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Sand Tufa.  Tufa is a form of limestone precipitated when carbonate-rich source waters, usually from a fresh-water spring, emerge into an alkaline soda lake. These fragile examples of sand tufa were formed when spring water percolated upward through lake-bed sands, creating tubes weakly held together by calcium carbonate.  Mono Lake Tufa State Reserve. Near Lee Vining, Mono Co., Calif.