The Conestoga Formation is a geologic formation in Pennsylvania. Description Light-gray, thin-bedded, impure, contorted limestone having shale partings; conglomeratic at base; in Chester Valley, includes micaceous limestone in upper part, phyllite in middle, and alternating dolomite and limestone in lower part. Type section Named from outcrops along Conestoga River, Lancaster County, Pennsylvania. References Cambrian geology of Pennsylvania Ordovician geology of Pennsylvania Geologic formations of Pennsylvania