The Hessisch-Niedersächsische Bergland (Hessian-Lower Saxony Highlands) are actually two regions, the Hessische Bergland (Hessian Highlands) and the Niedersächsische Bergland (Lower Saxony Highlands). However, both have very similar geography and geology, the separation into two regions is obviously only due to the political border. Since the number of listed sights is very small and very heterogeneous, we have refrained from separating them. We even went further and added a piece of Thuringia, which also belongs to it geologically. This is the Werra-Fulda-Kalirevier (Werra-Fulda potash district), to which the Merkers potash mine also belongs. The connection becomes quite clear when you realise that the mines on both sides of the German-German border were connected underground.
The Hessische Senke (Hessian Depression) is the northernmost part of the Rhone-Rhine Rift, a rift system that represents a divergent plate margin. Here one could assume that the European plate will break apart in a few million years and a sea will form in between, conceptually like the Atlantic Millions of years ago. However, the argument against this is that the graben or rift is developing very slowly, and it looks like it will slow down and maybe just stop at some point.
In general, the area has always been lowland during the Mesozoic, covered for long periods by an estuary connecting the North Sea and the Thetys. The rocks are marine sediments, but also continental sediments. There are salts from the Zechstein, as in the North German Plain and the Thuringian Basin. Lignite was formed in the Eocene. All these rocks have in common that they were formed in basins and never folded. The fact that they are now low mountain ranges is due to erosion.