The Silmarillion

This dividing fall was named Andram, the Long Wall, from Nargothrond to Ramdal, the Wall's End, in East Beleriand. But in the east it became ever less sheer, for the vale of Gelion sloped steadily southward, and Gelion had neither fall nor rapids throughout his course, but was ever swifter than was Sirion. Between Ramdal and Gelion there stood a single hill of great extent and gentle slopes, but seeming mightier than it was, for it stood alone; and that hill was named Amon Ereb. Upon Amon Ereb died Denethor, lord of the Nandor that dwelt in Ossiriand, who marched to the aid of Thingol against Morgoth in those days when the Orcs first came down in force, and broke the starlit peace of Beleriand; and upon that hill Maedhros dwelt after the great defeat. But south of the Andram, between Sirion and Gelion, was a wild land of tangled forest in which no folk went, save here and there a few Dark Elves wandering; Taur-im-Duinath it was named, the Forest between the Rivers.

Gelion was a great river; and he rose in two sources and had at first two branches; Little Gelion that came from the Hill of Himring, and Greater Gelion that came from Mount Rerir. From the meeting of his arms he flowed south for forty leagues before he found his tributaries; and before he found the sea he was twice as long as Sirion, though less wide and full, for more rain fell in Hithlum and Dorthonion, whence Sirion drew his waters, than in the east. From Ered Luin flowed the six tributaries of Gelion: Ascar (that was after named Rathlóriel), Thalos, Legolin, Brilthor, Duilwen, and Adurant, swift and turbulent streams, falling steeply from the mountains; and between Ascar in the north and Adurant in the south, and between Gelion and Ered Luin, lay the far green country of Ossiriand, the Land of Seven Rivers. Now at a point nearly midway in its course the stream of Adurant divided and then joined again; and the island that its waters enclosed was named Tol Galen, the Green Isle. There Beren and Lúthien dwelt after their return.

In Ossiriand dwelt the Green-elves, in the protection of their rivers; for after Sirion Ulmo loved Gelion above all the waters of the western world. The woodcraft of the Elves of Ossiriand was such that a stranger might pass through their land from end to end and see none of them. They were clad in green in spring and summer, and the sound of their singing could be heard even across the waters of Gelion; wherefore the Noldor named that country Lindon, the land of music, and the mountains beyond they named Ered Lindon, for they first saw them from Ossiriand.

East of Dorthonion the marches of Beleriand were most open to attack, and only hills of no great height guarded the vale of Gelion from the north. In that region, upon the March of Maedhros and in the lands behind, dwelt the sons of Fëanor with many people; and their riders passed often over the vast northern plain, Lothlann the wide and empty, east of Ard-galen, lest Morgoth should attempt any sortie towards East Beleriand. The chief citadel of Maedhros was upon the Hill of Himring, the Ever-cold; and that was wide-shouldered, bare of trees, and flat upon its summit, surrounded by many lesser hills. Between Himring and Dorthonion there was a pass, exceeding steep upon the west, and that was the Pass of Aglon, and was a gate unto Doriath; and a bitter wind blew ever through it from the north. But Celegorm and Curufin fortified Aglon and held it with great strength, and all the land of Himlad southward between the River Aros that rose in Dorthonion and his tributary Celon that came from Himring.

Between the arms of Gelion was the ward of Maglor, and here in one place the hills failed altogether: there it was that the Orcs came into East Beleriand before the Third Battle. Therefore the Noldor held strength of cavalry In the plains at that place; and the people of Caranthir fortified the mountains to the east of Maglor's Gap. There Mount Rerir, and about it many lesser heights, stood out from the main range of Ered Lindon westward; and in the angle between Rerir and Ered Lindon there was a lake, shadowed by mountains on all sides save the south. That was Lake Helevorn, deep and dark, and beside it Caranthir had his abode; but all the great land between Gelion and the mountains, and between Rerir and the River Ascar, was called by the Noldor Thargelion, which signifies the Land beyond Gelion, or Dor Caranthir, the Land of Caranthir; and it was here that the Noldor first met the Dwarves. But Thargelion was before called by the Grey-elves Talath Rhúnen, the East Vale.