Partially conjectural¹

A range of mountains running north to south in the distant eastern parts of Middle-earth, far beyond its more familiar regions. They were also known as the Orocarni, the Red Mountains. From their western slopes, many streams ran down into a great lake or inland sea, and it was on the shores of this lake that the first Elves awoke, in a place that they called Cuiviénen, the Water of Awakening. Beneath the Mountains of the East, Oromë encountered the Elves for the first time, and many of them left the land beneath the Mountains to pass into the West.

In the original, highly symmetrical, conception of Arda, Middle-earth had four great ranges of mountains. In the northwest, these were the familiar Blue Mountains, and the Red Mountains, or Mountains of the East, were their counterpart in the east. Two further ranges ran into the far south: the Grey Mountains (which were quite distinct from the Grey Mountains of the north) and the Yellow Mountains. The primeval destruction wrought by Melkor broke this design for the world, but it is clear that at least some part of the Mountains of the East survived at least until the time of the awakening of the Elves.