The Misty Mountains marched southwards through Middle-earth for hundreds of miles, forming a barrier between Eriador and the Vales of Anduin that could be crossed only by a few difficult passes. At their southern end, they diminished into a range of hills, and then came to an end altogether, leaving a flat plain that ran southwards for some fifty miles until the foothills of the White Mountains rose out of it in the south.

At the end of the Third Age, this opening was most usually called the Gap of Rohan, named for the country on whose western borders it stood, but for most of the earlier part of that Age it had been known as the Gap of Calenardhon. Calenardhon was the name given by the Gondorians to the wide green land that would be settled by the Rohirrim, but before their coming it was a northern fief of the Kings of Gondor, and through it ran the Great Road that passed through the Gap and ran on northwards across Enedwaith towards Arnor in the North.