Halleberg consists of Cambrian, Ordovician and Permian rocks. The Palaeozoic rocks are deposited directly on the crystalline peneplain bedrock, which in this range consists of göta granite. The Cambrian sedimentary rocks are sandstone and alum shale. The sandstone layer is on average thick. Above this there is then a thick slate which in turn covered by about of alum shale. The Ordovician is represented by a calcareous slate. During the Permian it was pushed into diabase which lays as a protective cover and thus protect the mountain from erosion. The diabase on Västgötabergen known locally for staircases.