Land plants and animals did not yet exist, but the Michigan Sea was full of life. On the bottom of the Michigan Sea, layers of sediment started to accumulate. The sedimentary material varied according to the changing times and conditions. One source of sediment was rivers which flowed into the sea, carrying sand and clay from nearby mountains. Another source of sediment was biological.
Some sea creatures have hard shells or skeletons of calcium carbonate.
Shellfish and corals are well-known examples. Less known are the foraminifers , single celled organisms, which produce a tiny shell of calcium carbonate. When foraminifers in the Michigan Sea died, their shells sank to the bottom and formed thick sediments of calcium carbonate. With the passage of millions of years, through the effects of pressure, heat and chemical reactions, the sedimentary layers of sand, clay and calcium carbonate became stone. Sand became sandstone.