An extensive fresh-water oolitic carbonate representing the remnant deposits of a Pliocene low-salinity lake occurs at the base of the Glenns Ferry Formation along the western margin of the Snake River Plain. This oolite crops out in a 45 kilometer-long belt that treads northwest-southeast and parallels the southwestern shoreline of fossil Lake Idaho. It consists almost entirely of ooids ranging in size from 0.5 mm to 6.0 mm in diameter. Optical and scanning electron microscopy reveal a random mixture of radial laths and equant blocky grains of low-magnesian calcite in the ooid cortices. The primary mineralogy of the lath-shaped crystals is interpreted as having been aragonite and the radial orientation of these crystals as having been synsedimentary based upon comparison with crystals in the cortex of Great Salt Lake ooids. Crenulation of concentric laminae is indicative of significant algal influence during ooid formation, suggesting accretionary processes more typical of algal oncoids. Cortex deposition took place in a shallow agitated environment on a nearshore lacustrine bench, while ooid deposition took place lakeward on the deeper water bench slope.