Before the development of the Molasse basin, in the Mesozoic era, the region was covered by a shallow sea that was the northern margin of the Alpine part of the Tethys Ocean (Valais Ocean in the west, Tethys Ocean proper in the east). The Molasse basin was formed when the orogenic wedge of the forming Alps was pushed north over the European continental margin, due to the convergent movement of the European and Apulian plates during the Paleogene period. The weight of the orogenic wedge made the European plate bend downward, forming a deep marine foredeep. In the Eocene epoch (55 to 34 million years ago) the basin became deeper until north of the developing orogen it formed a small oceanic trench, in which flysch sediments were deposited.