Experts see this as being the product of one or more lava eruptions that flowed on top or next to each other and which occurred about 290 million years after the eruption and deposit of the thick Rochlitz porphyry. However, not all the properties of the Gattersburg porphyry clearly indicate a lava flow. For example, the relatively high proportion of large crystals in a quite uniformly developed groundmass of rocks and visible hollow spaces (either filled or open) in the rocks indicate that igneous rock (magma) intruded into rock layers close to the surface in several phases.
It took the Mulde thousands of years to break through the rock barrier at the Gattersburg. This means that today’s town of Grimma (Slavic grim = deep lying ground surrounded by water) is squeezed in between cliffs, and that it was repeatedly flooded by the Mulde in its more than 800 year history, most recently in the years 2002 and 2013. As a precautionary measure an approx. two kilometres long flood wall with closable floodgates was built.