A study of the mineral constituents and textures of upper Chester sandstones in Illinois was undertaken to throw light on environments of deposition, tectonic activity, and possible source areas. Although there are many minor facies changes in all the sandstone beds, all except the Menard and Kinkaid sandstones are persistent over wide areas in the Eastern Interior Basin. Most of the sandstone beds are dominantly quartzose, with minor admixtures of clay minerals, carbonate, rock fragments, and accessory minerals. Most of the sandstones are well-sorted clean orthoquartzites, but many sandstones have a composition more like that of a subgreywacke. Many of the sands show evidence of having been winnowed and reworked at the site of deposition. The sedimentary environment is reconstructed from petrographic and stratigraphic data as dominantly sublittoral, near-shore, with brackish or marine waters, although there is evidence that some of the sands were deposited on a terrestrial coastal plain. The mineral composition of the sands, considered in conjunction with the amounts of coarse clastic material in other areas of deposition during Chester time, points to northern sources of detritus rather than sources to the east.