Mirabilite (Na2So4. 10H20) is widely distributed in the McMurdo Sound region in the beds of extinct lakes, around shores of existing lakes, and as a white alkali accumulation on the surface of the ground. Large blocks of almost pure mirabilite 5-25ft in diameter have been incorporated into glacial moraines, many of which are still ice-cored. Mirabilite was reported in this region more than 50 years ago by geologists of British Antarctic Expeditions, who thought the salt was or marine origin. The writers propose that at least some of the mirabilite in the McMurdo Sound region is a non-marine evaporate deposited from complete or partial evaporation of lakes during Pleistocene and Recent times. After deposition of the salt, glaciers advanced over many of the mirabilite deposits. The white alkali films on the present surface of the ground which consists mainly of glacial drift, is the result of evaporation of soil moisture, although some may be directly related to emanations from Mt Erebus, a nearby active volcano. The McMurdo Sound region has an arid climate, dry lake beds, and soda-rich rocks.