Limestone is an example of a sedimentary rock which has not formed from the processes of weathering . Limestone comes from the remains of millions of sea creatures which have died and fallen to the bottom of the sea . The creatures themselves have decayed , but their shells and skeletons remain . Compaction over millions of years have turned these remains into limestone rock .
Limestone is mainly calcium carbonate .
Other examples of sedimentary rock not formed by weathering are chalk , coal and rock salt . Chalk is a very pure form of calcium carbonate , and is made in a similar way to limestone . The formation of coal is outlined in the chapter Products from Oil . Rock salt is formed from salts which have crystallized out of sea water , as the water has evaporated . The process is the same as that given for general crystallization where evaporation of water from a saturated solution causes the salt to crystallize .