The German state of Thüringen (Turingia) has some mountaineous parts, but most of its surface is a huge plain called Thüringer Becken. It is bordered by the Harz to the north and the Thüringer Wald (Thüringen Forest) to the south. To the west it is bordered by the Hessisches Bergland. To the southeast lies the Thüringer Schiefergebirge, with a sharp border marked by a fracture zone. The total size of the area is about 75 km north-south and 100 km west-east. The biggest city in the area is Erfurt. Other famous cities are Weimar, Jena and Eisenach. The altitude lies between 150 m and 300 m asl, a few areas may reach 500 m asl.
The German term Becken is also used for the geologic structure of a syncline. And actually thats the geologic structure of this area: a deep crystalline basement forming a syncline filled with Mesozoic sedimentary rocks. The lowest layer is generally salt and anhydrite from the Late Permian (Guadalupian and Lopingian) which is called Zechstein in German. Above are the layers odf the Triassic, called Buntsandstein, Muschelkalk and Keuper in German. The structure is complicated by numerous fracture zones, most running in hercynian direction (northwest to southeast).
The layers are almost horizontal, but at the northern and southern border they go up like the rim of a bowl. Layers with different hardness were eroded and formed hogbacks. There are ridges like the Hainleite and the Dün which are formed by the layers of the Unterer Muschelkalk (Anisian) and the Windleite which is formed by layers of the Buntsandstein (Olenekian).
The ridges in the middle of the plain are formed ba salt tectonics. This process is described on the page of the Norddeutsche Tiefebene where it is even more common. The most important salt tectonic ridge in Thüringen are the Hainich, Finne and the Großer Ettersberg.
All those ridges, where limeston is lifted above the surrounding plain, are subject to karst processes and thr formation of caves. But those higher areas are pretty small, the surroundings are lowlands with very high ground water level which are not karstified.
However, the relief is still flat, the structures are still rather young, the karstification is in a very early stage and the caves are very little. They are generally inaccessible or extremely small. There is one famous exception, the Karsthöhle Dienstedt. But much more important and interesting is the great number of karst spring surrounding thos small karst areas, They formed huge layers of tufa aka travertine, which is actually a proff of existing but unknown caves: the limestone which is deposited around the spring originates from underground, and there is now a void, a cave.
At the multiple karst springs in the Thüringen plains, between Harz and Thüringer Wald, large deposits of travetine formed.
The water of karst springs is very rich in dissolved limestone. At the moment the water reaches the surface, the carbon dioxide(CO2) in the water starts to vanish into the atmosphere. Still more carbon dioxide is consumed by plants, moss and lichen, in the brook. Using photo synthesis, they split the CO2 into carbon, which they use to grow and oxyde, which is released into the atmosphere. But without carbon dioxide the water looses the ability to hold the limestone and it gets deposited. The limestone often forms huge terraces around the spring and along the brook.
During the deposition of tufa, the remains of plants and animals which fall into the brook, are covered with limestone and thus preserved. Sometimes the formation of the tufa also includes the formation of primary caves (see Tuffhöhlen), but from Thüringen travertines no such cave is known.
The common speech uses the word tufa for this limestone. In Thüringen it is called Travertin (travertine), which is also used by hewers, if they sell this rock for wall or floor cover. In geology or mineralogy the names freshwater limestone and sinter are also common.