How Rocks are Classified - Geology (U.S. National Park Service)
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. CLASTIC: Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported by erosion to some basin or depression where ...