Mountains of Iron

In the depths of time, when Melkor fought against the Valar for power over Arda, his great stronghold was at Utumno in the far North of the world. To protect Utumno, the Dark Lord raised a range of mountains that arced across Middle-earth, following a curve along the edges of the arctic frosts. These were Ered Engrin or the Mountains of Iron (also commonly called the Iron Mountains). In the western part of the range, where they came closest to Valinor, he reinforced his defences by building another great fortress, Angband, which in those early times was commanded by Melkor's lieutenant, Sauron.

At the end of the War of the Powers, the Valar overcame Utumno's defences and captured Melkor, taking him in chains back to Valinor. There he remained for three ages, until he returned once again to Middle-earth. Though Utumno had been destroyed, Angband remained behind the western end of the Mountains of Iron. There Melkor took up his power again, and tunnelled into the mountains. He created a way through to the south for his forces, and raised Thangorodrim, three immense towers as tall as the Iron Mountains themselves.

From Angband he made war against the Elves and Men to the south, and they in turn attempted to besiege him in Angband. That Siege was never complete because of the Iron Mountains, whose great curve across the northlands prevented Angband from being truly surrounded. As the First Age neared its close, a great battle was fought across the North in the shadow of the Mountains of Iron, and the Nirnaeth Arnoediad ended in total victory for Melkor. After that time his power spread southwards, apparently unstoppable, until the Valar sent their forces against him and broke Angband.

In the cataclysm brought about in that War of Wrath, Beleriand and its neighbouring lands were drowned by the Great Sea. Presumably the Mountains of Iron were also lost at this time, at least along the western part of their length. It is possible that the more easterly mountains of the range survived, but they would have stood far to the North of any known part of Middle-earth.