the Black Forest and the Vosges mountain range but forms a deep geosyncline in the Swiss Plateau and in the Jura (see also Jurassic). Around 2500 – 3000 metres below the surface, but considerably deeper near the Alps, the drillings have hit the crystalline basement. It is covered by unfolded strata of Mesozoic sediments, which are part of the Helvetic nappes. Its depth gradually decreases from about 2.5 km in the west to 0.8 km in the east. These layers, like the ones of the Jura Mountains, were deposited in a relatively shallow sea, the Tethys Ocean. Above the Mesozoic layers, is the Molasse, consisting of conglomerate, sandstone, marl and shale. The uppermost layer consists of gravel and glacial sediments that have been transported by the glaciers of the ice ages. 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. The eroded material has been sorted by grain size. The coarse material was predominantly deposited near the Alps. In the middle of the plateau, there are finer sandstones and near the Jura, clays and marl. During the Tertiary orogenic uplift, around 60 – 40 million years ago, the area of today's Swiss Plateau was a Karst plateau somewhat inclined to the south. Through processes of rising and lowering that were brought by the folding of the Alps, the area was twice flooded by a sea. The corresponding sediments are distinguished as sea molasse and freshwater molasse, even though the latter consists rather of fluvial and eolian sediments (a kind of mainland molasse). Lower sea molasse (around 37 - 30 million years ago): The limestone plateau subsided gradually, and a shallow sea invaded, spreading east to the Carpathian Mountains. The sediments consisted of fine-grained sands, clay and marl. There were no conglomerate fans since the proper Alpine folding began only at the end of that period.