Breakthrough valley

The notch that a mountain range experiences through a crossing river is referred to as a breakthrough . Often the breakthrough runs perpendicular to the strike of the rocks.

A breakthrough can almost selectively go through a ridge-like ridge (e.g. Porta Westfalica ). Punctual breakthroughs and bottlenecks within wider breakthrough valleys often have proper names that begin with porta or end with gate .

If a breakthrough is not punctual, but linearly through a larger mountain range, the resulting, often winding valley is called the breakthrough valley . The Upper Middle Rhine Valley breaks through the Rhenish Slate Mountains .

Causes of the formation of breakthrough valleys

The starting point for the creation of a breakthrough valley is usually a mountain range that is inclined or perpendicular to the large-scale flow direction of a river. Over the course of millions of years, the river erodes the mountains along a tectonic line of weakness or fault . At the breakthrough, the stratifications of the mountain range are therefore often clearly exposed ( exposure ) and allow conclusions to be drawn about the mountain formation . When mountain regions rise - often even today by 1 to 3 mm per year - some rivers deepen to the same extent. During floods, pebbles and gravel are deposited in the basins of the upper reaches or below the breakthroughs, while fine gravel and sand mostly end up in the main river. Due to the removal of material, the river continues to cut into the subsoil in the direction of its source . This process is known as retrograde erosion.

Upper Danube: View from the Knopfmacherfelsen towards Beuron

The Manawatu Gorge in New Zealand

Breakthrough valleys are divided into:

• antecedent breakthrough valleys
• epigenetic breakthrough valleys
• Overflow breakthrough valleys.

Antecedent breakthrough valley

Antecedent breakthrough valleys are valleys of rivers, the direction of which was determined before the onset of the mountain elevation. With the tectonic uplift, the river cut into the rising mountains instead of shifting its river bed. The Middle Rhine Valley between Bingen and Koblenz ( Upper Middle Rhine Valley ) is an antecedent breakthrough valley.

Epigenetic breakthrough valley

Epigenetic breakthrough valleys are valleys that were originally formed on loose sediments that covered a buried mountain or hardwood ridge. By lowering the surface through fluvial erosion and denudation , these ridges are exposed and thus freed from the less resistant sediment shell. Due to deep erosion, which marks the deepening of the river bed, the river was able to cut into the hardened ridge and thus shape the valley.

Overflow breakthrough valley

Overflow breakthrough valleys are caused by the fact that water accumulates behind a naturally deposited mass of debris (e.g. rockslide mass) and finally overflows at the lowest point. Due to the high gradient, this overflow is deepened by retreating erosion, which drains the reservoir that has formed. Lake sediments ( lacustrine sediments ) above such a breakthrough attest to this type of formation of breakthrough valleys.

Known breakthroughs

Danube

Well-known breakthroughs in the catchment area of ​​the Danube are z. B .:

• the Danube breakthrough near Beuron in the Upper Danube Nature Park , where the still young river has dug a deep bed through the Jura rocks of the Swabian Alb
• the Danube breakthrough at Weltenburg , a 180 m deep valley with steep rock faces
• the Neuburg Forest with the Lion Wall
• The breakthrough through the Czech mass between Passau and Mühlviertel which the Sauwald from the rest of Bohemia separated
• the Wachau in Lower Austria (from pin Melk to Creams )
• the Viennese gate near Klosterneuburg
• the Danube Bend below Esztergom (north of Budapest ) and
• Iron gate through the Southern Carpathians between Serbia and Romania .

Other rivers in Central Europe

• Elbe valley zone : breakthrough of the Elbe in the Elbe Sandstone Mountains of Saxon Switzerland
• Gesäuse : alpine breakthrough valley in the middle reaches of the Enns
• Middle Rhine Valley : breakthrough valley of the Rhine through the Rhenish Slate Mountains between Bingen and Rüdesheim am Rhein in the south and Bonn- Bad Godesberg and Bonn- Oberkassel in the north - Binger Loch
• Porta Hassiaca : breakthrough of the Eder through red sandstone in the Wabern plain
• Porta Westfalica : Breakthrough of the Weser through the Jura rocks of the Wiehen and Weser mountains
• Schwalmpforte : breakthrough of the Schwalm through red sandstone from the Löwensteiner Grund into the Schwalmaue
• Thuringian gate : breakthrough of the Unstrut through the shell limestone of the jewelry
• Wipper breakthrough: breakthrough valley of the Unstrut tributary Wipper through the shell limestone of the Hainleite
• Jura : breakthrough valley of the Aare through the limestone mountains to the Rhine ; Klus der Rhone near Bellegarde-sur-Valserine

Breakthroughs worldwide

• Kali Gandaki in Nepal, deepest breakthrough valley in the world, between the eight-thousanders Dhaulagiri and Annapurna
• The three gorges of the Yangtze River ( Qutang Gorge , Wu Gorge , Xiling Gorge ), China
• The Waag valley near Strečno ( Little Fatra , Slovakia )
• The Rotenturm Pass in the middle reaches of the Olt , Romania
• The Delaware Water Gap (Delaware breakthrough) of the Delaware River in the Appalachian Mountains between the US states New Jersey and Pennsylvania
• The Manawatu Gorge on the North Island of New Zealand
• The Iskar Gorge in Bulgaria

literature

• H. Zepp: Geomorphologie: Grundriß Allgemeine Geographie . 6th edition. Schöningh, Paderborn 2014, ISBN 978-3-8252-4030-1 .
• John Grotzinger, Thomas H Jordan, Frank Press, Raymond Siever: General Geology - Introduction to the Earth System . 5th edition. Spectrum, Berlin / Heidelberg 2007, ISBN 978-3-8274-1812-8 (Original title: Understanding earth . Translated by Volker Schweizer).