In and around Rome, coastal and fluvial sediments influenced by Quaternary sea-level changes interfinger with accurately datable, highly potassic pyroclastic volcanic units. We describe the complex internal geology of the Capitoline Hill, for centuries a place of leading importance in ancient Rome and more recently a focus of archaeological research. Outcrops and many new drill cores reveal at least four cycles of valley cutting and filling, probably recording the history of the Tiber River in response to changes in sea level. Single-crystal 40Ar/39Ar dates on sanidine give ages of 518 ± 6, 413 ± 5, and 353 ± 2 ka for three prominent volcanic units filling separate paleovalleys. The Roman region offers the potential for direct age calibration of the Quaternary sea-level record.