From a geological point of view the Brocken and its surrounding terrain, the Brocken massif, consists mainly of granite (called Brocken granite), an igneous rock. The granitic plutons of the Harz – the Brocken, Ramberg and Oker plutons – emerged towards the end of the Harz mountain-building phase of the Upper Carboniferous, about 300 million years ago. First, alkaline magma intruded into the overlying sediments, crystallized out and formed gabbro and diorite massifs, such as the Harzburg gabbro. A little later, silica-rich granitic magma rose, some intruding into voids and gaps in the older rocks, but most being created by the melting of existing sediments. On the boundary between granite and host rock, the so-called contact zone, a great variety of transitions may be seen. For example, the summit of the Achtermannshöhe consists of contact-metamorphosed hornfels of the contact zone that, here, lies over the Brocken granite. The subsequent erosion of the Harz mountains that followed the uplifting of the Harz during the Upper Cretaceous saw the disappearance of the protective hornfels summit, thus exposing the granite that had crystallized underground during the Upper Carboniferous. The alleged hardness of Brocken granite is not the reason for the height of the mountain, but the geological fact that it was well protected by its weather-resistant hornfels crest for a long time before erosion set in.