The Thüringer Wald (Thuringian Forest) is about 75 km long and 20 km wide. It runs in a west-northwest/east-southeast direction (Hercynian) from Eisenach via Suhl to the Thüringer Schiefergebirge (Thuringian Slate Mountains). It reaches altitudes of between 600 m and 800 m asl, the highest elevation being the Große Inselberg at 916 m asl.
Like the Harz Mountains, this is a horst structure. The plateau, which was uplifted in the Cretaceous and Tertiary era, lies between two faults. To the north, the fault is clearly visible as an escarpment. It runs roughly along the F88 from Eisenach to Ilmenau. The southern border is roughly indicated by the Werra river.
The Triassic strata have already been eroded, as they were subject to greater weathering due to the uplift. Therefore, the rocks of the Lower Permian and older are mainly present here. These are the Variscan folded basement rocks.
These old rocks are insoluble metamorhic rocks, mostly slate, and thus not karstified. On the other hand, younger strata may have been deposited or even overturned on the marginal faults. This exposes them to weathering and karstification (see Marienglashöhle). In other words, caves ar found at the edges, at the foot of the mountain range.