Numerous laterally extensive, thin fresh-water lacustrine phosphatic horizons characterize nearshore facies of the Pliocene Glenns Ferry Formation in southwestern Idaho. These units are restricted to, and formed on, hiatal surfaces in coarse sandstone and oolite which now crop out in a narrow northwest-trending belt along 50 km of the southwestern shoreline of Pliocene Lake Idaho. These hiatal surfaces developed during periods of transgression when sites of nearshore deposition migrated landward. Phosphatization occurred through the replacement of calcareous ooid and oncoid cortices and through the precipitation of fibrous isopachous cements around both terrigenous and coated grains. Francolite, a carbonate fluorapatite, is the primary phosphate phase present. Phosphatization was a synsedimentary diagenetic event; it occurred just below the sediment-water interface as a consequence of high phosphate concentrations in interstitial waters which resulted from intergranular decay and dissolution of organics during periods of low terrigenous sedimentation.