Most of Germany is covered by mountains and plateaus. But to the north, about the latitude of Braunschweig and Hannover, the north German coastal plains start. The area is covered by Quartary sediments like sand and gravel, deposited during the ice ages. At this time, more than 10,000 years ago, the plains were covered by glaciers which came from Scandinavia. The sand and gravel is not karstified, and the groundwater table is in the low plain very close to the surface, so there is not much chance for Karst.
The whole area is a Mesozoic basin, an area of continual downlift during the Mesozoic, which was compensated by sedimentation and resulted in 4 to 5 km of sedimentary rocks. The sedimentary rocks include terrestrial and marine sandstones, limestones and marls. Deep below the surface, the layers of the Zechstein (Upper Permian) consist of salt and gypsum. These layers are rather mighty. The specific weight of the salt is lower than that of the layers above. And at a pressure of 100 kg/cm² salt starts to flow. During the Jurassic and Cretaceous the salt started to flow, thickening at some points, thinning at others and forming pillows. Today most of those pillows formed salt diapirs, formed like mushrooms and sometimes extending up close to the surface. The forces of the salt pressing upwards forced the Jurassic and Cretaceous limestone and sandstone layers above to bend upwards. They form rather typical circular or elongated hills.
This development is mostly finished by now in the southern part of the plains. The salt reached an elevation where the forces of the process fade out. But to the north, at the North Sea coast, the process started later is not completed now. The salt diapir below Bad Segeberg was uplifted after the last Ice Age, which means in the last 8,000 years. Earlier uplifts would have been cut of by the glaciers anyway.
The uplift of hills by the salt is called halokinetic, where halo is derived from the greek word for salt, and kinetic means mechanical force. Another geographic feature is also formed by the salt, but not by halokinetic forces. Numerous lakes, like the Arendsee near Salzwedel are located on the top of a diapir. The groundwater dissolved the top of the salt and transported it away. The resulting cavern collapsed and thus formed a sort of doline which filled with groundwater soon. Gypsum is the diapir is not as soluble as salt, so it accumulates and saves the diapir against further solution.
All in all the northern plains of German, although formed of carbonate rock, have very little caves. This is because of the geography, the flat surface and the high groundwater. There are several other karst features. Caves like the Kalkberghöhle are very rare and fascinating.