Mountain Wall

From Tolkien Gateway

From Evereven's lofty hills
where softly silver fountains fall
his wings him bore, a wandering light,
beyond the mighty Mountain Wall.

The Mountain Wall was a term that appears just once, in Bilbo Baggins's poem Eärendil was a mariner quoted above. The meaning is not completely certain, but it seems to describe the Pelóri, the Mountains of Defence that ranged along the eastern borders of Valinor.