Marl, city, North Rhine–Westphalia Land (state), western Germany. It is situated in the Ruhr industrial district, just northwest of Recklinghausen. First mentioned about 800 as a relatively large settlement, the Marl district was sold to the archbishops of Cologne about 1000 and thereafter was part of the “Vest Recklinghausen” of the prince electors. After 1802 it passed to the dukes of Arenberg, who held it as a fief of Prussia from 1815. It grew with the development of coal and iron ore mining in the late 19th century, and the town was chartered in 1936. Chemical factories and heavy industry traditionally supplemented coal mining, but these declined in the late 20th century. Pop. (2003 est.) 91,748.