Crinoidal Limestone

Crinoidal limestone is a fossiliferous limestone composed mainly of the broken skeletal remains of crinoids, stalked marine echinoderms commonly called sea lilies and feather stars. Their calcite skeletons break into characteristic disc and ring-shaped plates that accumulate into rock.

The rock is calcium carbonate and often shows a sparkling appearance on fresh or polished surfaces because each crinoid plate is a single calcite crystal that reflects light. When abundant the rock is also called encrinite.

Colors range from gray and cream to brown and pink. Crinoidal limestones are widespread in Paleozoic strata and record times when vast crinoid meadows carpeted shallow sea floors.

Crinoidal limestone forms in shallow, warm, clear marine settings where crinoids flourished in dense colonies on the sea floor. When these animals died, their multi-plated skeletons disarticulated into countless small calcite ossicles.

Wave and current action concentrated and sorted these plates into shoals and banks, which were then cemented by calcium carbonate into solid limestone. High crinoid productivity and limited mud input favored thick, relatively pure crinoidal deposits.

Such rocks are especially common in Paleozoic strata such as the Carboniferous limestones of Britain and the Mississippian limestones of North America, which preserve enormous quantities of crinoid debris.

Crinoidal limestone is identified by its abundant disc-shaped, ring-shaped, and columnar crinoid ossicles, which look like tiny buttons, washers, or stacked coins (the source of the folk name screwstone for some columnar forms).

Fresh surfaces often sparkle as light reflects off the single-crystal calcite plates. As a carbonate it fizzes in dilute acid and is about Mohs 3. Color is commonly gray, cream, brown, or pink.

Look-alikes include shelly limestone (dominated by mollusk shells) and oolitic limestone (made of round ooids). The distinctive round and star-centered crinoid plates and the sparkling cleavage of calcite confirm crinoidal limestone.

Crinoidal limestone is quarried as building and ornamental stone, prized for the attractive fossil patterns revealed when it is cut and polished. Polished slabs are sometimes sold as a fossil-rich decorative marble.

It is also crushed for aggregate, cement raw material, and agricultural lime like other limestones. Historic buildings in regions such as northern England feature crinoidal limestone facing and flooring.

For collectors and paleontologists the rock is a rich source of crinoid fossils, and it is an important indicator of ancient shallow marine ecosystems dominated by these once-abundant echinoderms.