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Marl is an earthy material rich in carbonate minerals,
clays 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 ...
, and
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 hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
. Marl makes up the lower part of the cliffs of Dover, and the
Channel Tunnel The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone (Kent, England, UK) with Coquelles ( Hauts-de-France, France) beneath the English Channel at the Strait of Dover ...
follows these marl layers between France and the United Kingdom. Marl is also a common sediment in post-
glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10, ...
s, such as the marl ponds of the northeastern United States. Marl has been used as a soil conditioner and neutralizing agent for acid soil and in the manufacture of
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixe ...
.


Description

Marl or marlstone is a
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
-rich mud or
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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology.' ...
which contains variable amounts of
clays 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 ...
and
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 ...
. The term was originally loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions. These typically contain 35–65% clay and 65–35% carbonate. The term is today often used to describe
indurated Friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under duress or contact, especially by rubbing. The opposite of friable is indurate. Substances that are designated hazardous, ...
marine deposits and lacustrine (lake)
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 ...
s which more accurately should be named 'marlstone'. Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl. This is more correctly described as an earthy or impure argillaceous
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
. It has a blocky subconchoidal fracture, and is less fissile than shale. The dominant carbonate mineral in most marls is
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
, but other carbonate minerals such as aragonite or
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
may be present. ''Glauconitic marl'' is marl containing pellets of glauconite, a clay mineral that gives the mark a green color. Glauconite is characteristic of sediments deposited in marine conditions.


Occurrences

The lower stratigraphic units of the
chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. C ...
cliffs of Dover consist of a sequence of
glauconitic Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek () me ...
marls followed by rhythmically banded limestone and marl layers. Such alternating cycles of chalk and marl are common in
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
beds of northwestern Europe. The
Channel Tunnel The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone (Kent, England, UK) with Coquelles ( Hauts-de-France, France) beneath the English Channel at the Strait of Dover ...
follows these marl layers between France and the United Kingdom. Upper
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
cyclic sequences in
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...
and marl–
opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline form ...
-rich Tortonian- Messinian strata in the Sorbas basin related to multiple sea drawdown have been correlated with Milankovitch orbital forcing. Marl as lacustrine sediment is common in post-
glacial lake A glacial lake is a body of water with origins from glacier activity. They are formed when a glacier erodes the land and then melts, filling the depression created by the glacier. Formation Near the end of the last glacial period, roughly 10, ...
-bed sediments. '' Chara'', a macroalga also known as stonewort, thrives in shallow lakes with high pH and alkalinity, where its stems and fruiting bodies become calcified. After the alga dies, the calcified stems and fruiting bodies break down into fine carbonate particles that mingle with silt and clay to produce marl. Marl ponds of the northeastern United States are often kettle ponds in areas of limestone bedrock that become poor in nutrients ( oligotrophic) due to precipitation of essential
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...
. Normal pond life is unable to survive, and skeletons of freshwater molluscs such as '' Sphaerium'' and '' Planorbis'' accumulate as part of the bottom marl. In Hungary, Buda Marl is found that was formed in the Upper Eocene era. It lies between layers of rock and soil and may be defined it as both "weak rock and strong soil."


Economic geology

Marl has been used as a soil conditioner and neutralizing agent for acid soil and in the manufacture of
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...
. Because some marls have a very low permeability, they have been exploited for construction of the
Channel Tunnel The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone (Kent, England, UK) with Coquelles ( Hauts-de-France, France) beneath the English Channel at the Strait of Dover ...
between England and France and are being investigated for the storage of nuclear waste.


Historical use in agriculture

Marl is one of the oldest soil amendments used in agriculture. In addition to increasing available calcium, marl is valuable for improving soil structure and decreasing soil acidity and thereby making other nutrients more available. It was used sporadically in Britain beginning in prehistoric times and its used was mentioned by
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ' ...
in the 1st century. Its more widespread use from the 16th century on contributed to the early modern agricultural revolution. However, the lack of a high-energy economy hindered its large-scale use until the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
. Marl was used extensively in Britain, particularly in
Lancashire Lancashire ( , ; abbreviated Lancs) is the name of a historic county, ceremonial county, and non-metropolitan county in North West England. The boundaries of these three areas differ significantly. The non-metropolitan county of Lancas ...
, during the 18th century. The marl was normally extracted close to its point of use, so that almost every field had a marl pit, but some marl was transported greater distances by railroad. However, marl was gradually replaced by lime and imported mineral fertilizers early in the 19th century. A similar historical pattern was seen in Scotland. Marl was one a few soil amendments available in limited quantities the southern United States, where soils were generally poor in nutrients, prior to about 1840. By the late 19th century, marl was being mined on an industrial scale in
New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delawa ...
and was increasingly being used on a more scientific basis, with marl being classified by grade and the state geological survey publishing detailed chemical analyses.


Modern agricultural and aquacultural uses

Marl continues to be used for agriculture into the 21st century, though less frequently. The rate of application must be adjusted for the reduced content of calcium carbonate versus straight lime, expressed as the calcium carbonate equivalent. Because the carbonate in marl is predominantly calcium carbonate,
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
deficiency may be seen in crops treated with marl if they are not also supplemented with magnesium. Marl has been used in Pamlico Sound to provide a suitable artificial substrate for
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not ...
s in a reef-like environment.


Portland cement

Marl has been used in the manufacture of Portland cement. It is abundant and yields better physical and mechanical properties than metakaolin as a supplementary cementitious material and can be calcined at a considerably lower temperature.


Civil engineering

The Channel Tunnel was constructed in the West Melbury Marly Chalk, a
geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...
containing marl beds. This formation was chosen because of its very low permeability, absence of
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...
, and lack of fissures found in overlying formations. The underlying Glauconitic Marl is easily recognizable in core samples and helped establish the right level for excavating the tunnel. Marl soil has poor engineering properties, particularly when alternately wetted and dried. The soils can be stabilized by adding pozzolan (
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...
) to the soil.


Nuclear waste storage

Some marl beds have a very low permeability and are under consideration for use in the storage of nuclear waste. One such proposed storage site is the Wellenberg in central Switzerland.


Marl lakes

A marl lake is a lake whose bottom sediments include large deposits of marl. They are most often found in areas of recent glaciation and are characterized by alkaline water, rich in dissolved calcium carbonate, from which carbonate minerals are deposited. Marl lakes have frequently been dredged or mined for marl, often used for manufacturing
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19t ...
. However, they are regarded as ecologically important, and are vulnerable to damage by silting, nutrient pollution, drainage, and invasive species. In Britain, only the marl lakes of the more remote parts of northern Scotland are likely to remain pristine into the near future.


See also

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References


Citations


Bibliography

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Further reading

* Schurrenberger, D., Russell, J. and Kerry Kelts. 2003. ''Classification of lacustrine sediments based on sedimentary components''. Journal of Paleolimnology 29: 141–154.


External links


Chalk of Kent by C. S. Harris

Palaeoenvironmental Interpretation of the Early Postglacial Sedimentary Record of a Marl Lake
{{Authority control Limestone Sedimentary rocks