The Kinnikinic Formation of central Idaho is a sedimentary unit consisting principally of orthoquartzite. From its maximum measured thickness of 2285 feet in the central Lemhi Range near Gilmore, Idaho, it thins westward to 376 feet at the type section and southward to 326 feet near Arco, Idaho. Northeast of the Lemhi Range it occurs only as erosional remnants, due to pre-Devonian erosion. Both the lower and upper contacts of the Kinnikinic Formation are disconformable. The Kinnikinic Formation is light colored, predominately fine to medium grained, thin to medium bedded, and largely cemented by silica overgrowths. Some metamorphic recrystallization has occurred locally. Although parallel laminae and structureless beds predominate within primarily parallel bedding, omikron-type (underwater dune) cross-laminae are locally abundant. The sediments are moderately sorted to well sorted and both positively and negatively skewed. Cumulative-frequency probability curves illustrate traction, "saltation," and "suspension" populations; some thin sections indicate two "saltation" populations. An open-marine, shallow-shelfal environment influenced by high-energy (tidal?) currents is postulated for deposition of the Kinnikinic Formation on the basis of the lithlogic uniformity, lateral extent, sedimentary structure, trace fossils, and paleogeographic setting. The Swan Peak Formation of southeastern Idaho and north-central Utah is divisible into a lower member of dark gray shale and quartz silitite, a middle member of brown orthoquartzite and light-colored shale, and an upper member of white orthoquartzite. Correlation of the white, fine- to medium-grained quartzite in the Raft River Range with the Ordovician Eureka Quartzite, considered probable by Compton (1972), is here accepted. The relatively thin nature of the Eureka Quartzite in this area reflects lesser sedimentation rates relative to areas farther south and/ or post-depositional erosion. The Dayley Creek Quartzite of Armstrong (1968) in the Albion Range is complexly faulted; as mapped, all or most of it probably is not correlative with the Kinnikinic Formation. Quartzites in the lower thrust plate of the Phi Kappa Formation in central Idaho are not environmental equivalents of the Kinnikinic Formation to the east, even though they apparently are approximately time equivalent (Churkin, 1963a, pp. 1612-1615). Middle Ordovician, shallow-shelfal orthoquartzit.es (Kinnikinic Formation, upper member of the Swan Peak Formation, Eureka Quartzite, and Mount Wilson Quartzite of southern Canada) are considered to be one originally continuous genetic unit, based on age determinations of overlying and underlying units, disconformable lower and upper contacts, close physical resemblances, and the high probability they were principally derived from the same source area, possibly Cambrian sandstones in the Peace River-Athabasca Arch area of northern Alberta, Canada. The shelfal environment of the Middle Ordovician Cordilleran miogeosyncline can be divided into at least five major, distinctly separated, basins of deposition: (1) Alberta-British Columbia Basin; (2) Central Idaho Basin; (3} Southeastern Idaho Basin (herein proposed); (4) Northeastern Nevada Basin; and (5) Ibex Basin. Anomalously high percentages of undulatory quartz grains in Middle Ordovician orthoquartzites of the Cordilleran miogeosynclinal shelf are highly suggestive of straining in situ and are probably related to post-depositional conditions ranging from purely tectonic to intricate combinations of tectonic and nontectonic events.