The Cenozoic era (around 66 million years ago) witnessed the happening of two tectonic phenomena. The collision of the Atlantic Ocean with the Alps caused the Rhine ditch to collapse. As a result, the Vosges mountains and the Black Forest, until then united, rose. They were separated by a sea that, when retreating, left a filling of sediments known today as the plain of Alsace. A network of mountainous rivers brought sandstones there that would be used to build the Haut-Koenigsbourg castle millions of years later.