The River Sirion, the chief river of Beleriand, running north to south, divided it into West and East Beleriand. Crossing it east to west was a series of hills and a sudden drop in elevation known as Andram, the Long Wall. The river sank into the ground at the Fens of Sirion, and re-emerged below the Andram at the Gates of Sirion. To the east of the Long Wall, was the River Gelion and its six tributaries draining the Ered Luin, in an area known as Ossiriand, "Land of Seven Rivers".