Shale is a sedimentary rock, created by the compaction of thick layers of clay deposited at the bottom of a lake or ocean. In marine (salt water) or salt lake environments, a dry period can expose the depositing wet clay (mud) to evaporation; evaporation of salt water can allow cube-shaped salt (halite) crystals to form within the mud. Although these crystals may later re-dissolve, their imprint is sometimes found preserved in the hardened claystone. In this case the clay is a 'pseudomorph' (false-form) of the original halite crystals.