An Upper Cenozoic (Late Pliocene-middle Pleistocene) non-marine sedimentary sequence--the St. David Formation--crops out in badland topography on both sides of the San Pedro River near St. David, Arizona. It consists chiefly of fluvial floodplain silts and clays with smaller amounts of sand, gravels, fresh-water limestones, pyroclastic units, and paleosols. An immature mineral suite and angularity of the mineral grains suggest that source areas for sediments in the St. David Formation were nearby. X-ray analyses show that the clay suite is dominated by illite in the lower part, and by montmorillonite in the middle and upper parts; the change was probably initiated in part by volcanic activity. Red beds, which are common in the lower part of the formation, may have been formed in part diagenetically. Very fine crystalline limestones and green clays contain fresh-water algae, ostracods, and molluscs suggesting deposition in lakes, ponds, and swamps. Results of the sedimentologic and petrographic studies indicate that the St. David Formation was deposited by a large, slow-moving, through-flowing stream system which spread fine-grained sediments on an aggrading floodplain, and at times in lakes and ponds.As indicated by the vertebrate fossils, savannas and open-grasslands probably prevailed in areas away from the streams.