Geographical error in the Silmarillion?

The following passage makes no sense:

for nigh and two years after the Dagor Bragollach the Noldor still defended the western pass about the sources of Sirion, for the power of Ulmo was in that water, and Minas Tirith withstood the orcs.

This makes no sense because Sirion is in the middle of Beleriand, and according to The Lord of the rings map, Minas Tirith is in the south-east, near Mordor. where there two towers named Minas Tirith? Or am I missing something?