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Two small perennial salt lakes in a salar of the Chilean altiplano have indurated bottom sediments and are bordered by damlike ramparts composed of almost pure gypsum. The bottom of one lake is constituted of polygonal plates (5-15-m diam) of gypsum buckled upward into pressure ridges and overthrust slabs. Overthrust slabs that have reached the lake surface have developed massive iceberglike accumulations of gypsum on their leading edges. Expansion of anhydrite ( CaS04) on alteration to gypsum ( CaS04 *2H,O) is speculated to be involved in formation of the pressure ridges. The two lakes are about four times deeper (2 m) than any other lakes in the salar, and their surfaces are elevated as much as 1.2 m above the adjacent salar ,crust, only the gypsum ramparts intervening. The ramparts range to 2 m high. The buildup of these ramparts via evaporative precipitation of wave-wash, spray, and capillary brine, elevation of the lip of the small outflow channels, and deflation of the salar crust probably account for the lakes' depths and elevated positions.