Several geologic formations can be found within watershed of Darby Creek. The Wissahickon Formation is the most prevalent formation in the watershed. It is derived from derived from sandstones and mudstones and is composed of mica schist. Metamorphic felsic gneiss and mafic gneiss formations are common in the northern parts of the watershed. The Bryn Mawr Formation and the Bridgeton Formation are also present and are unconsolidated deposits of rock that rest on top of the dense crystalline bedrock. Mica slate is present in Marple Township and was manufactured to form "Darby Creek scythe stones" in the 1860s.