The Matterhorn's faces are steep, and only small patches of snow and ice cling to them; regular avalanches send the snow down to accumulate on the glaciers at the base of each face, the largest of which are the Tiefmattengletscher to the west, part of the Zmutt Glacier, and the Matterhorn Glacier to the north. Smaller glaciers lie at the base of the south face (the Lower Matterhorn Glacier) and the east face (unnamed). In this area, the border between Switzerland and Italy coincides with the main Alpine watershed, separating the drainage basin of the Rhone on the north (Mediterranean Sea) and that of the Po on the south (Adriatic Sea). The north side is drained by the Zmuttbach (west and north faces) and the Gornera through the Furggbach (east face), tributaries of the Rhone through the (Matter) Vispa. The south side and face is drained by the Marmore torrent, tributary of the Po through the Dora Baltea (or Doire baltée). The Theodul Pass, located on the watershed between the Matterhorn and the Breithorn, at 3,295 metres, is the easiest passage between the two valleys and countries (the slightly lower Furggjoch not being used as a pass). The pass was used as a crossover and trade route for the Romans and the Romanised Celtic population Salassi between 100 BCE and 400 CE. The area is now heavily glaciated and covered on the north side by the Theodul Glacier.