Helcaraxë

In the fiction of J. R. R. Tolkien, Helcaraxë or the Grinding Ice was an icy waste between the lands of Aman and Middle-earth. Its exact nature is left a little unclear, but it seems to have been broken and shifting pack ice[?] covering the northernmost parts of the Great Sea Belegaer.

Warning: Wikipedia contains spoilers

Helcaraxë is first recorded as having been crossed by Melkor and Ungoliant after the darkening of the Trees, then later by Fingolfin and his people.