The top area of a recultivated landfill in the northern area of Bielefeld was used to lay out a garden with 100 large glacial erratic blocks as well as 25 local limestone blocks. The erratic blocks were all originally found during excavation work in preparation of the landfill. They have been transported to the area of Bielefeld by the glaciers of the Saale Ice Age (Drenthe stadial). For this paper, the glacial erratics were petrographically described and the current overgrowth of pioneer vegetation (lichens and moss on the blocks and vascular plants on the area around the blocks) was mapped. As a comparison, 6 large erratics from nearby small arrangements that have been exposed at the surface much longer were also mapped. The moss species found on the erratic blocks belong to the Tortuletum muralis Waldheim 1944, a plant society previously described for basic substrates. We propose the thesis that the Tortuletum muralis is also a typical pioneer society for erratic blocks of any composition in Northern Germany, which are freshly excavated from the ground. It is proposed to extensively investigate the moss flora on these stones in the future. The mapping of lichens showed a stock of pioneer species as well as species typically found at eutrophicated sites. This points towards extensive use of the rock garden by visitors and walkers. The surrounding grassland is unfertilized, so the input of nutrients most likely comes from wind and rain, together with external contribution by humans and animals.