Thuringian forest is surrounded on three sides by triassic rocks: the Thuringian Basin in the northeast, the Hesse Highlands in the west, and the northeastern parts of the South German Scarplands in the south, and by the Variscan rocks of the Thuringian Highland towards the east. The geological borders differ from the geographical ones insofar, as the Rotliegend rock of the Thuringian forest finds its continuation in the Masserberg and Crock block in Hildburghausen district, southeast of the main range, and the rock types of the Thuringian Highland are also found in the Schleuse horst between Schönbrunn reservoir and Schönau, and in the Vesser complex near Schmiedefeld, an island of Variscan rocks embedded in Rotliegend. While the near-surface rocks of the Thuringian Highland comprise mostly the folded Variscan basement, the oldest unfolded overlying strata (dating from the Permosilesian age) of this basement are exposed in the Thuringian forest. Common to both ranges, but also to other low mountain ranges in Central Europe uplifted at the same time, are the bordering Zechstein deposits which contain Bryozoa reefs. These stretch especially wide on the northwestern edge of the Thuringian forest, where the landscape park of Altenstein Palace is located on one of the largest Zechstein reefs in Germany.