What is the type of rock that most often contains fossils?
Fossils will be found mainly in sedimentary rocks. Sedimentary rocks are formed by the compression and cementation of sediments over millions of years. Organisms that die on, in, or near a body of water have a better chance of becoming fossils due to the fact that fossilization requires that the organism be covered relatively quickly in sediments. Once covered in sediments, some of these remains remain intact and can be discovered to have formed fossils or fossilized remains within the layers of the rock. Shale is a particularly good rock for this to occur in, and some limestones like chalk, are composed almost entirely of fossilized marine animal remains.
