Lower freshwater molasse (around 30 - 22 million years ago): The sea receded because of uplift, but also because of a worldwide lowering of the mean sea level. The initiation of the Alpine orogeny and subsequent folding and uplift resulted in rapid erosion accompanied by the deposition of the first conglomerate fans. Upper sea molasse (around 22 - 16 million years ago): For a second time, a shallow sea invaded. The formation of the conglomerate fans of the Napf and of the Hörnli began. Upper freshwater molasse (about 16 - 2 million years ago): The sea receded as the formation of the Napf and Hörnli fans continued (along with other minor fans). At the end of this period, the thickness reached about 1500 meters. In the following time, especially the western part of the plateau was again significantly risen, so that in this area, the sediments of the upper sweetwater molasse and the upper sea molasse have been largely eroded. A characteristic of the sea molasses is fossil snails, shells and shark teeth, whereas in the sweetwater molasse, fossils of typical land mammals and former subtropical vegetation (for instance palm leaves) are found. Ice ages The contemporary landscape of the Swiss Plateau has been shaped by the ice age glaciers. During all the known alpine glaciations (Günz glaciation, Mindel glaciation, Riss glaciation and Würm glaciation), huge glaciers penetrated the Swiss Plateau. During the warm interglacials, the glaciers receded to the high alps (sometimes more than today) and subtropical vegetation spread in the plateau. During the ice ages, the Rhône glacier split into two branches when leaving the Alps, covering the whole western Swiss Plateau and reaching today's regions of Solothurn and Aarau. In the region of Bern, it merged with the Aar Glacier. The glaciers of the Reuss, the Limmat and the Rhine advanced sometimes as well until the Jura. The glaciers formed the land by erosion, but also by base moraines (very fine stone meal) often several meters thick, and by the meltwater streams depositing gravel. Traces of the older Günz and Mindel glaciation are only left in a few places because most have been removed or transferred by the later glaciations. The greatest extension was reached by the glaciers of the Riss glaciation, when the entire Swiss Plateau was covered with ice except for the Napf and Töss regions. Most notable are the traces of the Würm glaciation