Modern oceanographic studies in estuaries and other restricted arms of the sea have shown that a characteristic circulation pattern exists. Surface currents flow from regions of low salinity to regions of higher salinity in response to hydrostatic head and are accompanied at depth by oppositely directed currents flowing from high to low salinity regions because of density distribution. Salts are deposited in restricted estuaries where evaporation exceeds precipitation plus runoff. The necessary restrictions of the estuary or basin are in part dynamic and in part static. Dynamic restriction is caused by the hydrostatic head and by frictional stresses between the oppositely directed surface and bottom currents, and between the bottom current and the channel floor. Static re triction is produced by topographic confinement.

When high concentrations are developed a strong horizontal salinity gradient exists which produces lateral segregation of different salts during precipitation. The escaping deep current returns to the sea those salts which have not been precipitated. Fluctuations in equilibrium caused principally by changes in excess of evaporation or in degree of channel closure cause migrations of the horizontal salinity gradient along the longitudinal axis of the basin which produce vertical differentiation of salts. The order in which salts occur in the vertical column can be predicted from oceanographic and chemical data and agrees well with many vertical sequences which have been described.