"Sedimentary rocks are formed by the pressure over millions of years of organic material, inorganic material, sediment of course , and earth piling up over itself. Metamorphic rock and igneous rock originate either below the surface where plants lived and creatures roamed, or the heat from volcanic activity transformed everything. Sedimentary rock formation is the only geologic activity that permits and allows organic material preservation and hence fossil formation." - Charles Snider, Co-Founder of American Geode (on Quora.com)