In 1779, Col. Daniel Brodhead was in command of Fort Pitt at the present site of the city of Pittsburgh. Col Brodhead begged Gen. Washington to allow him to lead an expedition against the Seneca Indians who were raiding settlements in the area. The troops encountered the Indians and were forced to flee for their lives.
In pursuit the soldiers crossed a creek at a place where the stream bed was comprised of large smooth rocks. Wearing heavy boots, the soldiers were able to cross the creek safely, but the Seneca Indians slipped and fell on the rocks – enabling the cavalry to make their escape.
Historically, the Indians called the stream “Wechachapohka” which means “a slippery rock.” Once Slippery Rock Creek was christened, the adjoining township and borough later became known by the catchy name as well.
The Slippery Rock is in the Slippery Rock Gorge, between McConnells Mill State Park and Wurtemburg, in Lawrence County. The rock was especially slippery because of an oil seep on it, and was at a place where a ford crossed the stream. See http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_20027551.pdf.