Hohentwiel is an extinct volcano in the Hegau region of Baden-Württemberg in southern Germany The mountain is west of the city of Singen and 20 miles from Lake Constance.
Hohentwiel began forming, along with the chain of volcanoes in the Hegau region, about 7–8 million years ago, when a layer of volcanic ash and stone was laid down. The magma consists of phonolite. In the following millions of years, the core was opened 260 metres beneath the surface by the glaciers from the ice age. This formed the core that is now exposed, after the ensuing millions of years of erosion.
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