The Cambrian is represented in Southern Ardenne by phyllads, quartzites and quartzophyllads. The sedimentary rocks of the Ordovician are mainly schists, phyllads and quartzophyllads. In the Ardenne, schists and sandstones are nutrient-poor, particularly sandstone and generate very poor and acid soils. These soils are especially poor in phosphorus. However, the decomposition of schists produces soils with good potash availability. The Ardennes are part of the Ardenno-rhenan schistose massif (with the Eifel in Germany and the Oesling in Luxemburg). This massif has been formed in two phases of orogeny during the Caledonian (–444 to –416 million years) and Hercynian periods (–400 to –245 million years), and was severely eroded afterwards. At the extreme southeast of the country, the sedimentary rocks of the Lower Jurassic are dominant in the Belgian Lorraine or Jurassic Region. Some Higher Triassic terrains are present in the northeast of this region. These Mesozoic formations belong to the geological entity of the ‘Gulf of Luxemburg’ that constitutes a junction between the Paris Basin and the Germanic Basin. The Trias period is represented by a clay-sand complex of fluvial origin, surmounted by red and green marls including gypsum nodules and whitish dolomite layers. In other places, soft sandstones alternate with black marl and clay. This lagoon formation is overtopped by a red clay layer: Levallois Clay. The Lias (Jurassic) deposits are characterized by alternating sand and marl-clay mixtures. Erosion has resulted in ridge formation that give a characteristic structure to the landscape of this region: the cuestas.