Sediments and sedimentary rocks cover about 80% of all continental areas but are less than 1% of the volume of the Earth’s crust. They are, in effect, a thin blanket on top of igneous and metamorphic basement rocks. Yet, we interact with them more than with any other geological materials except, perhaps, soils. Sediments, and rocks derived from sediments, are mostly recycled materials that come from preexisting igneous, metamorphic, or sedimentary rocks. Petrologists usually divide sedimentary rocks into two main groups based on the kinds of material they comprise: clastic rocks and chemical rocks.