is a variety of limestone, composed of shells or shell fragments, more or less cemented to form a rock.

limestone composed predominantly of clastic sand-size grains of calcite, or rarely aragonite, usually as fragments of shells or other skeletal structures. Some calcarenite contain oolites (small, spherical grains of calcium carbonate that resemble roe) and may be termed oolite limestone. Calcareous sandstone, in which the calcium carbonate is present chiefly as bonding material, are not included in this category. [Go to source

An indurated (qv) sand composed mainly of detrital (qv) calcium carbonate fragments. A sand sized limestone.