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Sedimentary Rocks
Of the three types of rocks -- igneous, metamorphic, and sedimentary -- only sedimentary rocks contain fossils. These types of rocks form over thousands and millions of years. As pieces of sediment, that is, miniscule pieces of earth, are eroded by water or wind, they settle at the bottoms of rivers and lakes, where they get compressed into rocks. Over millions of years, the pressure causes the pieces of sediment to form rocks.
Limestone
Limestone, a type of sedimentary rock, is effective at preserving fossils and fossil remains. For example, a limestone cave near Chicago contained a fossil trove. Some of the fossils dated back more than 310 million years. Limestone typically forms in marine waters that are shallow, calm and warm, which makes them ideal for preserving shells and shell fragments because it is in these waters where organisms that are capable of forming calcium carbonate shells thrive.
Shale
Shale, a type of sedimentary rock, preserves the shape of dead life forms. As life forms settled on developing shale, fine-grain rocks and sediment settled on and around the forms, creating a kind of mold of the dead life form. For example, the Burgess Shale in Alberta, Canada includes fossils from the early- to middle-Cambrian period; most of the fossils here are of animals that do not have hard skeletons, indicating that the fossilization process had to have occurred quickly, as soft tissue decomposes more quickly than hard remains.
Sandstone
Sandstone, a type of sedimentary rock, does not commonly include as many fossils as limestone and shale because of its coarse grains, which don't retain faithful impressions of decomposing lifeforms. In Arizona, for example, sandstone captured the remains of life forms from more than 175 million years ago; the fossils include vertebrate skeletal fossils, Moab specimens and freshwater deposits.