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One of the marl lions at the entrance of Castle Valkenburg,...

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[1]. 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.

 

Source: Wikipedia.org

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Taken on March 28, 2009