Geology
Hohentwiel is located in a geological unit that encompasses Hegau and the western portion of Lake Constance. Its multiple geological layers were formed through sedimentation and volcanism, as evidenced by the Rhine Rift Valley, where the Alps were formed. A remnant of this activity is the Ur-Hohentwiel vent, which is filled with depositing tuff, as well as phonolite that rises out of the eastern terrain.
Sedmentation since the Jurassic Period
During the Jurassic period (201-145 million years ago), the continental shelf of Central Europe was submerged under the primeval Tethys Ocean, resulting in sedmentation convering the area and formation of the South German Jurassic. With the beginning of the Alpine Uplift Event, which transpired approximately 65 million years ago, land rose from the sea due to the collision between the African and European continents. To compensate for the elevation of land, the area inbetween the newly formed Alps and Swabian Alb subsided, and with it, the Hegau region. This erosive processes of the subsidance caused much of the surrounding dry surface area to wear away. The material was deposited in the still-flooded depression, forming a molassae basin on the sea floor (marine molasse). With a thickness of roughly 5000 metres, the resulting molasse blanket filled the space between the two continents.
Volcanism in the Miocene Period
During the Miocene period (c. 23-5.3 million years ago), after the flood waters receded, the deep fractures and stresses caused from the uplift of the Alps and Swabian Alb resulted in intraplate volcanism. This type of volcanism is not caused by the movement of a tectonic plate, but rather is a result of uplift with a single continental plate. This volcanism is believed to have been caused by the convergence of the Bonndorfer Graben fault, running northwest-southeast, and a north-south fault extending from Höwenegg to Riedheim.
Active volcanoes were present for a duration of 6-7 million years. The time span can be divided into two sections. In the first three million years, eruptions of volcanoes such as Ur-Hohentwiel reached the earth's surface, creating a tuff cone mountain which towered over its surroundings by 100-200 metres. In the course of time and with a decrease in eruptions, the thickness of the covering tuffs reduced due to erosion. During the Tortonian period of the late Miocene, the second phase of volcanic activity took place. Rising magma no longer reached the surface land area, but solidified in ascending passages through the molasse. The composition of the magma likely had a higher silica content, causing the mass to have increased viscosity and durability. This resulted in the formation of a vent plug under the tuff layer, approximately 100 metres thick.[2][3]
The Quaternary Glaciation
The Quaternary period was the last strongly formative period in Earth's glacial history, characterised by four major glacial periods: Günz (400,000 - 300,000 years ago), Mindel (300,000 - 200,000 years ago), Riß 200,000 - 130,000 years ago) and Würm (20,000 years ago). During these periods, Alpine glaciers continued to erode the landscape. The Würm glaciation saw the Rhine glacier advance northward. The Lake Constance foreland glacier, part of the Rhine glacier, eroded the molasse and tuff layers around the Hohentwiel plug, causing the mountain summit to become exposed as the ice receded. Tuff is only present in the western portion, due to the neutralising effect of the ice masses in their direction of advance.
Stratification
Stratification of the Hohentwiel landscape is characterised by layers of sedimentary rocks - including Brown Jurassic, White Jurassic, Kimmeridgian, Tithonian, Lower Freshwater Molasse, Brackish Molasse, Upper Seawater Molasse, Jurassic Gelfluh of the Upper Freshwater Molasse, phonolite debris, cover tuff, mica sands, and Jurassic nape. These sedimentary layers range in thickness from 100 to 150 metres, with an additional 70 metres of basic facies at the lower level. This is topped by an upper layer of moraine material in the far western portion of the mountain.[2]