Gefjon's myth explains two real places at once, and you can still stand in both of them.
Gylfaginning opens with it, before any of the gods have been introduced. King Gylfi of Sweden rewards a travelling woman who has entertained him with a ploughland — as much as four oxen could plough in a day and a night. She is of the Æsir. She goes north to Jötunheimr, gets four sons on a giant, turns them into oxen, and sets them to the plough. The share cuts so deep and hard that the land tears loose, and the oxen drag it westward into the sea and set it down in a sound. She named it Selund, and where the land had been taken up there was afterwards water: that is the lake now called Lögrinn, and the inlets in the lake match the headlands of Selund.
Selund is Zealand, the Danish island Copenhagen stands on. Lögrinn is Lake Mälaren in Sweden. The story is a piece of comparative topography — the claim being that the island's coastline and the lake's shores fit each other — and it is the earliest known example of anyone in Scandinavia noticing that landforms might be explained by displacement.
The observation does not survive scrutiny; the two do not actually correspond. But somebody looked.
The oldest witness
Snorri does not just tell the story, he quotes his source: a stanza by Bragi Boddason, the ninth-century skald, describing Gefjun drawing the land from Gylfi, glad of the increase, deep-grown, so that steam rose from the running beasts, and the oxen bearing eight eyes and four heads.
That verse is roughly three and a half centuries older than the Prose Edda and is the earliest datable reference to any specific Norse myth in the skaldic corpus. The four-headed, eight-eyed team is Bragi's image, not Snorri's, and it is a fine one: the poet counts the animals by their parts because the whole is a single machine.
Ynglinga saga retells the episode and adds that Gefjun then married Skjöldr, son of Odin, and settled at Hleiðra — Lejre, the site of a genuinely major Iron Age and Viking-age royal hall complex on Zealand, extensively excavated since the 1980s. The myth is being used to attach a Danish dynasty to a piece of land and to a god.
The problem of what she is
Her name comes from the verb gefa, to give, and means something like "the giver" or "the generous one." Snorri lists her among the goddesses and says she is a virgin, and that all who die maidens attend her.
That last claim is difficult to hold alongside the myth in which she has four sons by a giant. It is also difficult to hold alongside Lokasenna, where Loki accuses her of having been bought by a white-haired boy who gave her a jewel and whom she laid her thigh over — and Odin's response, which is not a denial but a warning that Gefjun knows the fates of all as well as he does himself.
That is a striking line. Gefjun is put on a level with Frigg as a possessor of complete foreknowledge, in a single stanza, and then never mentioned in that connection again.
The virginity claim may be Snorri importing a classical or Christian category. Alternatively Gefjun may be a byname rather than an independent goddess: Gefn is one of Freyja's recorded names, both derive from the same verb, and a "giver" goddess associated with ploughed land and with fate has a good deal of Freyja about her. The identification is proposed and not established, and the family tree keeps them separate because the sources do.
Gefjun's plough in the twentieth century
In 1908 the Carlsberg Foundation unveiled a bronze fountain at Langelinie in Copenhagen, by the sculptor Anders Bundgaard: Gefjun standing over four straining oxen, whip raised, water pouring from the group. It is one of the most-photographed monuments in Denmark and it depicts an origin story for the ground the city sits on.
A ninth-century skald's image, preserved by a thirteenth-century Icelander, reproduced in bronze beside a modern harbour, is about as continuous a transmission as Norse mythology ever manages — which does not make the story old in the way people assume, only well-travelled.
Where else the name appears
The Old English poem Juliana contains the word geofon meaning ocean, which has been connected to her name; the Danish Gefjunarbrunnr place-name has been proposed and disputed; and the name appears in a handful of skaldic kennings. Modern usage is slight — occasional Danish and Icelandic given names, a scattering of ships and institutions.
For the tradition she belongs to and how much of it rests on one systematiser, see the Norse overview; for the poet who preserved her, Bragi.
Why does a goddess turn her own sons into oxen?
The story treats it as a resource decision rather than a cruelty, and the sons are never mentioned again. Their giant father is unnamed, and their transformation is one line. What the episode needs is a plough-team strong enough to cut a landmass free, and Gefjun produces one by the most direct means available. It is worth noticing how differently this reads from the Greek habit of turning transformation into punishment or rescue: here it is simply how the work gets done.
Is the Gefjun story evidence for a real land-taking?
Not geologically, no. Zealand did not come out of Mälaren and the coastlines do not match. What the story may encode is a political claim rather than a physical one — a Danish assertion of ownership over territory once held or claimed by Swedes, told as a divine transfer with a legal frame: Gylfi grants the land, the terms are met literally, and the recipient takes possession. It is a property-conveyance myth, and the deed is drawn very carefully.