How do mountain passes form between peaks?

Mountain passes are gaps or low saddles in ridges that allow people, animals, and roads to cross mountains more easily. They can form when erosion removes rock in the weakest part of a range, when glaciers carve through high valleys, or when older river routes are uplifted by tectonics. Many famous trade and travel routes developed through passes because they were more practical than climbing over the highest peaks. The Brenner Pass in the Alps and the Khyber Pass between Afghanistan and Pakistan are historic examples. Pass elevation can be hundreds or thousands of meters above sea level, but still lower than the surrounding peaks.