The Pass of Sirion.
The Narrow land was the land between the Mountains of Mithrim, the Shadowy Mountains and the steep cliffs of Dorthonion. The pass between Anfauglith and Beleriand where Sirion came from the Marshes of Serech into the land between Dimbar to Brithiach and river Malduin to the eaves of Brethil. In the midst of Sirion in the narrow pass to Anfauglith was the isle Tol-Sirion. The narrow vales sheer walls were clad with pines but the vale itself was green because of Sirion.