Searles Lake is a playa or dry lake which occupies the central floor of Searles basin, a northward-trending graben of the Basin and Range province in southeastern California. During pluvial periods of the Pleistocene epoch, deep-water lakes filled Searles basin, and in 2 of these lakes more than 500 pinnacled masses of calcareous tufa accumulated in an armlike bay at the SW. end of the basin. Pinnacles are tower-, tombstone-, and cone-shaped masses and large limestone ridges. Most of the pinnacles are 10 to 40 ft. high, but a few reach heights between 100 and 140 ft. Basal diameters or widths range from about 10 ft. to as much as 500 ft. and average 20 to 30 ft. Seven varieties of tufa compose the pinnacles; one of these varieties is also found in lenticular bodies buried in the lacustrine sediments underlying the pinnacles at the northern end of the bay. It is proposed that the pinnacles were precipitated by algae about the orifices of sublacustrine springs issuing along faults striking N.65 degrees W., N.50 degrees W., N.30 degrees E., N.55 degrees E., and N.65 degrees E. in the underlying basement rocks. Deposition of the pinnacles at the SW. end of the bay took place in a lake of Tahoe age which filled Searles basin more than 32,000 years ago. Pinnacles midway along and at the northern end of the bay formed in a lake of Tioga age which initially flooded Searles basin about 23,000 years ago and lasted until about 10,000 years ago. Tioga Searles Lake at its maximum stand was approximately 460 ft. deep and at its highest stage the surface reached an elevation of about 2,000 ft.