Chalcedon
The site of Chalcedon is located on a small peninsula on the north coast of the Sea of Marmara, near the mouth of the Bosphorus. A stream, called the Chalcis or Chalcedon in antiquity and now known as the Kurbağalıdere (Turkish: "stream with frogs"), flows into Fenerbahçe bay. There Greek colonists from Megara in Attica founded the settlement of Chalcedon in 685 BC, some seventeen years before Byzantium.
Megara
In historical times, Megara was an early dependency of Corinth, in which capacity colonists from Megara founded Megara Hyblaea, a small "polis" north of Syracuse in Sicily. Megara then fought a war of independence with Corinth, and afterwards founded Chalcedon in 685 BC, as well as Byzantium (c. 667 BC).