Marl, a natural mixture of clay and calcium carbonate. It is a loose, earthy substance, usually gray or bluish in color. Marl deposits are formed in shallow lakes. The calcium carbonate marl is derived from the biological action of certain aquatic plants on dissolved carbonates. Shell marl contains shellfish remains. Greensand marl contains glauconite, or greensand, a silicate of potassium and iron. Marl is used to neutralize acid soils. Portland cement is sometimes made with marl instead of limestone.