§ Of the Noldor in Beleriand The Silmarillion

It has been told how by the guidance of Ulmo Turgon of Nevrast discovered the hidden vale of Tumladen; and that (as was after known) lay east of the upper waters of Sirion, in a ring of mountains tall and sheer, and no living thing came there save the eagles of Thorondor. But there was a deep way under the mountains delved in the darkness of the world by waters that flowed out to join the streams of Sirion; and this way Turgon found, and so came to the green plain amid the mountains, and saw the island-hill that stood there of hard smooth stone; for the vale had been a great lake in ancient days. Then Turgon knew that he had found the place of his desire, and he resolved to build there a fair city, a memorial of Tirion upon Túna; but he returned to Nevrast, and remained there in peace, though he pondered ever in his thought how he should accomplish his design.