Hohenhewen

Transcription

Hegau Volcanoes Seven million years ago the last magmatic melts did rise here. Since that time the Hegau volcanoes are extinct. From Hohentwiel Mountain, the most famous relic of „God’s bowling”, the view goes out over the city of Singen and northern Lake Constance. Where once glowing lava did flow, today the vineyards flourish. The Hegau region extends from Lake Constance and the Aach-Source to the Donauversickerung (Danube percolation near the southwestern Swabian Alb). Here the Danube is tapped and diverted to the largest karstic spring in Germany, the Aachtopf (Aach-Source). This is an impressive proof of the constant changing watersheds and stream networks. This change is especially true for the Middle Miocene, a period which brought increased tectonic activity to southwestern Germany. During this geological period, about 20 million years ago, tectonic movements triggered intense volcanic activity. Since the eruptions started several million years before the impact of the Ries meteorite, a connection can be ruled out. In quick succession, at least in geological terms, because the intervals were about 1 million years, the volcanic fields of the Swabian volcano (near Urach and Kirchheim), the Kaiserstuhl mountain range and Hegau volcanoes formed. Magmatic eruptions and lava flows characterized Kaiserstuhl and Hegau volcanism. The volcanism in the Hegau region persisted for about 10 million years, going through several phases of activity. Even today the volcanic relics shape the landscape. But this landscape that exists today has been transformed several times over the course of the last million years. Not only volcanism, but also erosion and deposition, especially during the ice ages, were significantly involved in its formation. During the Middle Miocene an intense volcanism developed in the Hegau region. Tectonic lowering of the Upper Rhine rift valley and the uplift of the Alpine mountains are seen as possible causes. About a dozen volcanoes erupted and deposited thick tuff layers. Rain and wind eroded the volcanic cones and the whole area was covered later by the fast debris fans of the Alpine Molasse. Alpine Molasse refers to those sediments, which were deposited during the Alpine uplift in the northern foreland basin. Therefore, before the Ice Age, the Hegau appeared as a high plateau interspersed with low hills. But the major glacier advances acted as a huge plane. They eroded the Molasse sediments and the tuff layers and carved out the volcanic cones of the former volcanoes. The Hegau volcano from which the volcanic vent has been carved out especially well by glacial erosion is Hohenkrähen. This mountain is situated 5 kilometers north of Singen near the motorway A8. At its summit plateau are the ruins of Hohenkrähen Castle. The castle dates back to about 1200 A.C. It has been a robber knights den and was burned down several times. The mountain is 644 meters high and composed of phonolite („sound stone”), a volcanic rock that is characteristic of the eastern volcanic vents in the Hegau region. To the northern side of Hohenkrähen one can see nearby Mägdeberg and a bit farther away Hohenhewen. The first one is composed of basalt, the other one of phonolite. Various relics in the Hegau region suggest that in the extinct volcanoes Maar lakes developed. In these lakes a unique flora and fauna flourished. It is particularly well documented due to its deposition conditions. A very rough idea how such a crater lake might have looked conveys Höwenegg. It is far in the North of the Hegau region, between the village Mauenheim and Immendingen on the Danube. Höwenegg is one of the basaltic volcanoes and was formerly exploited for basalt as a road building material. The digging, which lasted until 1979, produced a 65 meter deep crater. Following closure of the quarry a lake formed fed by rain- and groundwater. During the exploitation of the basalt some important fossil finds have been made in the area, for example the skeleton an extinct equid, now kept in the natural history museum of Karlsruhe. The volcanism in the Hegau region continued - with interruptions - for almost 10 million years. That means it lasted twice as long as the volcanism of the Swabian Volcano or the Kaiserstuhl Mountain. It is characterized by a greater complexity and shows several clearly distinguishable phases of development. This can be illustrated by the volcanic composition of the most famous Hegau volcano, Hohentwiel. Only during the first 3 million years the volcanic flows, that started about 15 million years, reached the surface. The volcanic ashes formed a tuff cone, primordial Hohentwiel. About 8 million years ago, there was a second phase of volcanic activity. However, the magma did not reach the surface. A phonolit intrusion developed, which became stuck in the tuff layers. These tuff layers on the one hand were removed by weathering but one the other hand were buried under the debris of the molasse. The mighty glaciers of the Ice Ages, in particular the Rhine Glacier, transformed the landscape and eroded the unconsolidated sediments. The phonolit intrusion of Hohentwiel was excavated during this transformation. Today the ruins of the fortress Hohentwiel stand on its top. The foothills are formed by former tuff layers. Today they bear the highest-located vineyards in Germany, 562 meters above sea level. These belong to the Staatsweingut Meersburg (state vinery Meersburg) and the vinery Vollmayer. Westwards from Hohentwiel one notices the twin peaks of Hohenstoffeln. Originally this mountain had three peaks, but the northernmost was removed by the basalt quarrying. The quarry has been abandoned in 1939 and the whole mountain was declared a nature reserve. Today one can see the typical basalt columns at the place of the former quarry. Two of the original three castles on top of Hohenstoffeln have been abolished. Isolated remnants of the third castle can be found in the forest. 5 kilometers to the north of Hohentwiel the small town of Engen is situated. The town’s landmark mountain (Hausberg) is called Hohenwehen. The historic old town has been completely renovated since the 70s and shines in high medieval splendor. Engen has an interesting city museum. Many Stone Age discoveries from nearby Brudertal are on display. The everyday objects and the objects of art were excavated by the amateur archaeologist Peter between WW1 and WW2. These objects prove that about 12 000 years ago people settled here. By that time the ice of the great glaciers had decreased far enough to allow tundra like conditions to prevail. From Engen to the foot of Hohenhewen Mountain the distance is about 2 km. Hohenhewen belongs to the western basaltic volcanoes. At the foot of the mountain I pass a large gravel pit. Here that material is removed, which has been deposited by the glaciers and the meltwater streams during the last ice age. Hohenhewen is the second highest of the Hegau mountains (846 meters). But it has never been covered completely with ice, even during the peak of the last ice age it towered above the Rhine Glacier as a nunatak. On top of Hohenhewen is a ruined castle by the same name. Today an observation tower is integrated into the ruins. From the tower one has a beautiful view over the old volcanic cones lining up in the South in front of Lake Constance: Hohenkrähen, Hohentwiel and Hohenstoffeln. When they spat flames in their youth even the formation of Lake Constance was still far in the future.