§ Of Beleriand and Its Realms The Silmarillion

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.