§ Of Beleriand and Its Realms The Silmarillion

In the south-west of Doriath, where Aros flowed into Sirion, lay great pools and marshes on either side of the river, which halted there in his course and strayed in many channels. That region was named Aelin-uial, the Twilight Meres, for they were wrapped in mists, and the enchantment of Doriath lay over them. Now all the northern part of Beleriand sloped southward to this point and then for a while was plain, and the flood of Sirion was stayed. But south of Aelin-uial the land fell suddenly and steeply; and all the lower fields of Sirion were divided from the upper fields by this fall, which to one looking from the south northward appeared as an endless chain of hills running from Eglarest beyond Narog in the west to Amon Ereb in the east, within far sight of Gelion. Narog came through these hills in a deep gorge, and flowed over rapids but had no fall, and on its western bank the land rose into the great wooded highlands of Tauren-Faroth. On the west side of this gorge, where the short and foaming stream Ringwil tumbled headlong into Narog from the High Faroth, Finrod established Nargothrond. But some twenty-five leagues east of the gorge of Nargothrond Sirion fell from the north in a mighty fall below the Meres, and then he plunged suddenly underground into great tunnels that the weight of his falling waters delved; and he issued again three leagues southward with great noise and smoke through rocky arches at the foot of the hills which were called the Gates of Sirion.