Animal remains are most likely to be fossilized if their hard parts are covered by layers of sediments soon after death. With time and pressure, these sediments, such as sand, plant debris, or ash, become compressed into rock. Therefore, fossils are found in SEDIMENTARY ROCK, like sandstone, shale, limestone and coal.

IGNEOUS ROCK, like granite and basalt, is formed by molten rock erupting from deep within the earth. METAMORPHIC ROCK, such as marble, is formed by tremendous heat and pressure. Fossils are not usually found in either igneous or metamorphic rocks.