The formation of the Danube Valley is closely linked to the geology that today contributes to the very special climate in the Danube Valley.
Formation of the Danube Valley
The history of the valley began with the uplifting of the Bohemian Massif 17-18 million years ago during the Tertiary period. However, the original structures of the valley had already been formed during this time. This is because the structures and directions that are still visible in the mountain range were created in the early history of the Earth, during the Precambrian period 1,000 million years ago.
The tectonic faults that run through this area have always played an important role in the course of the Danube. From Regensburg onwards, the Danube follows the direction of the main fault lines from west-northwest to east-southeast. Until it enters the crystalline rock at the Hilgartsberg ruins, the Danube follows the so-called Danube edge fault. This is a fault line where the bedrock has been raised by over 1000 meters compared to the southern Tertiary foreland. While the Danube Marginal Fault deviates to the south at Hofkirchen and disappears at Griesbach and Pocking, the Danube continues its course to the southeast. A precursor of the original Danube, coming from Upper Austria, probably cut further into the valley to the west through regressive erosion, thus determining the current course of the Danube.
According to another theory, the course of the ancient Danube from Osterhofen to Passau could also have followed the Hengersberg Tertiary Bay and then the Aicha-Halser side channel, the “little brother” of the Pfahlstörung. The continuation to the east could then have led from Passau to the south and via the Schärding area south of the crystalline rock towards Austria. It was probably not until the end of the Tertiary period that the Danube found its present bed. Above Passau, it was pushed to the southern edge of the crystalline rock by erosion. And below Passau, the Danube now follows the Donauleiten fault in direct continuation of the Danube edge fault and the Aicha-Halser secondary fault.
During the Pleistocene (approx. 1.5 million years ago), i.e. during the Ice Age, the continuous uplift of the entire Bohemian Massif led to the significant clearing and formation of today’s Danube Valley. This resulted in the creation of the epigenetic breakthrough valley that exists today, which means that the deepening of the valley occurred at the same time as the uplift of the mountains. The incision of the Danube was facilitated by the tectonic lines already described.
The narrowness of the Danube valley is repeatedly interrupted by flat areas where the river has deposited gravel and sand. These areas are important settlement areas, but their storage capacity also makes them important groundwater reservoirs. The city of Passau, as well as the municipalities of Obernzell and Jochenstein, obtain their drinking water from the gravel deposits in the Danube floodplains.
Geology in the Bavarian Forest and Danube Valley
The history of the Danube Valley begins with the uplifting of the Bohemian Massif 17-18 million years ago in the Tertiary period. As early as the Precambrian period 1 billion years ago, the structures and directions that are still visible in the mountain range today were formed.
On both sides, the valley is formed by ancient crystalline rocks, which can be divided into gneisses and granites. The gneisses, which are rocks deformed by pressure and heat, can be divided into two groups: a so-called “monotone group,” which originates from a metamorphic transformation of sandstone, clay, or marl, and a “colorful group,” which is interspersed with volcanic deposits (e.g., the graphite shale in the Obernzell area).
During the Variscan orogeny in the Carboniferous period, which led to the intrusion of granites, the Bavarian Forest developed into a high mountain range. This period also saw the formation of the Pfahl (e.g., the Aicha-Halser Nebenpfahl from Iggensbach to Passau), a quartzitic fissure filling.
In the subsequent geological epochs of the Permian and Triassic periods, the high mountains were eroded away by profound erosion, leaving only a mountain ridge. During the Jurassic period, only the “Bohemian Island” remained, surrounded by shallow seas and coral reefs.
The formation of the Alps became apparent with the beginning of the collision between the African and European plates, which led to the uplift of the Bohemian Massif. Multiple marine inundations in the basins in front of the emerging Alps still covered the bedrock. Remnants of these marine deposits, now known as molasse, are still present on the bedrock. The tectonic faults that run through the area play an important role in the course of the Danube.
The Danube Fault, which deviates southward at Hofkirchen, is a fault line where the bedrock was raised to over 1,000 m above the southern Tertiary foreland. A precursor of the Urdonau probably cut further into the valley to the west through regressive erosion, thus determining the current course of the Danube.
However, there are other theories on this subject. It was probably not until the end of the Tertiary period that the Danube found its present bed. Below Passau, the Danube now follows the Donauleiten fault in direct continuation of the Danube margin fault and the Aicha-Halser side fault. During the Ice Age, the uplift of the mountains coincided with the deepening of the valley, which contributed to the formation of today’s Danube Valley.
The floodplains, where the river has deposited gravel and sand, are important settlement areas as well as important groundwater reservoirs.
Eozoon bavaricum
In the mid-19th century, geologists and paleontologists around the world were searching for the oldest living creatures on Earth. In 1865, they believed they had found what they were looking for in the “primitive limestone” of Canada. The fossil found there was named “Eozoon canadense” (“Canadian dawn animal”).
In the same year, Bavaria’s most important geologist, Carl Wilhelm Von Gümbel (1823–1898), also found this “fossil” in marble near Obernzell/Passau. In 1866, he discovered a smaller and much more intricate form here in Hohenberg a.d. Eger in the quarries that no longer exist today, just below the location of the plaque. He named it “Eozoon bavaricum,” meaning “Bavarian animal of the dawn.”
However, a discussion soon arose as to whether Eozoon was actually an organic entity or a mineral formation.
Today, it is certain that the Eozoon is a purely inorganic formation. It consists of the greenish magnesium silicate serpentine, which has a fibrous and mesh structure. This mimics the structure of an organism. It is assumed that the serpentine was formed by the thermal effect of the intrusive granite, i.e., by contact metamorphism from the mineral forsterite (Mg2[SiO]4). Forsterite is often found as a relic inside the “chambers” of the supposed primordial animal.
Mineralogically/petrographically, the marbles colored greenish by the serpentine are called ophicalcites. They occur worldwide.
Climate in the Danube Valley
Between Passau and Linz, the Danube Valley cuts through the Bohemian Massif, which, as the foothills of the Bavarian Forest and the Sauwald, creates a harsh low mountain climate.
The area has a distinct climate gradient: precipitation increases significantly from west to east, and the local climate becomes noticeably more humid (average annual precipitation: Passau: 889 mm, Obernzell: 922 mm, state border: 1000 mm; figures from 1952).
The climate in the Danube Valley is mild and characterized by an early spring, warm summers, and a long frost-free period. Reasons for this include:
- The high heat storage capacity of water results in mild winters.
- The south-facing slopes make optimal use of solar radiation and convert it into heat.
- Reduction of nighttime radiation due to the continuous forest edge.
- Reflection of solar radiation by the water surface.
Stream valleys form a connection between the Danube Valley and the low mountain ranges and act as cold air corridors. Where these so-called Dobeln flow into the Danube Valley, cold air areas with corresponding flora and fauna are created.
This is illustrated by a comparison of weather data from Rollhäusl/Untergriesbach (560 m above sea level) (www.meteo24.de) and Engelhartszell (295 m above sea level) (www.wetter.at): on December 5, 2008, the wind speed in Rollhäusl was five knots with gusts of 10 knots at a temperature of 0.1°C, while in Engelhartszell there was no measurable wind at a temperature of 4°C.
The following climate zones apply in the region:
Southern German-Austrian zone (mild transition zone, 380–600 m): average annual temperature: 6–7°C, annual precipitation: 800 to 900 mm, number of frost days: 90–100, duration of snow cover/days: 100
Southern German-Austrian region (harsh stage, 600–900 m): average annual temperature: 5–6°C, annual precipitation: 900–1000 mm, number of frost days: 100–120, duration of snow cover/days: 130