Geologically the most important layer of the Swiss Plateau is the thick molasse sequence that accumulated at the border of the Alps due to the rapid erosion of the concurrently uplifted mountains. The thickness of the molasse increases from west to east (at the same distance from the Alps). The former alpine rivers built huge fans of sediment at the foot of the mountains. The most important examples are the Napf fan and the Hörnli fan; other sedimentary fans exist in the Rigi region, in the Schwarzenburg region and in the region between the eastern lake Geneva and the middle reaches of the Saane/Sarine.