100 million years ago the extension of the Tethys Ocean stopped and the Apulian plate broke from Gondwana and moved toward the European continent. This resulted in the closure of the western Tethys by subduction under the Apulian plate (with the Piemont-Liguria Ocean first and Valais Ocean later). The subduction of the oceanic crust left traces still visible today at the base of the Matterhorn (accretionary prism). The orogeny itself began after the end of the oceanic subduction when the European continental crust collided with the Apulian continent, resulting in the formation of nappes.