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The Ered Lithui, or the Ash Mountains, was a mountain range on the northern border of Mordor. It went 500 miles from east to west, and Barad-dûr stood at the end of a spur of the Ash Mountains. Gorgoroth was enclosed by another spur of these mountains. The west end of the Ash Mountains met the Mountains of Shadow. Between the Mountains of Shadow and the Ash Mountains was Cirith Gorgor, another name for "The Haunted Pass".
Name and etymology
The name "Mountains of Ash", or "Ashy Mountains", suggests an ash-grey hue.
Perhaps the range is downwind of Mount Doom and collects its ash fallout.