sentences:  Gradually hardening into limestone and drifting northward, the limestone of Stora Alvaret contains a rich fossil record of some of the marine creatures who contributed to it.For example, orthoceratites are found in some of the present day structures on the island.Only as recently as 11,000 years ago did the first portions of the island of Öland emerge from the Baltic Sea, after the overpressure of the last glaciers was relieved by melting.