Serving as the northern boundary of Mordor, the Ash Mountains (also called the Ashen Mountains, Ashy Mountains or the Ered Lithui) ran almost due east from their start at Morannon until they ended in the eastern land of Rhûn.

There was a narrow gap where the Ash Mountains met the Mountains of Shadow (the Ephel Dúath), and it was in this gap that Sauron had the Black Gate of Mordor erected (the Morannon).

Three spurs ran to the southwest from the Ash Mountains. The first of these spurs met a similar spur from the Ephel Dúath at the gap of Carach Angram, also known as the Isenmouthe. The ancient castle of Durthang sat where the second spur met the main range, and was converted by Sauron into an orc-hold. By the end of the Third Age the entire length of this spur was covered with the forts and towers of Sauron's troops. This spur on which Durthang sat was simply a part of the greater range known as the Ephel Dúath.

At the end of the second spur Sauron erected the tower of Barad-dûr. It was from here that he ruled his lands. Mount Doom, where the One Ring was forged and the only place where it could be destroyed, was situated between this spur and the Ephel Duath.

The third spur also ran towards a similar spur from the Mountains of Shadow and separated the plateau of Gorgoroth from Nurn and the south of Mordor.