The Derby-Doerun Dolomite is a Cambrian geologic formation exposed in southeast Missouri. Once classified as the separate Derby and Doerun formations, it is now recognized as a single unit. Its combined name reflects the region’s mining past, preserving connections to the Derby Mine and the Doe Run Lead Company in Missouri’s Old Lead Belt. The formation is also a fossil-bearing stratigraphic unit, tying it to the area’s much older geological history.