Most fossils are found in sedimentary rocks. These rocks form when layers of silt, precipitates from water, or volcanic ash build up on the bottom of a lake, pond, marsh or sea. As the layers build up, they compress and harden into sedimentary rocks. There are four different types of fossils that all form in their own special way: petrified fossils, carbon fossils, cast fossils and trace fossils. (Roadcut in Morrison Formation)