The Kinnikinic Quartzite (Middle Ordovician) forms distinctive white cliffs in much of east-central Idaho. It is a fine- to medium-grained, silica-cemented, supermature quartz arenite. Structureless beds and thin to medium parallel bedding with parallel laminae predominate. Locally abundant, however, are omikron-type cross-laminae, probably subaqueous dunes, formed by predominantly southwestward-flowing paleocurrents. Small-scale ripples and shallow scours are rare. Rare Skolithos tubes. concentrated at local horizons, dominate the sparse trace-fossil assemblage. Anomalously high amounts of undulatory quartz indicate widespread in situ tectonic straining of quartz grains following deposition. Deposition on a shallow, open-marine shelf, swept by moderate to high-energy currents, is indicated by physical and biological sedimentary structures, grain-size trends, lithologic uniformity through considerable lateral extent, and overall paleogeographic setting. In the type area, near Clayton, Idaho, interbeds of shale in generally very fine-grained quartz arenite indicate lower energy conditions westward. This area probably marks the transition from the outer shelf to the continental slope. Differential subsidence during deposition resulted in large lateral differences in thickness of this shallow-water unit. From a maximum preserved thickness of more than 1,500 feet at Mahogany Hills, at Hawley Mountain, and in the central Lemhi Range, the Kinnikinic thins southward to 326 feet near Arco, Idaho, and westward to 266 feet near Ketchum, Idaho. In the northeast, depositional onlap and subsequent pro-Devonian erosion also contributed to this variability in thickness.