Marl is a loose, soft, brittle and relatively young sedimentary rock composed of silt, muddy carbonate and clay minerals. It is a transition rock type between clay rock and carbonate rock. It is usually grey, yellow and green and can also be darker. Marl is mainly composed of carbonate (30–70%), and the dominant minerals are calcite with small amounts of dolomite, aragonite and siderite. The main clay mineral is illite, and kaolinite and montmorillonite are uncommon. Marl is usually distributed in the transition zone between clay rock and limestone and appears in thin layers or lenticular formations between these two types of rocks.