The frontier between Gubbio (ancient Umbria) and Perugia (ancient Etruria), in the northeast part of the modern region of Umbria, was founded in the late sixth century bc. The frontier endured in different forms, most notably in the late antique and medieval periods, as well as fleetingly in 1944, and is fossilized today in the local government boundaries. Archaeological, documentary and philological evidence are brought together to investigate different scales of time that vary from millennia to single days in the representation of a frontier that captured a watershed of geological origins. The foundation of the frontier appears to have been a product of the active agency of the Etruscans, who projected new settlements across the Tiber in the course of the sixth century bc, protected at the outer limit of their territory by the naturally defended farmstead of Col di Marzo. The immediate environs of the ancient abbey of Montelabate have been studied intensively by targeted, systematic and geophysical survey in conjunction with excavation, work that is still in progress. An overview of the development of the frontier is presented here, employing the data currently available.