Marl or
marlstone is a
calcium carbonate or
lime-rich mud or
mudstone which contains variable amounts of
clays and
silt. The dominant carbonate mineral in most marls is
calcite, but other carbonate minerals such as
aragonite,
dolomite, and
siderite may be present. Marl was 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. It also describes a habit of coralline red alga. 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 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 "lake chalk") are used in European references.