The Harz Mountains were uplifted by tectonic forces, so that today rocks of the bedrock, which originate from a depth of 4.5 km, protrude far beyond the surrounding area. However, the uplift was not uniform, it was greatest in the north, becoming continuously smaller towards the south. On the southern edge of the Harz we find an area about 5 km wide that runs parallel to the southern edge of the Harz for almost 100 km. Here the Zechstein gypsum occurs with great thicknesses.
During the Zechstein (Late Permian, Lopingian), more than 225 million years ago, there was a large geosyncline (depression) in the area of northern Germany. This area had desert climate, but had a connection to the sea, through which seawater entered from time to time and filled the whole depression. When the inflow stopped, a large amount of water evaporated and in the process limestone, anhydrite, rock salt and potash salt were deposited. The deposition of these four salts in exactly this order is due to their different solubility. Such a sequence is also called a series, each time seawater entered the depression again, a new series began. During the Zechstein, four series are distinguished, named after local rivers:
Especially due to the great gypsum thicknesses of the Werra Series, the most important gypsum karst area in Germany was able to form here. This thickness of around 100 m can often even be seen on the surface, as it results in a small stratification step. This can be seen particularly clearly at the Düna nature reserve, for example. The Zechstein strata strike hercynian (west-northwest to east-southeast) and fall variscan (northeast to southwest), the step therefore runs in a Hercynian direction.
The anhydrite is finely layered, with the individual anhydrite layers, about 2 cm to 3 cm thick, separated by a dark, very thin clay layer. Obviously, the anhydrite layers correspond to a period of evaporation and the clay layers to a period of quiescence, during which the suspended matter (clay minerals) could be deposited. In addition, the anhydrite has weak folding, especially near the resin, caused by the rise of the resin.
For almost its entire length, the southern edge of the Harz is accompanied by a gypsum karst area. Unfortunately, only a few remnants of it remain. Massive mining of the anhydrite as a building material has transformed large areas into huge quarries over decades. And even today, in times when flue gas desulphurisation plants produce gypsum in large quantities and in excellent quality as a waste product, the last gypsum karst areas are disappearing at an increasing rate. At the same time, the gypsum from the desulphurisation plants has to be disposed of in landfills. The reason is as simple as it is incomprehensible: mining costs a few cents less per tonne.
About 5 km south of the Harz Mountains is the Kyffhäuser. This mountain range is about 12 km long and 6 km wide, a pultscholle that was formed at the same time as the Harz and by the same forces. Therefore its geological structure is related. Here, too, we find a large gypsum karst area on the southern edge. The elevation itself, on the other hand, consists mainly of the sandstones of the Rotliegend and is therefore not capable of karstification. There are numerous caves, sinkholes, water vents, underground watercourses and dry valleys here. Currently, about 40 caves and over 80 sinkholes are known.
The most important cave is the show cave Barbarossa Cave. North of Rottleben is the Prinzenhöhle, which is freely accessible and easily navigable. The second largest cave in the Kyffhäuser after the Barbarossa Cave is the Schusterhöhle near Tilleda. The largest sinkhole is the Äbtissinnengrube near Bad Frankenhausen, it was probably first formed in the 14th or 15th century, has a diameter of 100 m and is 20 m deep. Near the Barbarossa Cave is another large sinkhole called Pfannenspring.