§ Of the Sun and Moon and the Hiding of Valinor The Silmarillion

But seeing the assault upon Tilion the Valar were in doubt, fearing what the malice and cunning of Morgoth might yet contrive against them. Being unwilling to make war upon him in Middle-earth, they remembered nonetheless the ruin of Almaren; and they resolved that the like should not befall Valinor. Therefore at that time they fortified their land anew, and they raised up the mountain-walls of the Pelóri to sheer and dreadful heights, east, north, and south. Their outer sides were dark and smooth, without foothold or ledge, and they fell in great precipices with faces hard as glass, and rose up to towers with crowns of white ice. A sleepless watch was set upon them, and no pass led through them, save only at the Calacirya: but that pass the Valar did not close, because of the Eldar that were faithful, and in the city of Tirion upon the green hill Finarfin yet ruled the remnant of the Noldor in the deep cleft of the mountains. For all those of elven-race, even the Vanyar and Ingwë their lord, must breathe at times the outer air and the wind that comes over the sea from the lands of their birth; and the Valar would not sunder the Teleri wholly from their kin. But in the Calacirya they set strong towers and many sentinels, and at its issue upon the plains of Valmar a host was encamped, so that neither bird nor beast nor elf nor man, nor any creature beside that dwelt in Middle-earth, could pass that leaguer.