There are numerous mineralogical references of the early presence of liquid water in and around chaotic terrains on Mars. Sulphate-containing layers of sediment identified in some of the basins that host chaotic terrains point to evaporative processes. Clay-bearing deposits on the plateau units of Margaritifer Terra, predating the sulfate deposits in the chaotic terrains, probably resulted from fracturing and groundwater outflow. The formation of clay minerals would have required the presence of standing, pH-neutral water.