An assortment of fossil animals Ð mostly brachiopods, bryozoans and crinoids Ð lie in a fine-grained, sparkly background of petrified mud. The animals perished in the mud a little less than 400 million years ago. Their shells remain as fossils. At first, there were numerous mini-caves in areas sheltered between the shells. Coarse crystals of calcite spar eventually filled these holes. The sparry crystals grew outward from the shells into the open spaces until they met mud. Although the fossils and mud are also made of calcite, each element retains its own identity.

Crossed polarizers