at the foot of the

Sangre de Cristo Range , country= United States , subdivision1= Colorado , subdivision2_type= Counties , subdivision2= , parent= Sangre de Cristo Mountains, Rocky Mountains , borders_on= , geology= , age= , orogeny= Fault-block mountains , area_mi2= 1 ...

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

San Luis Valley The San Luis Valley is a region in south-central Colorado with a small portion overlapping into New Mexico. The valley is approximately long and wide, extending from the Continental Divide on the northwest rim into New Mexico on the south. It co ...

. The Crestone Conglomerate consists of poorly sorted fanglomerates that accumulated in prehistoric alluvial fans and related fluvial systems. Some of these rocks have hues of red and green. Conglomerate cliffs are found on the east coast of

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...

from

Arbroath Arbroath () or Aberbrothock ( gd, Obar Bhrothaig ) is a former royal burgh and the largest town in the council area of Angus, Scotland, with a population of 23,902. It lies on the North Sea coast some ENE of Dundee and SSW of Aberdeen. The ...

northwards along the coastlines of the former counties of

Angus Angus may refer to: Media * ''Angus'' (film), a 1995 film * ''Angus Og'' (comics), in the ''Daily Record'' Places Australia * Angus, New South Wales Canada * Angus, Ontario, a community in Essa, Ontario * East Angus, Quebec Scotland * An ...

and

Kincardineshire Kincardineshire, also known as the Mearns (from the Scottish Gaelic meaning "the Stewartry"), is a historic county, registration county and lieutenancy area on the coast of northeast Scotland. It is bounded by Aberdeenshire on the north and w ...

Dunnottar Castle Dunnottar Castle ( gd, Dùn Fhoithear, "fort on the shelving slope") is a ruined medieval fortress located upon a rocky headland on the north-eastern coast of Scotland, about south of Stonehaven. The surviving buildings are largely of the 1 ...

sits on a rugged promontory of conglomerate jutting into the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

just south of the town of

Stonehaven Stonehaven ( , ) is a town in Scotland. It lies on Scotland's northeast coast and had a population of 11,602 at the 2011 Census. After the demise of the town of Kincardine, which was gradually abandoned after the destruction of its royal cast ...

. Copper Harbor Conglomerate is found both in the

Keweenaw Peninsula The Keweenaw Peninsula ( , sometimes locally ) is the northernmost part of Michigan's Upper Peninsula. It projects into Lake Superior and was the site of the first copper boom in the United States, leading to its moniker of "Copper Country." As o ...

and

Isle Royale National Park Isle Royale National Park is an American national park consisting of Isle Royale – known as Minong to the native Ojibwe – along with more than 400 small adjacent islands and the surrounding waters of Lake Superior, in the state of Michigan ...

Lake Superior Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...

. Conglomerate may also be seen in the domed hills of

Kata Tjuta Kata Tjuṯa / The Olgas (Pitjantjatjara: , lit. 'many heads'; ) is a group of large, domed rock formations or bornhardts located about southwest of Alice Springs, in the southern part of the Northern Territory, central Australia. Uluṟu / Aye ...

, in Australia's

Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an states and territories of Australia, Australian territory in the central and central northern regions of Australia. The Northern Territory ...

or in the

Buda Hills The Buda Hills ( Hungarian: ''Budai-hegység'') are a low mountain range of numerous hills which dot the Buda side of Budapest, capital of Hungary. The most famous ones located within city limits are Gellért Hill, Castle Hill, Rózsadomb, , J ...

in Hungary. In the nineteenth century a thick layer of Pottsville conglomerate was recognized to underlie

anthracite coal Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the high ...

measures in Pennsylvania.

Examples on Mars

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

, slabs of conglomerate have been found at an

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...

named " Hottah", and have been interpreted by scientists as having formed in an ancient streambed. The gravels, which were discovered by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

's Mars rover

Curiosity Curiosity (from Latin '' cūriōsitās'', from ''cūriōsus'' "careful, diligent, curious", akin to ''cura'' "care") is a quality related to inquisitive thinking such as exploration, investigation, and learning, evident by observation in humans ...

, range from the size of sand particles to the size of golf balls. Analysis has shown that the pebbles were deposited by a stream that flowed at walking pace and was ankle- to hip-deep.

Metaconglomerate

Metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...

alteration transforms conglomerate into

metaconglomerate Metaconglomerate is a rock type which originated from conglomerate after undergoing metamorphism. Conglomerate is easily identifiable by the pebbles or larger clasts in a matrix of sand, silt, or clay. Metaconglomerate looks similar to conglome ...

References

External links

Conglomerate at Cushendun, Northern Ireland
{{Webarchive, url=https://web.archive.org/web/20160303200906/http://www.geographyinaction.co.uk/Assets/Photo_albums/Seven/pages/Conglomerate.html , date=2016-03-03 Sedimentary rocks