The small pieces of rock are called sediment when they are carried along by water in rivers or the sea. Sediment forms because other rocks are eroded (worn down) by rain, wind, waves or huge rivers of ice called glaciers. Sedimentary rocks form where rivers meet the sea, or in shallow lakes where there is a limit to the amount of sediment the water can hold. Lakes of salty water evaporate to form rock salt, a sedimentary rock made largely of the same type of salt that we use to flavour our food.