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 Fellowship of the Ring, Many Meetings - Song of Eärendil

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.