The Golconda Group was deposited during the early to middle Chesterian, a time of transition in the Mississippian from when conditions favored ooid formation (Valmeyeran) to when conditions were unfavorable (late Chesterian). As a result, the Golconda is one of the youngest Chesterian carbonate units in the Illinois basin containing abundant ooids. Oolitic limestone is common along the western, southern, and eastern margins of the basin. Elsewhere, oolitic deposits are more sporadic and thin. The presence of a positive area (i.e., the Ozark dome) in Missouri induced conditions appropriate to ooid formation in the southwestern part of the basin. Antecedent topography formed by bioaccumulation also favored oolite deposition. Terrigenous influx and storm activity tended to d scourage ooid formation. Although there are similarities between Valmeyeran and Chesterian oolite characteristics, notable differences exist.