Marl or Marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl is originally 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 freshwater conditions; specifically an earthy substance containing 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 marlstones. 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. The term marl is widely used in English-language geology, while the terms Mergel and Seekreide (German for "sea chalk") are used in European references.
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. Similar upper Cretaceous cyclic sequences in Germany have been correlated with Milankovitch orbital forcing.
Marl as lacustrine sediment 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. It 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).