Ammianus Marcellinus Res Gestae 22.8.14

[14] The right-hand curve of the Thracian Bosporus begins with the shore of Bithynia, which the men of old called Mygdonia, containing the provinces of Thynia and Mariandena, and also the Bebrycians, who were delivered from the cruelty of Amycus through the valour of Pollux; ¹ and a remote station, a place where the menacing harpies fluttered about the seer Phineus and filled him with fear. ² Along these shores, which curve into extensive bays, the rivers Sangarius and Phyllis, Lycus and Rheba pour into the sea; opposite them are the dark Symplegades, twin rocks rising on all sides into precipitous cliffs, which were wont in ages past to rush together and dash their huge mass upon each other with awful crash, and then to recoil with a swift spring and return to what they had struck. ³ If even a bird should fly between these swiftly separating and clashing rocks, no speed of wing could save it from being crushed to death.