§ Of Beleriand and Its Realms The Silmarillion

Southward lay the guarded woods of Doriath, abode of Thingol the Hidden King, into whose realm none passed save by his will. Its northern and lesser part, the Forest of Neldoreth, was bounded east and south by the dark river Esgalduin, which bent westward in the midst of the land; and between Aros and Esgalduin lay the denser and greater woods of Region. Upon the southern bank of Esgalduin, where it turned westward towards Sirion, were the Caves of Menegroth; and all Doriath lay east of Sirion save for a narrow region of woodland between the meeting of Teiglin and Sirion and the Meres of Twilight. By the people of Doriath this wood was called Nivrim, the West March; great oak-trees grew there, and it also was encompassed within the Girdle of Melian, that so some portion of Sirion which she loved in reverence of Ulmo should be wholly under the power of Thingol.