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A sample of marl

Marls are calcium carbonate or lime-rich muds or mudstones which contain variable amounts of clays and calcite or aragonite. The term is most often used to describe lacustrine (lake) sediments but may also be used for marine deposits. The term 'marl' is widely used in English-language geology, while the term seekreide ("sea chalk" in German) is used in European references.

The lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marl followed by rhythmically-banded limestone and marl layers. Similar upper Cretaceous cyclic sequences in Germany have been correlated with Milankovitch orbital forcing.

Marl is common in post-glacial lake bed sediments, often found underlying peat bogs. It has been utilized as a soil conditioner and acid soil neutralizing agent.

Marl is a soft, loose, earthy, material that consists of varying amounts of calcium carbonate, clay, and silt size material and is formed primarily in freshwater conditions (Hubbard and Herman, 1990). Marl and marlstone are fine grained sediments and rocks that are composed of 50% or more carbonate (Potter et al., 2005). Marl is an old term 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 marine or especially freshwater conditions; specifically an earthy substance containing 35-65% clay and 65-35% carbonate (Pettijohn, 1957, p. 410). Marlstone is an indurated 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.

• Schurrenberger, D., Russell, J. and Kerry Kelts. 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29: 141-154.
• Chalk of Kent by C. S. Harris Accessed 11/06/2005
• Geochemistry and time-series analyses of orbitally forced Upper Cretaceous marl–limestone rhythmites, abstract Accessed 11/06/2005
• Palaeoenvironmental Interpretation of the Early Postglacial Sedimentary Record of a Marl Lake Accessed 11/06/2005

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• Agricultural lime