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 and AMSL. It is by far the most densely populated region of Switzerland, the center of economy and important transportation.
Geography
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, 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 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, and its width increases from the west to the east: In the Geneva region, it is about, at Bern about and in eastern Switzerland about.
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.
Geology
Geological layers
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. 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. 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.
- Lower sea molasse : 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.
- Lower freshwater molasse : 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 : 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 : The sea receded as the formation of the Napf and Hörnli fans continued. At the end of this period, the thickness reached about 1500 meters.
A characteristic of the sea molasses is fossil snails, shells and shark teeth, whereas in the freshwater molasse, fossils of typical land mammals and former subtropical vegetation 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, huge glaciers penetrated the Swiss Plateau. During the warm interglacials, the glaciers receded to the high alps and subtropical vegetation spread in the plateau.Image:Picswiss BE-92-04 Lüderenalp.jpg|thumb|left|The Napf region in the higher Swiss 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 towards the Jura. The glaciers formed the land by erosion, but base moraines often several meters thick, and the deposition of gravel by meltwater streams also shaped the land.
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 extent 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 about 15 000 years ago. The end moraines of different glacial retreats have been conserved.
Glacial landscapes
A look at a map still reveals the directions where the ice age glaciers ran. The furthest expansion of the Rhône Glacier to the northeast is indicated by way the western Swiss Plateau valleys trend: the valleys of the Broye and the Glâne as well as Lake Murten, Lake Neuchâtel, and Lake Biel all trend northeast, parallel to the Jura and to the Alps. The glaciers of the Reuss and the Limmat have carved the valleys of the central Swiss Plateau trend northwest. The Rhine Glacier has mostly left traces that trend west, such as the eastern Swiss Plateau of the Thur Valley and Lake Constance. In certain places, there are characteristic drumlins of the base moraine, often clustered, especially in the highlands of Zürich, in the Hirzel region, in the Lake Constance region and between the Reuss Valley and Lake Baldegg.Another reminder of the glaciation are glacial erratics which are found all over the Swiss Plateau. These rocks, sometimes of enormous size, are of alien stones, mostly granite and gneiss from the central crystalline Alps. Taken together, they were clues that led to the substantiation of the glaciation theory in the 19th century, since transport by water or by volcanism was physically impossible.
Gravel deposits in the bottoms of the valleys are another testimonial of the glaciation. During the advances and retreats of the glaciers, gravel layers were deposited in the valleys, sometimes quite thick, though most of it eroded in the subsequent interglacials. Therefore, many valleys have characteristic terraces, the lower terraces consisting of Würm glaciation gravel, the higher terraces of Riss glaciation terraces. Sometimes, there is also gravel from older glaciations.
Topography
Even though the Swiss Plateau forms a basin, it is by no means a flat territory, but depending on the region, it has a manifold structure. Important elements are the two big lakes, Lake Geneva and Lake Constance that delimit the Swiss Plateau in the southwest and the northeast. The western plateau is stamped by the Gros-de-Vaud plateau and the Jorat molasse hills but is sometimes intersected by deep valleys. Near the Jura, there is an almost continuous dip consisting of the Venoge and the Orbe valleys which are separated by the Mormont hill, the main watershed between Rhône and Rhine, at only 500 m AMSL. The Seeland, characterized by the Murten, Neuchâtel and Biel lakes, represents the largest plain of the Swiss Plateau, though it is also interrupted by isolated molasse ranges. In the east, it is neighboured by various hill countries the height of which decreases to the north. Another major plain is the Wasseramt where the Emme runs. In a broad valley alongside the Jura, the Aare collects all the rivers that come down from the Alps.Image:Mauensee.jpg|thumb|left|Central Swiss Plateau near Sursee
The central Swiss Plateau is characterised by a number of ranges and broad valleys, some of them with lakes, that run northwest. The last of them is the Albis range, which together with the Heitersberg range forms a bar from the Alps to the Jura. Major transportation routes cross it only in a few places, mostly via tunnels.
The eastern Swiss Plateau is structured by the valleys of the Limmat, the Glatt, the Töss, the Murg, the Thur, and the Sitter. Between them there are hill countries. In the canton of Thurgau there are also the broad molasse ranges of Seerücken and Ottenberg north of the Thur, and the hilly ranges between the Thur and the Murg. This area is colloquially also known as Mostindien.
Two hill countries are out of sync with the above-mentioned landscapes: the Napf region and the Töss region ; both are the remains of Tertiary conglomerate sediment fans. Since they were not glaciated, they have only been eroded by water, resulting in a dense net of deep, narrow valleys.