The most famous mountain in the Hegau region is Hohentwiel (2250 ft/686 m above sea level), which rises 820 ft/250m above the Aach river valley. It stands like a mighty block in the basin, which is also populated by numerous other mountains. This characteristic originally gave rise to the field name "Hegau" from "Höhen" (heights).
The mountains and hills of the Hegau region are volcanic chimneys. Hohentwiel is the remains of a so-called cinder cone, which forms when magma is forced up from within the earth, does not erupt and then cools. Hohentwiel formed from phonolite, a very hard rock resulting from the high temperature and enormous pressure of the magma. During the ice ages the hard cinder cones withstood the glacier flows toward the southwest, while softer rock and earth were washed away.
Hohentwiel is an important natural monument, its terraced structure (field, meadow, shrubby heath, forest, rock) is a habitat for diverse, rare flora and fauna. Botanists prize the plants which, with their broad range of origins - from the Alps (rosa alpina), the Mediterranean (moschatel), from the area around the Black Sea (alyssum montanum), from the Atlantic-Continental area (yellow winged broom) - offer a unique diversity.
For zoologists as well, Hohentwiel offers a broad range of interesting insects and animals: dragonflies, butterflies, beetles, snails, slugs and lizards live on the mountain.
Since 1941 it has been the center of a nature preserve, a fact which has increasing lead to conflicts with the touristic use of the attractive cultural monument in the course of the 20th century, and has meanwhile been routed in ecologically more acceptable directions.