Many sedimentary rocks can be grouped according to how their material formed. Learning these categories is a useful first step when examining a rock collection.
Clastic sedimentary rocks
Clastic rocks form from pieces of older rocks and minerals that accumulate, compact, and become cemented together. Sandstone, shale, and conglomerate are familiar examples. Grain size, rounding, and sorting offer clues about how the sediment was transported.
Chemical sedimentary rocks
Chemical rocks form when dissolved minerals precipitate from water. Some limestone, rock salt, and gypsum deposits form this way.
Biochemical or biological sedimentary rocks
Biochemical sedimentary rocks form largely from accumulated shells, skeletal remains, or other biologically produced materials. Fossil-rich limestone is a common example. Some rocks involve more than one process, so classifications can overlap.
To identify a specimen, consider texture, grains, layering, fossils, and the location where it was collected. A photograph alone may not always establish a reliable identification.
Illustrative sandstone photograph: Phillip Flores / Unsplash.