The Younger Edda

King Gylfe ruled the lands that are now called Svithjod (Sweden). Of him it is said that he gave to a wayfaring woman, as a reward for the entertainment she had afforded him by her story-telling, a plow-land in his realm, as large as four oxen could plow it in a day and a night But this woman was of the asa-race; her name was Gefjun. She took from the north, from Jotunheim, four oxen, which were the sons of a giant and her, and set them before the plow. Then went the plow so hard and deep that it tore up the land, and the oxen drew it westward into the sea, until it stood still in a sound. There Gefjun set the land, gave it a name and called it Seeland. And where the land had been taken away became afterward a sea, which in Sweden is now called Logrinn (the Lake, the Malar Lake in Sweden).