Abstract: Mud crack is a general term used to describe a fissure that forms in finegrained soils and sediments as voiume is reduced during dewatering.The presencc of the crack structure in mudstone and limestone is usually considered by many geologists in China as evidence of an air-drying during a period of surface exposure and is often called sun crack or mud crack. The current study shows,however, that crack formed in the geologic past might be usually generated in argillaceous deposits during a period of uninterrupted water cover by increasing the salinity of the aqueous milieu （the salt concentration was not very high）. So this crack should be termed subaqucous syneresis crack or subaqueous shrinkage crack.The term mud or sun crack is sometimes used interchangeably with syneresis crack,but is misleading.