The Basin is surrounded by a wide outer girdle of limestone (Muschelkalk) ridges (including Hainich, Dün, Hainleite, Hohe Schrecke, Schmücke, Finne), and to the southwest by the Thuringian Forest and to the southeast by sharply divided terraces (the Ilm-Saale and Ohrdruf Muschelkalk plateaus, and the Saale-Elster Bunter sandstone plateau). The Thuringian Basin belongs to the triassic period, during which horizontal beds of Bunter sandstone, Muschelkalk and Keuper were laid down. Below those lie the salt and gypsum layers of Magnesian Limestone (Zechstein). In the Cenozoic era the surrounding ridges were uplifted, whilst the Thuringian Basin sank to form a saucer-shaped depression.