The Aux Vases Sandstone is named for the Aux Vases River in Ste. Genevieve County, Missouri, and the type section consists of outcrops in the Mississippi River bluffs at the mouth of the Aux Vases River. The Aux Vases consists of sandstone, siltstone, and minor amounts of shale and, locally, dolomite and limestone. It occurs in much of the area of the Chesterian Series. It crops out along the Mississippi River Valley in St. Clair, Monroe, and Randolph Counties, being particularly well exposed in the bluffs 2-3 miles southeast of Prairie du Rocher, Randolph County. In southern Illinois it crops out principally in Union, Johnson, and Hardin Counties.