Bithynia was an ancient region, kingdom and Roman province in the northwest of Asia Minor, adjoining the Propontis, the Thracian Bosporus and the Euxine (today Black Sea).
Several major cities sat on the fertile shores of the Propontis (which is now known as Sea of Marmara): Nicomedia, Chalcedon, Cius and Apamea. Bithynia also contained Nicaea, noted for being the birthplace of the Nicene Creed.
According to Strabo Bithynia was bounded on the east by the river Sangarius (modern Sakarya river), but the more commonly received division extended it to the Parthenius, which separated it from Paphlagonia, thus comprising the district inhabited by the Mariandyni. On the west and southwest it was separated from Mysia by the river Rhyndacus, and on the south it adjoined Phrygia and Galatia.
It is occupied by mountains and forests, but has valleys and coastal districts of great fertility. The most important mountain range is the (so-called) "Mysian" Olympus (8000 ft., 2500 m), which towers above Bursa and is clearly visible as far away as Istanbul (70 miles, 113 km). Its summits are covered with snow for a great part of the year.
East of this the range extends for more than 100 miles (160 km), from the Sakarya to Paphlagonia. Both of these ranges are part of the border of mountains which bounds the great tableland of Anatolia, Turkey. The broad tract which projects towards the west as far as the shores of the Bosporus, though hilly and covered with forests — the Turkish Ağaç Denizi, or "The Ocean of Trees" — is not traversed by any mountain chain. The west coast is indented by two deep inlets, the northernmost, the Gulf of İzmit (ancient Gulf of Astacus), penetrating between 40 and 50 miles (65–80 km) into the interior as far as İzmit (ancient Nicomedia), separated by an isthmus of only about 25 miles (40 km) from the Black Sea; and the Gulf of Mudanya or Gemlik (Gulf of Cius), about 25 miles (40 km) long. At its extremity is situated the small town of Gemlik (ancient Cius) at the mouth of a valley, communicating with the lake of Iznik, on which was situated Nicaea.
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