Sedimentary rocks are usually formed in several layers called strata . The different layers can be recognized by color, composition or the grain size. In a group of several, the oldest bed is at the bottom and the youngest is found at the top. Fossils are important components found in many sedimentary layers. Fossils represent a record of past life because it is a bone, shell or even a body part.
Limestone generally has many fossils while shale and sandstone generally have fewer.
Of the three kinds of rock, metamorphic rock is the least abundant. Metamorphic rock, unlike sedimentary and igneous rock forms only at great depths or in high temperatures. Usually metamorphic rock is formed at temperatures between 150C and 800C and at depths from one mile to several miles. It is believed that the heat originates with the decay of radioactive elements and the pressure results from the weight of sediments lying above or from all the forces that accompany mountain building. Metamorphic rocks are divided into groups depending on whether or not they have bands.