The Symplegades or ‘Clashing Rocks’, situated at the Bosporus, are known in Greek mythology as one of the challenges encountered by Jason and the Argonauts. Pindar describes Jason’s journey to the ‘Inhospitable Sea’ (Black Sea) via the Clashing Rocks [1787.61].
Rosario Vieni has suggested that the Symplegades, were precursors of the Pillars of Heracles, John Lundwall in Mythos and Cosmos [1747.305-6] also suggests a linkage between the Symplegades and the Pillars of Herakles!(a)
(a) https://www.quora.com/Where-are-the-pillars-of-Hercules (just over halfway down the page)