The oldest rocks in the Paradox Basin of the southwestern United States are an Early Proterozoic crustal sequence of gneiss and schist, approximately 1,800-1,740 Ma. The complex was intruded by Early to Middle Proterozoic (1,730-1,700 Ma and 1,435-1,400 Ma) plutonic igneous rocks and is overlain by supracrustal Middle Proterozoic (1,695-1,435 Ma) sedimentary rocks in some places. A younger Middle to Late Proterozoic (1,250-800 Ma) sequence of metasedimentary rocks may be present in parts of the western Paradox Basin. Early Proterozoic rocks in the Paradox Basin and adjacent areas accumulated in a convergent plate setting on the edge of the Archean craton. The possible Middle to Late Proterozoic rocks may have been deposited in a lacustrine setting. A wedge of clastic and carbonate Cambrian rocks unconformably overlies basement rocks. Cambrian rocks are thickest on the west side of the study area and thin eastward. From oldest to youngest, Cambrian units are the Tintic Quartzite, Ophir Formation, Maxfield Limestone, Lynch Dolomite, and Ignacio Quartzite. In the Paradox Basin, Upper Devonian rocks unconformably overlie Cambrian strata; Ordovician and Silurian rocks are not known in this area. A basal Devonian unit, the Aneth Formation, is areally restricted and only is present near the Four Corners. Overlying the Aneth, probably unconformably, is the Elbert Formation. In much of the Paradox Basin the basal member of the Elbert is the McCracken Sandstone Member. Overlying the McCracken is a shale and dolomite member known informally as the upper member. The youngest Devonian unit in the basin is a carbonate rock, the Ouray Limestone. An unconformity separates Devonian from Mississippian rocks in the Paradox Basin. In the eastern part of the basin Mississippian rocks are known as the Leadville Limestone, and in the western part this carbonate unit is known as the Redwall Limestone. Mississippian rocks are in turn unconformably overlain by Pennsylvanian rocks in the Paradox Basin. In most areas the Molas Formation, which includes a basal regolith, overlies Mississippian strata. In a few areas the Molas is missing, and Mississippian strata are overlain by carbonate rocks of the Hermosa Group. All of the Phanerozoic sedimentary rocks of the Paradox Basin were deposited on a stable cratonic shelf on the trailing edge of the continent. Uppermost Cambrian, Devonian, and Mississippian rocks were deposited in warm, shallow-marine environments. Low to moderate topography east of the study area, associated with the Transcontinental arch, provided elastics to the shelf during deposition of the Tintic and Ignacio Quartzites and the Elbert Formation. The pre-Pennsylvanian sedimentary wedge thickens markedly to the west into the Cordilleran miogeocline. J.A. Campbell and K.B. Ketner greatly improved the manuscript. Discussions of Cambrian and Devonian rocks with J.A. Campbell and C.A. Sandberg were very helpful in my gaining an understanding of those units.