Two major tectonic movements took place during the Cenozoic era (around 66 million years ago). The recently formed Atlantic Ocean collided with the Alps. The shock collapsed the Rhine ditch, causing both the Vosges and the Black Forest to rise. They were separated by a sea that, when retreating, left a filling of sediments known today as the plain of Alsace. At the same time, a network of mountainous rivers brought sandstones there that would be used to build the Haut-Koenigsbourg castle millions of years later.