Boulders and blockfields

The Achtermannshöhe / Achtermann's Hight is the third highest mountain in Lower Saxony, at an elevation of 926 metres above sea level, and the fourth highest point in the Harz. Its treeless rocky hilltop rises steeply above the forest, about 15 to 20 metres high, offering one of the best views in the national park. From here you can see the Brocken, Bruchberg and Wurmberg. The geology of the Achtermannshöhe / Achtermann's Hight is of special interest. The underlying rock is hard Hercynian granite, whereas its treeless hilltop is capped by a layer of hornfels about 10 metres thick. This very hard rock formation once originated from greywacke coming into contact with very hot granitic magmas. Today's blockfields were formed at the end of the last ice age, dating back for at least 10,000 years. Strong forces were involved in their formation, with weathering due to frost wedging playing a pivotal role. Blockfields show peculiar microclimate conditions. Whereas a dry and warm climate prevails at its surface, with temperatures varying up to 60 degrees, it is cooler at the lower ends of the boulders and the climate altogether is more balanced. Conditions in the inner spaces of the blockfield resemble those of natural cavities. Throughout the year, it is comparatively cool and moist and, of course, dark. Only specialists are able to survive in this bleak sea of rocks. This is the realm of rare moss and lichens – a territory which recalls the Alps or the high north. Dwarf shrubs cling to rock crevices wherever some fine soil has accumulated. As regards animals, the blockfields appear virtually void at first sight. But this is not true. In fact, there are numerous inhabitants to be discovered in the blockfields of the Harz National Park. Common lizards can be seen basking on sunny rocks and quickly taking to the gaps between stones and boulders, as do the Fire salamanders. These species are assumed to hide in the deeper and warmer gaps of the blockfields in winter. This seems also true for a number of butterfly species, including the Comma and the European peacock, which may be found basking in the sun especially in autumn. Bats, such as the Northern bat, use the large rock accumulations as a day roost.