The western extent of the Iron Mountains, where Angband stood beneath Thangorodrim, were destroyed by the assault of the Valar and the inrushing Sea at the end of the First Age. It is known that the range ran farther into the east, and it is conceivable that some of its eastern parts, which would lie to the north of the known regions of Middle-earth, survived the cataclysm. If so, they are never mentioned in any account after the end of the First Age.

It's tempting to imagine that the Iron Hills to the east might have been a remnant of the great range of the Iron Mountains, but this does not seem to be the case. Following the track of the original Mountains from Beleriand in the First Age, they would have run far to the north of the Iron Hills and so, apart from their similar names, these two ranges do not seem to have been historically connected.

This rather vague map follows a rough sketch map from volume IV of The History of Middle-earth. Though lacking in specific detail, this is the clearest picture we have of the entire range of the Iron Mountains as they ran across the northern continent of Middle-earth. For a closer view of the detail at the important western end of the Mountains, see the entry for Ered Engrin.