In the eddic poem Grottasöngr, the Lay of Grótti, the magical mill ground by Fenja and Menja for the legendary Danish king Fróði.
Grottasöngr tells the tale of two young slave girls who were bought by king Fróði when he was on a visit to king Fjölnir in Sweden. At this time two mill-stones were found in Danmark which were so large that no one could drag them. These stones possessed the property of grinding whatever the grinder wished. Fróði set Fenja and Menja to work at the quern, or mill, and commanded them to grind gold, peace and prosperity for him. They were not given rest or sleep longer than the time of a song or the silence of the cuckoo.
During their work they sang the song called Grottasöngr, and before they ceased their singing they had ground an army into existence against Fróði. The maidens, who were now in jötunn-mood, continued to ground even harder, until the stone split into fragments.
According to the prose header of Grottasöngr, and to Snorri Sturluson in Skáldskaparmál, the army was led by the sea-king Mýsingr. He slew Fróði and took all his wealth. Mýsingr brought the giantesses and the mill with him and bade them to ground white salt in his ships. When asked if he had enough salt, Mýsingr told them to grind more, and the weight of the salt caused his ships to sink in Pentland Firth. A huge maelstrom was created where the seawater rushed through the center of the mill stone. It was then that the sea turned salt.
Snorri adds that the mill was given to Fróði by Hengikjöptr.
- Skáldskaparmál, 42.