The Michaud and Pocatello quadrangles comprise an area in southeastern Idaho extending eastward from the American Falls Reservoir, in the Snake River Plain, and including the Pocatello Range and the northern part of the Bannock Range, in the Basin and Range physiographic province. The mountainous parts are underlain principally by marine sedimentary rocks of Precambrian and Cambrian age but also contain a fairly complete section of Paleozoic rocks. Tertiary rocks crop out extensively in the foothills. Quaternary deposits cover more than half the area and include basalt, pediment gravel, loess, and lacustrine and fluvial deposits.More than 20,000 feet of sedimentary rocks, principally marine, is assigned to the Precambrian System. The oldest rocks, which make up the Pocatello Formation, are divided into four members. An unnamed lower member, at least 1,000 feet thick, is mainly slate and slaty argillite. Several thousand feet of interbedded quartzite and submarine-deposited diamictite, later metamorphosed, constitutes the overlying Scout Mountain Member. Metavolcanic rocks, principally greenstone and greenstone schist, compose the Bannock Volcanic Member, which tongues or lenses into the Scout Mountain Member. The upper member of the Pocatello, at least 1,800 feet thick, consists of slaty to phyllitic argillite that is interbedded with quartzite in the upper baH. The Blackrock Canyon Limestone, commonly several hundred feet thick, overlies the Pocatello Formation and forms a distinctive marker. Overlying quartzitic rocks are also divided into four formations. The Papoose Creek Formation (oldest) consists of 600 to perhaps 2,500 feet of distinctively, irregularly bedded very fine grained quartzite and siltite. The overlying Caddy Canyon Quartzite consists of several thousand feet of quartzite, separated by about 50 feet of dolomite or limestone. The succeeding Inkom Formation is a distinctive clastic marker unit characterized by fine grain size and pale-green color. The Precambrian unit, the Mutual Formation, consists of 2,500-3,000 feet of medium-to coarse-grained crossbedded quartzite, much of which is grayish red or purple, and red and green argillites and slates.The Precambrian and Cambrian Camelback Mountain Quartzite conformably overlies the Mutual Formation. It consists of as much as 3,500 feet of light-colored vitreous orthoquartzite. It is overlain by the Gibson Jack Formation, 1,500-2,300 feet of mainly argillaceous siltstone and shaly argillite that contains the arthropod Naraoia, which is thought to be Early Cambrian but may be Middle Cambrian. The succeeding Elkhead Limestone, more than 2,000 feet thick, consists principally of gray limestone. This formation is Middle Cambrian.