From Tolkien Gateway

Harrowdale was the name given to the deep north-south valley cut out of the White Mountains by the Snowbourn River. At its southern end, beneath the mountain Starkhorn, stood Dunharrow, an ancient structure of the Men of the Mountains, later used as a refuge of the Rohirrim. On its east side stood the saw-toothed peak Írensaga, while on its far south-east there stood Dwimorberg, the Haunted Mountain, where lay the entrance to the Paths of the Dead.

Hamlets in the dale were Underharrow and Upbourn. At its northern end, where the Snowbourn issued onto the plains of Rohan, stood Rohan's capital, Edoras.

During the War of the Ring the Lord of Harrowdale was Dúnhere. It was there that the muster of Rohan took place on 10 March[1] before Théoden led his riders to Minas Tirith[2] and the Battle of the Pelennor Fields, where Théoden and Dúnhere were killed.