The formation and geological folding of the Harz hills began during a prominent phase of the Palaeozoic era, in the course of the Hercynian mountain building of the Carboniferous period, about 350 to 250 million years ago. At that time in the history of the Earth, numerous high mountains appeared in Western Europe, including the Fichtelgebirge and Rhenish Massif. They were, however, heavily eroded due to their height (up to 4 km) and were later covered over by Mesozoic rocks. From the Early Cretaceous and into Late Cretaceous times the Harz was uplifted in a single block by tectonic movements and, particularly during the Tertiary period, the younger overlying strata were eroded and the underlying base rock left standing as low mountains. The most important uplift movements were during the sub-Hercynian phase (83 mya), when the northern edge was steeply tilted. This formed a fault zone on the northern border of the Harz (the Northern Harz Boundary Fault or Harznordrandverwerfung).