The White Mountains (Ered Nimrais) ran westward from the Anduin almost to the Great Sea.

When Gondor was founded in SA 3320 it was bisected by the White Mountains. A northern spur of the range ran towards the Misty Mountains, stopping at the Gap of Rohan, and the River Isen flowed through the gap.

In TA 2510, all of Gondor north of the mountains, save for AnĂ³rien, was given to the Rohirrim and became the kingdom of Rohan.

Only one pass through the White Mountains was known. Starting in Harrowdale, the pass ran through the Valley of Dimholt to the Dark Door. From there, the pass entered the passage known as the Paths of the Dead, passing beneath Mount Dwimorberg to emerge in the Blackroot Vale at the Hill of Erech, where the Stone of Erech was placed.

Throughout the Third Age no living man travelled the Paths of the Dead until 3019 when Aragorn took the Grey Company through the pass, and at the Stone of Erech called upon the Dead Men to fulfill the oath they broke over 3000 years earlier and finally come to the aid of Gondor.
Encyclopedia entry originally written by PotbellyHairyfoot