Wednesday, August 5, 2020

How the Main found his bed

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The Main shapes the Franconian landscape. A long time ago it flowed over the Regnitz valley to the Ur-Danube. Main triangle and Mainviereck formed much later.

The Main is the heart of the Franconian landscape. Sometimes he carries more water, sometimes less. He keeps flowing as if he had always existed. But the Main, as we know it, only found its present bed between the last two cold periods. And the Main Triangle and Mainviereck have only been in existence for a short geological period.

Palontologist Gerd Geyer researches the history of the Main. Photo: Geyer

How the Main and its forerunners burrowed into the landscape and changed their course of the river can be seen, for example, at the Achtelsberg near Hafenlohr. This is a circulating mountain, so called because it used to be surrounded by the river. This can be clearly seen from above, says Dr. Gerd Geyer, professor of geology and palontology at the University of Wrzburg, and points to an aerial picture he made of himself, showing how the Main must have had its bed in a wide loop there.

Rainberg near Kreuzwertheim and Romberg near Sendelbach are also such circulating mountains. Due to constant erosion, the Main shortened its path at some point and buried itself in today’s bed. What remained are the river loops of a river that used to be heavily mandaring, which today are often only drained by a stream or are even completely dry.

In geological terms, such processes are very young: they are among the rather minor modifications to the course of the river that took place in the post-ice age (Holozn). According to Geyer, during his trip to the Netherlands in 1520, Albrecht Drer experienced parts of the Main in a completely different form than today.

Significant changes between the last two cold periods

More significant changes must have happened between the last two periods of the Pleistocene Ice Age. This means the two ice ages Ri (300,000 to 130,000 years ago) and Wrm (115,000 to 12,000 years before today). In between there was the so-called Ri / Wrm interglacial, also called Eem warm time. At temperatures comparable to today’s temperatures, the ice sheets over Scandinavia and the Alps were largely defrosted at that time, the rivers were full of water and therefore had the strength to look for new ways.

But in order to understand the history of the Main, you have to travel much further back in time. A terrifying amount is unclear and still needs to be researched, says Geyer. And even the known data would only give an unclear picture of the origin of the Main. What is certain is that a few million years ago (in the so-called Pliozn) the land surface in what is now northern Bavaria was much higher and had a relatively low relief. Only then was today’s layered country prepared. Today’s river network was only formed in the course of this formation phase.

Ur-Main flowed in the opposite direction

The Main Valley section between Wrzburg and Gemnden also formed at that time. The part of the Ur-Main flowing there, however, flowed in the opposite direction from north to south. This so-called Wernfeld river had its bed at today’s Edelweies near Karlstadt, around 100 meters above today’s Main level, Geyer estimates. He had no connection with the other sections of today’s Main.

The original Ur-Main, which springs from the Weier Main in the Fichtelgebirge and the Roter Main south of Bayreuth, then flowed far east over today’s Regnitz Valley to the south to the Ur-Danube. Geyer assumes that back then, at the beginning of the quarters roughly two million years ago, the region of today’s Lower Franconia was drained to the south, for example by continuing the Wernfeld river mentioned above: it flowed across the valley of the now completely insignificant Breitbach Market width across the Altmhl Valley to the Ur-Danube.

River history: How the Main found its bed

Only later did the geological plate in southern Germany tilt slightly, so that the flow directions of several rivers shifted (Figure A). Even small changes in the roof caused the water networks to change completely, according to Geyer, and within a very short period of time. A sequence of episodic erosion processes shaped the current shape. The enormously changeable climatic conditions during the Pleistocene Ice Age largely controlled these processes by removing and transporting rocks and sediments and the different amounts of water in the rivers.

Typical pebbles from the Pleistocene gravel of the river Main show the transport history of the river.  On the right two white quartz crystals, above a black pebble slate (Lydite) from the Franconian Forest, in front in the middle a rubble made of banded gneiss, which comes from the basement of Eastern Bavaria;  on the left a brown-red sandstone.

Typical bells from the Pleistocene gravel of the Main show the transport history of the river. On the right two white quartz crystals, above a black pebble slate (Lydite) from the Franconian Forest, in the front in the middle a rubble made of banded gneiss, which comes from the basement of Eastern Bavaria; on the left a brown-red sandstone. Photo: H. Metzner

In order to reconstruct the development of river streams, gerls made of characteristic rocks help, since they reveal their origin from the areas of origin of these rocks. For the Main, for example, the Kieselschiefergerle (or Lydite) are such certificates. They occur in the source region of today’s Main (Franconian Forest) and were only shipped from there to the mouth of the Main near Frankfurt since the Main had created its continuous course from east to west. In the earlier gravel accumulations on the sections flowing from north to south, there are no Lydit-Gerlle because the rock was not available in their source areas at that time.

Thanks to this detective work by the geologists, it can be seen that the Main made its breakthrough between Habergen and Steigerwald in the section between Bamberg and Schweinfurt (Figure B) very late. Geyer assumes that the newly formed, west-facing river initially used the long-standing valley of the Nassach and drained westwards over the Wern. The enormous amounts of water that flowed there temporarily can still be seen today on the extraordinarily wide valley of today’s Wernbach on the Autobahn near Werneck.

The Main had thus drained into the Rhine instead of the Danube, but the Main Triangle did not yet exist. Science assumes that the run along the Steigerwald stage developed during the Ri / Wrm interglacial (Figure C). Geyer believes it was likely that the river was then developed in a tangle of interlaced waterways along the Keuper strata. A large number of different collections of Gerll in a wide range indicate this.

The Main moved its bed further and further west from the foot of the Steigerwald in the following millennia (Figure D). The Main gradually dug sideways through the Keuper’s little weather-resistant rocks and was eventually stopped by the harder rocks in the Muschelkalk / Keuper border area.

River history not sufficiently researched

Unfortunately, says Geyer, the river history of the Main has not yet been sufficiently researched. Even the best opportunities were missed. It commemorates the important finds of fossils during the construction of the Wrzburg mental clinic, in which around 600,000-year-old bone remains were found in former mud pools in the Main around 40 meters above today’s river level. But instead of a thorough salvage, there was only a short emergency dig, in which hippos and lions were found in addition to deer. They allow information about the enormous climate fluctuations during the Pleistocene Ice Age.

What is certain is that the course of the river will continue to change in the next millennia, but of course depending on the intervention of the person and thus on the length of his existence. How long will the Main Loop at Urphar last? However, Geyer emphasizes that despite all its complexity, the Main story is a blink of an eye in Earth’s history. Contrary to what is often said in the media, rivers seldom dig in millions of years, says Geyer. The Main in its current form is probably the youngest of the major rivers in Central Europe.

Literature: Gerd Geyer with Hermann Schmidt-Kaler: Along the Main through the Franconian strata.

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