Interpretations of provenance and depositional environments of the Upper Cambrian Lamotte Sandstone provide a new model of basal Paleozoic sedimentation in the Ozark region. Detailed examination of sedimentary structures and lithologies in exposures and cores, as well as compositional and textural analyses of 110 thin sections, provide the data on which the model is based. The Lamotte can be divided into three major sandstone lithologies: a feldspathic type containing significant amounts of polycrystalline quartz, K-feldspar, plagioclase, and plutonic rock fragments derived from local granites; a lithic type containing significant amounts of volcanic rock fragments derived from local felsites; and a quartzitic type dominated by well rounded monocrystalline quartz grains derived from a quartz-rich source located northwest of the Ozarks. The feldspathic and lithic detritus was shed off the ancestral St. Francois Mountains and deposited on alluvial fans and in braided fluvial systems in topographic lows. The bulk of the quartzitic detritus was transported eastward by braided streams, resulting in the construction of an alluvial plain which thickened to the east, burying the flanks and lower portions of the ancestral St Francois Mountains. Local deposits of interbedded lithologic types resulted from mixing of locally derived detritus with quartzose detritus in braided fluvial settings. An Upper Cambrian sea eventually transgressed the region, resulting in contemporaneous deposition of alluvial, marginal marine and shallow marine facies (including local fan-delta development). The marginal marine deposits constitute a conformable transition zone between the Lamotte and the overlying Bonneterre formation in the study area.