Most sedimentary rocks fall into one of two categories:
- Clastic sedimentary rocks are formed from pieces of other rocks that are glued together. For example, a sandstone is made up of pieces of sand.
- Chemical sedimentary rocks are formed by precipitation of different elements dissolved in water. For example, halite (rock salt) forms when water evaporates away and leaves the dissolved salt behind.
There are also organic sedimentary rocks, which are made up of plant material. Coal is considered an organic sedimentary rock.