§ Of Tuor and the Fall of Gondolin The Silmarillion

But when Tuor had lived thus in solitude as an outlaw for four years, Ulmo set it in his heart to depart from the land of his fathers, for he had chosen Tuor as the instrument of his designs; and leaving once more the caves of Androth he went westwards across Dor-lómin, and found Annon-in-Gelydh, the Gate of the Noldor, which the people of Turgon built when they dwelt in Nevrast long years before. Thence a dark tunnel led beneath the mountains, and issued into Cirith Ninniach, the Rainbow Cleft, through which a turbulent water ran towards the western sea. Thus it was that Tuor's flight from Hithlum was marked by neither Man nor Orc, and no knowledge of it came to the ears of Morgoth.