Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. 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 marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'.
Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl, more correctly called an earthy or impure argillaceous limestone. It has a blocky subconchoidal fracture, and is less fissile than shale. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, 
The lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marls followed by rhythmically banded limestone and marl layers. The Channel Tunnel follows these marl layers between France and the United Kingdom. Upper Cretaceous cyclic sequences in Germany and marl–opal-rich Tortonian-Messinian strata in the Sorbas basin related to multiple sea drawdown have been correlated with Milankovitch orbital forcing.
American types used in agriculture
This section needs to be updated.February 2020)(
Marl was extensively mined in Central New Jersey as a soil conditioner in the 1800s. In 1863, the most common marl was blue marl. While the specific composition and properties of the marl varied depending on what layer it was found in, blue marl was generally composed of 38.70% silicic acid and sand, 30.67% oxide of iron, 13.91% carbonate of lime, 11.22% water, 4.47% potash, 1.21% magnesia, 1.14% phosphoric acid, and 0.31% sulphuric acid.
Marl was in high demand for farms. An example of the amount of marl mined comes from a report from 1880, from Marlboro, Monmouth County, New Jersey, which reported the following tons of marl sold during the year:
- OC Herbert Marl Pit – 9961 tons
- Uriah Smock Marl Pit – 4750 tons
- CM Conover Marl Pit – 760 tons
In the Centennial Exhibition report in 1877, marl is described in many different forms and came from 69 marl pits in and around New Jersey. The report identified a number of agricultural marls types, including clay marl, blue marl, red marl, high bank marl, shell layer marl, under shell layer marl, sand marl, green marl, gray marl, and clayey marl.
- Pettijohn (1957), pp. 368–369
- Pettijohn (1957), p. 410-411.
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- Annual report of the Agricultural Experiment Station (1896). sfnp error: no target: CITEREFAnnual_report_of_the_Agricultural_Experiment_Station1896 (help)
- Report of the New Jersey Commissioners (1877). sfnp error: no target: CITEREFReport_of_the_New_Jersey_Commissioners1877 (help)
- New Jersey. State Centennial Board (1877). Report of the New Jersey Commissioners on the Centennial Exhibition. Naar, Day, & Naar, printers. p. 203. Retrieved 2017-01-06.
- Geological Survey of New Jersey (1880). Annual Report of the State Geologist. p. 184. Retrieved 2017-01-06.
- Annual Report of the State Geologist. 1887. pp. 193–.
- University of Wisconsin. Agricultural Experiment Station; Wisconsin. Agricultural Experiment Station, Madison (1896). Annual Report of the Agricultural Experiment Station of the University of Wisconsin. 13. Democrat Printing Company, state printers. p. 295. Retrieved 2017-01-06.
- Cameron, Roderick (1951). Pioneer Days in Kansas. Belleville, Kansas: Cameron Book Company. p. 29.[self-published source]
- Harris, C.S.; et al., eds. (1996). Engineering Geology of the Channel Tunnel. London: Thomas Telford. p. 57. ISBN 0-7277-2045-7.
- Krijgsman, W. (2001). "Astrochronology for the Messinian Sorbas basin (SE Spain) and orbital (precessional) forcing for evaporite cyclicity" (PDF). Sedimentary Geology. 140 (1–2): 43–60. Bibcode:2001SedG..140...43K. doi:10.1016/S0037-0738(00)00171-8. hdl:1874/1632.
- Murphy, David H.; Wilkinson, Bruce H. (April 1980). "Carbonate deposition and facies distribution in a central Michigan marl lake". Sedimentology. 27 (2): 123–135. doi:10.1111/j.1365-3091.1980.tb01164.x.
- Parker, Alan (24 July 2005). "There[s Marl in ThemThar Ponds". Northern Woodlands. Center for Northern Woodlands Eduction. Retrieved 25 September 2020.
- Perri, Francesco; Dominici, Rocco; Critelli, Salvatore (March 2015). "Stratigraphy, composition and provenance of argillaceous marls from the Calcare di Base Formation, Rossano Basin (northeastern Calabria)". Geological Magazine. 152 (2): 193–209. doi:10.1017/S0016756814000089.
- Pettijohn, F. J. (1957). Sedimentary Rocks (2nd ed.). New York: Harper & Brothers. OCLC 551748.CS1 maint: ref=harv (link)
- Wiik, Emma; Bennion, Helen; Sayer, Carl D.; Davidson, Thomas A.; McGowan, Suzanne; Patmore, Ian R.; Clarke, Stewart J. (November 2015). "Ecological sensitivity of marl lakes to nutrient enrichment: evidence from Hawes Water, UK". Freshwater Biology. 60 (11): 2226–2247. doi:10.1111/fwb.12650.
- Schurrenberger, D., Russell, J. and Kerry Kelts. 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29: 141–154.
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