marlstone
/ˈmɑːlstəʊn/noun
Rock consisting of, or having the composition of, consolidated marl; marlite. Also: specifically an iron-rich oolitic limestone of the Middle Lias of England, or the group of iron-rich rocks in which it occurs.
Origin
Mid 18th century; earliest use found in Thomas Amory (d. 1788), novelist. From marl + stone.