Marl is an extremely, fine-grained sedimentary rock that is intermediate between a clay and a limestone [calcium carbonate (CaCO3)]. If the former predominates, it is a mudstone; if the latter (containing up to 60% of minerals such as calcite, with some quartzite and particles of silt), it is termed calcareous.
Red marl owes its color to the presence of iron oxide, whereas green marl contains chlorite and glauconite and hardly any calcium carbonate. Marls formed from deposits at the bottom of lakes or the sea. Those of marine origin may occur in beds with gypsum and halite.
Marls are used as soil conditioners and in making portland cement.