Swiss Plateau

The Swiss Plateau or Central Plateau is one of the three major landscapes in Switzerland, lying between the Jura Mountains and the Swiss Alps. It covers about 30% of the Swiss surface area, and is partly flat but mostly hilly. The average height is between 400 metres (1,300 ft) and 700 metres (2,300 ft) AMSL. It is by far the most densely populated region of Switzerland, the center of economy and important transportation.

In the north and northwest, the Swiss Plateau is sharply delimited geographically and geologically by the Jura Mountains. In the south, there is no clear border with the Alps. Usually, the rising of the terrain to altitudes above 1500 metres AMSL (lime Alps, partly sub-alpine molasse), which is very abrupt in certain places, is taken as a criterion for delimitation. Occasionally the regions of the higher Swiss Plateau, especially the hills of the canton of Fribourg, the Napf region, the Töss region, the (lower) Toggenburg, and parts of the Appenzell region are considered to form the Swiss Alpine foreland in a narrow sense. However, if a division into the three main regions Jura Mountains, Swiss Plateau and Alps is considered, the Alpine foreland belongs clearly to the Swiss Plateau. In the southwest, the Swiss Plateau is confined by Lake Geneva, in the northeast, by Lake Constance and the Rhine.

Geologically, the Swiss Plateau is part of a larger basin that extends beyond the border of Switzerland. At its southwestern end, in France, the plateau, in the Genevois, ends at Chambéry where Jura and Alps meet. On the other side of Lake Constance, the plateau continues in the German and Austrian Pre-Alps.

Within Switzerland, the Swiss Plateau has a length of about 300 kilometres (190 mi), and its width increases from the west to the east: In the Geneva region, it is about 30 kilometres (19 mi), at Bern about 50 kilometres (31 mi) and in eastern Switzerland about 70 kilometres (43 mi).

Many cantons of Switzerland include a part in the Swiss Plateau. Entirely situated within the Swiss Plateau are the cantons of Zürich, Thurgau and Geneva; mostly situated within the Swiss Plateau are the cantons of Lucerne, Aargau, Solothurn, Bern, Fribourg and Vaud; small portions of the Swiss Plateau are situated in the cantons of Neuchâtel, Zug, Schwyz, St. Gallen and Schaffhausen.

The geological layers of the Swiss Plateau are relatively well known. The base level is crystalline basement which outcrops in the central crystalline Alps as well as in 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.