The Prose Edda 1.8

Then said Gangleri: "What was done then by Borr's sons, if thou believe that they be gods?" Hárr replied: "In this matter there is no little to be said. They took Ymir and bore him into the middle of the Yawning Void, and made of him the earth: of his blood the sea and the waters; the land was made of his flesh, and the crags of his bones; gravel and stones they fashioned from his teeth and his grinders and from those bones that were broken." And Jafnhárr said: "Of the blood, which ran and welled forth freely out of his wounds, they made the sea, when they had formed and made firm the earth together, and laid the sea in a ring round about her; and it may well seem a hard thing to most men to cross over it." Then said Thridi: "They took his skull also, and made of it the heaven, and set it up over the earth with four corners; and under each corner they set a dwarf: the names of these are East, West, North, and South. Then they took the glowing embers and sparks that burst forth and had been cast out of Múspellheim, and set them in the midst of the Yawning Void, in the heaven, both above and below, to illumine heaven and earth. They assigned places to all fires: to some in heaven, some wandered free under the heavens; nevertheless, to these also they gave a place, and shaped them courses. It is said in old songs, that from these the days were reckoned, and the tale of years told, as is said in Völuspá:

The sun knew not where she had housing; The moon knew not what Might he had; The stars knew not where stood their places. Thus was it ere the earth was fashioned."

Then said Gangleri: "These are great tidings which I now hear; that is a wondrous great piece of craftsmanship, and cunningly made. How was the earth contrived?" And Hárr answered: "She is ring-shaped without, and round about her without lieth the deep sea; and along the strand of that sea they gave lands to the races of giants for habitation. But on the inner earth they made a citadel round about the world against the hostility of the giants, and for their citadel they raised up the brows of Ymir the giant, and called that place Midgard. They took also his brain and cast it in the air, and made from it the clouds, as is here said:

Of Ymir's flesh the earth was fashioned, And of his sweat the sea; Crags of his bones, trees of his hair, And of his skull the sky. Then of his brows the blithe gods made Midgard for sons of men; And of his brain the bitter-mooded Clouds were all created."