The Rhenohercynian Zone or Rheno-Hercynian zone in structural geology describes a fold belt of west and central Europe, formed during the Hercynian orogeny (about ). The zone consists of folded and thrust Devonian and early Carboniferous sedimentary rocks that were deposited in a back-arc basin along the southern margin of the then existing paleocontinent Laurussia. The Rhenohercynian Zone, named for the Rhine River and the Hercynian Forest of Antiquity, forms a narrow zone through western and central Europe, from Cornwall and Ireland in the west to the Harz mountains of central Germany in the east, including the Rhenish Massif (Ardennes, Taunus, Eifel and Hunsrück). The total length of this ancient basin (the Rhenohercynian Basin) could have been more than 2500 km. In the east the basin merges with the East-Silesian basin of southern Poland. The sedimentary rocks that were laid down in the basin are often weakly metamorphic (greenschist facies). Most geologists consider the South-Portuguese zone to be a continuation of the zone to the west. Whether the Rhenohercynian Basin was a continuous feature or rather a string of temporaneously interconnected smaller basins is not well understood, because in many places the Devonian and Carboniferous rock strata are covered with younger deposits. Parts of the basin have their own names, like the Cornwall basin in Cornwall, the Munster basin in Ireland or the Rhenisch basin in Belgium and Germany. Tectonic structure and metamorphism The Rhenohercynian Zone is a part of the northern foreland of the Hercynian orogeny. It has a lower grade of metamorphism than the Saxothuringian Zone to the south, meaning its rocks have generally been at smaller depths and under lower temperatures. The Subvariscan Zone north of the Rhenohercynian Zone was untouched by Hercynian metamorphism. During the Hercynian orogeny, the Rhenohercynian zone was folded and thrust internally. It was thrust over the foreland to the north (the London-Brabant Massif and other Avalonian terranes). From the south it was overthrust by the Mid-German Crystalline High, part of the Saxothuringian Zone. The metamorphic grade or degree of metamorphism increases towards the south or southeast. The southern edge of the Rhenish Massif lies in the Northern Phyllite Zone, which has a higher grade than other parts of the zone. Geodynamic history The Devonian basin The Rhenohercynian basin was situated north of the Rheic Ocean, the ocean that spread between the continents of Laurussia (north) and Gondwana (south) from the Silurian onwards.