Flysch

Flysch is a special kind of rock made from layers of different sediments. It forms when layers of deep-water mud and sand are carried by strong underwater currents and settle on the ocean floor. Over time, these layers change from deep-water deposits to shallower water shales and sandstones.

This type of rock usually forms when a deep basin quickly appears on the side of a continent during mountain-building events. You can find examples of flysch near big mountain ranges like the Alps, the Pyrenees, the Carpathians, and the North American Cordillera. These rocks tell us important stories about how mountains and oceans change over millions of years.

Sedimentological properties

Flysch is made of layers of sedimentary rock that repeat in cycles. At the bottom of each cycle, there may be coarse rocks that change into sandstone and shale. Flysch usually has layers of shale mixed with thin, hard sandstones. The shales often do not have many fossils, and the sandstones can contain small pieces of minerals.

Tectonics

When two big parts of Earth's crust crash together in a continental collision, one part pushes under the other, called subducting. This makes the rocks fold and can create tall mountain chains. Between the mountains and the flat land, a valley called a foreland basin forms.

If this valley forms slowly, like in the northern Appalachians, it fills with mud and sand from shallow water. But if it forms quickly, like on the east side of the North American Cordillera, water from the sea can rush in. This creates deep water deposits. Fast-moving underwater flows called turbidity currents carry sand and mud downhill, forming layers known as turbidite deposits. As the valley fills up, sand and mud from rivers create shallow water sandstones and other rocks. Near the mountains, these rocks can bend and shift, but most stay mostly unchanged. Finally, after the valley is full, more layers of sand and mud called molasse sit on top of everything else.

Name and use

The name flysch was first used by a Swiss geologist named Bernhard Studer in 1827. He saw that these rocks, made of layers of sandstone and shale, were common in front of the Alps. The word comes from a German word meaning "to flow," because Studer thought these rocks were laid down by rivers. We now know that flysch forms deep under the ocean where Earth's plates move.

Today, flysch is found in many mountain areas around the world, especially in places like the Pyrenees, the Carpathians, and parts of Italy, the Balkans, and Cyprus. In the northern Alps, flysch is also used as a special name for these rock layers.