Prepaleozoic red quartzites in the north-central United States are among the world’s oldest red beds. Not only do they constitute evidence about Proterozoic weathering and the advent of significant atmospheric free oxygen but they also provide important evidence bearing upon the sedimentary and tectonic history of North America. All of the red quartzites postdate the Penokean orogeny (1,760 to 1,860 m.y., broadly defined), and isotopic evidence permits that some could conceivably be as young as 1,200 m.y. (Keweenawan basalt eruption). Although these different quartzites may not be strictly correlative, it is probable that those south of Lake Superior (Baraboo, Sioux, Barron) fall within the narrower span of 1,450 to 1,750 m.y., here called the Baraboo interval for the best-dated sequence of sedimentation, deformation, and metamorphism. It has been suggested that their deposition probably predates a major 1,615 to 1,630-m.y. Rb-Sr resetting event, which would narrow the depositional age span considerably more. The severity of deformation of the quartzites is enigmatic in terms of their maturity and present intracratonic location. Sources of the red quartzitic sediments were quartz-bearing silicic volcanic rocks, such as underlie the Baraboo Quartzite, and older sedimentary, plutonic, and minor metamorphic rocks. Predominance of aluminous kaolinite and pyrophyllite in fine red argillaceous strata and near-absence of feldspar in the sandstones suggest mature chemical weathering in the source. Without vegetation to stabilize soils, this seems possible only on a stable landscape with little topographic relief and in a warm, humid climate. Braided rivers apparently drained the hinterland and formed a dominantly sandy coastal plain. Marine transgression may have begun during deposition of the sandstones, for the youngest strata preserved, which are black slates and iron-formation-bearing dolomite overlying the Baraboo Quartzite, are considered to be marine. Deposition of a red, mature quartzose (90% to 95%) sedimentary wedge up to 2,000 m thick points to a stable, passive continental margin on the southern edge of a Proto-North American craton. Later deformation was intense and greenschist metamorphism was pervasive, implying compressive orogenesis. The hypothesis is proposed that a major, hitherto unrecognized plate suture exists beneath Iowa and Illinois. Southward subduction during or after deposition of the red quartzites caused consumption of a sea floor that lay to the south of Proto-North America and culminated in collision either of a volcanic island arc or of another continent with southern Proto-North America about 1,615 to 1,630 m.y. ago. Foliated granitic plutons 1,625 m.y. old are known in northern Kansas and Missouri, and the Mazatzal belt of Arizona and New Mexico that contains plutonic and volcanic rocks of 1,610 to 1,680-m.y. ages also seems to be related to some such event. Apparently emplacement of the anorogenic Wolf River batholith in Wisconsin (about 1,500 m.y.) and eruption of widespread rhyolites to the south (1,380 to 1,480 m.y.) postdated the collision. Whatever may be the merits of this particular hypothesis, the red quartzites provide important constraints upon concepts of North American continent-building during Proterozoic time.