The southern margin of Laurussia was formed during the Caledonian orogeny of the Silurian period, about 420 million years ago. In the Gedinnian/Lochkovian (Early Devonian) the southern part of the Caledonian mountain belt became a region of north–south extension. An elongated basin was formed parallel to the continental margin. It separated the London-Brabant Massif to the north from the Normannian and Mid-German Highs to the south. In the Middle Devonian (from 390 million years ago) a subduction zone existed south of Laurussia, where oceanic lithosphere of the Rheic Ocean subducted beneath the Mid-German/Normannian highs. Volcanism above the subduction zone created a cordillera-type mountain chain, the Ligerian cordillera. In the Siegenian/Pragian and Emsian the Rhenohercynic basin was a back-arc basin behind this cordillera. Tectonic subsidence in a system of horsts and grabens together with basaltic volcanism resulted in the creation of new oceanic lithosphere. In the Middle Devonian a second basin, the Saxothuringian/Armorican basin, developed south of the Rhenohercynian basin. To the west some crustal convergence took place, and the Normannian High was partly thrust over the sedimentary basin fill of the Rhenohercynian basin. Carboniferous compression The Rhenohercynian basin disappeared when the continent Gondwana collided with Laurussia in the course of the Carboniferous period (the Hercynian orogeny). The sedimentary rocks in the basin were thrust in a series of piggyback basins over the northern foreland (the London-Brabant Massif). These rocks now form the folded sequences of Cornwall, the Ardennes, the Eifel and the Harz. From the Frasnian age (380 million years ago) the mafic volcanism ended, and the basin came locally under compressional stress, which led to folding and thrusting in the sedimentary rocks. Somewhere near the end of the Devonian, a subduction zone developed under the Mid-German/Normannian highs and Rhenohercynian crust began to subduct. This was the short Bretonnic phase of the Hercynian orogeny. It was followed, from the Tournaisian (early Carboniferous, 355 million years ago) till the end of the Visean by a new period of extension. During the Sudetic (main) phase of the Hercynian orogeny (330-320 million years ago, Late-Visean and Namurian/Serpukhovian) compressional tectonics had the upper hand again. In the Namurian age full-scale continental collision between Laurussia and Gondwana resulted in the destruction of the last oceanic crust of the basin. Its sedimentary fill was, however, not (totally) subducted but instead thrust northward. During the later part of the Carboniferous period (Westphalian and Stephanian) the Rhenohercynian zone