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The Swiss Plateau or Central Plateau (plateau suisse in French, Schweizer Mittelland in German, altipiano svizzero in Italian) constitutes one of the three major landscapes in Switzerland alongside the Jura mountains and the Swiss Alps. It covers about 30% of the Swiss surface. It comprises the regions between the Jura and the Alps, partly flat but mostly hilly, and lies at an average height between 400 and 700 meters AMSL. It is by far the most densely populated region of Switzerland, and the most important with respect to economy and 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 meters AMSL (lime Alps, partly sub-alpine molasse), which is very abrupt in certain places, is taken as a criterion for delimination. Occasionally the regions of the higher Swiss plateau, especially the hills of the canton of Fribourg, the Napf region, the Töss region 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, Swiss plateau and Alps is considered, the Alpine foreland belongs clearly to the Swiss plateau. In the southwest, the Swiss plateau is confined by the Lake Geneva, in the northeast, by the 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. At the other side of the Lake Constance, the plateau continues in the German and Austrian Prealps.
Within Switzerland, the Swiss plateau has a length of about 300 kilometers, and its width increases from the west to the east: In the Geneva region, it is about 30 kilometers, at Bern about 50 kilometers and in eastern Switzerland about 70 kilometers.
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 (see also Jurassic). Around 2500 – 3000 meters 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 kilometers in the west to 0.8 kilometers 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.
Molasse
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.
History of the Swiss plateau molasse
During the Tertiary orogenic uplift, around 60 – 40 millions 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 millions 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.
- Lower freshwater molasse (around 30 - 22 millions 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 deposition of the first conglomerate fans.
- Upper sea molasse (around 22 - 16 millions 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 millions years ago): The sea receded as the formation and 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 are 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.
The 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.
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During the ice ages, the Rhône glacier split into two branches when leaving the Alps, covering the whole western Swiss plateau 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 has 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 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 farthest 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 that trend all 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 that trend northwest (among others including the valleys of the Wigger, the Suhre, the Seetal, the Reuss and the Limmat). The Rhine glacier has mostly left traces that trend west: The eastern Swiss plateau Thur valley and lake Constance. In certain places, there are characteristic drumlins of 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 the Lake Baldegg.
Another reminder of the glaciation are glacial erratics which are found all over the Swiss plateau. These rocks, sometimes of an enormous size, are of alien stones, mostly granite and gneiss from the central crystalline Alps. Taken together, they were one of the clues that led to the substantiation of the glaciation theory in the 19th century since a 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 withdrawals 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 (up to 600 meters AMSL) and the Jorat molasse hills (up to 900 meters AMSL) but sometimes intersected by deep valleys. Only 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 meters AMSL. The Seeland ('lake land'), characterized by the Murten, Neuchâtel and Biel lakes, represents the biggest plain of the Swiss plateau, though it is also irrupted 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 river runs. In a broad valley alongside the Jura, the Aar collects all the rivers that come down from the Alps.
The central Swiss plateau is characterized 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 that the major transportations bypass only in a few places, mostly in tunnels.
The eastern Swiss plateau is structured by the valleys of the Limmat (including lake Zürich), the Glatt, the Töss and the Thur. Between them there are hill countries, in the canton of Thurgau also the broad molasse ranges of Seerücken and Ottenberg.
Two hill countries get out of line of the mentioned landscapes: The Napf region (with 1408 meters AMSL the highest point of the Swiss plateau) and the Töss region (up to 1300 meters AMSL), both of them 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.
Climate
The Swiss plateau is situated within a transition zone between humid oceanic climate and continental temperate climate. The predominant wind comes from the west. In the lower plateau, the mean annual temperature is about 9 – 10 °C. In January, the lake Geneva region and the watersides of lake Neuchâtel and lake Biel have the highest mean temperature of about +1 °C. At the same height AMSL, the temperature is decreasing towards the east. In the lake Constance region, the mean temperature of the coldest month is -1 °C. In July, the mean temperature of Geneva is 20 °C, alongside the southern edge of the Jura it is 18 – 20 °C, and in higher regions 16 – 18 °C. With regard to mean sunshine duration, the lake Geneva region is again advantaged with more than 1900 hours, whereas in the rest of the Swiss plateau, it is between 1600 (especially in the east) and 1900 hours.