Much of the eastern part of the county lies within the Triassic Basin, a subregion of the Piedmont. Much of the bedrock in the county is volcanic in origin and formed during the Triassic period (hence the name). The Triassic origins have led to the formation of coal deposits in the southern part of the county. The "Boren Clay Products Pit" just north of Gulf in extreme southern Chatham County is a place where Triassic flora fossils persist. The volcanic origins also led to the creation of high amounts of metamorphic-based rocks in the county. The county lies on the Carolina Slate Belt. Soils in the county are mostly clay based and have a deep red color, as do most soils in the Piedmont. Groundwater in the county is generally full of minerals and tends to be "hard" if not softened. Mineral-based water was the attraction at Mt. Vernon Springs during the latter part of the 19th century and the early part of the 20th century. A resort spa was established at the mineral springs. Visitors would drink the water in the hopes of curing ailments and diseases. The resort closed in the early 20th century and is now gone. The springs are still there and are maintained by a local church.