developing orogen it formed a small oceanic trench, in which flysch sediments were deposited. The huge amounts of sediments eroded from the forming mountain chain filled the basin and made it shallower. During the Oligocene and Miocene epochs (more exactly, between 10 and 30 million years ago), shallow marine to continental molasse was deposited in the basin. Around 10 to 5 million years ago, tectonic uplift had raised the basin so high that net sedimentation stopped. From the south, the molasse deposits were overthrust about 10 kilometers by the Helvetic nappes, which caused the deformation in the Subalpine Molasse zone. The Jura mountains, a fold and thrust belt along the present Swiss-French border, also originated from this tectonic phase. In some places in the Jura mountains, molasse deposits were folded together with older Mesozoic limestones. In present-day central Switzerland, however, the molasse formed a thick competent mass that was thrust northward in one piece over a decollement horizon at the base of the Mesozoic, in Triassic evaporites. Deformation instead localized further north, thus forming the relatively flat Swiss Mittelland between the Alps and the Jura Mountains. The Swiss part of the Molasse basin is now between the Alps and the Jura mountains, as a large piggyback basin. In the Eastern Alps an external mountain range such as the Jura Mountains never developed. Due to the last phase of tectonic uplift around 5 million years ago, the molasse in the Swiss Plateau, the South Bavarian plain and Eastern Austria is now 350 to 400 meters above sea level at its northern rim, slowly rises southwards and can reach more than 1,000 m at its contact with the Alps. Stratigraphy The molasse deposits in the basin can be up to 6 kilometers thick. Lithostratigraphically, this molasse is treated as a group divided into four formations. This division is made on whether the sedimentary facies is continental or marine. The lowermost formation is the Lower Marine Molasse (in German: Untere Meeresmolasse). Its age is Rupelian (early Oligocene, 34 to 28 million years old) and it consists of shallow marine sand, clay and marl. On top of this is the Lower Freshwater Molasse (German: Untere Süsswassermolasse) of Chattian and Aquitanian age (late Oligocene-early Miocene, 28 to 22 million years old). Due to a eustatic drop of the sea level combined by tectonic uplift, the basin was now above sea level. This second formation therefore