Large, isolated, silica-cemented orthoquartzite boulders and blocks lie on a surface underlain by unconsolidated sands and other materials in southwestern Union County and in other areas along the Midway-Wilcox contact across the state. The boulders and blocks have long interested geologists, but little has been published concerning them. Their large size indicates a nearby origin, and they contain suites of minerals nearly identical with the southern Appalachian sequence found in the body of underlying sands. A petrographic study of the boulders and underlying Wilcox sands in Union County revealed that the environment of deposition of the boulders and the subjacent sands was radically different, both physically and chemically. The Wilcox sand is texturally submature and is a river sand. The sand in the boulders is texturally supermature and is a beach sand.