This rock formed in a warm, salt sea in the area known as the Michigan Basin. Limestone is formed by the accumulation of the skeletons and shells of small sea life. This material was changed by dolomitization. Magnesium is added to the limestone causing it to recrystalise. This destroys the microscopic fossils and makes the larger ones difficult to identify. Holes are also formed during this process. These holes sometimes fill with other minerals. In the Wiarton area these holes, known as vugs, fill with sphalerite (zinc sulphide) and other minerals.