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This word, at first strange sounding and hard to pronounce, comes from the German “flissen” and it means to flow or slide.

The flysch started forming under the sea 110 Ma ago with the accumulation of sediment over several million years, approximately 60 Ma.

Like we said, the sediment was deposited very slowly, forming alternating layers with a different composition. Depending on the kind of flysch, the hard layers can be limestone or sandstone and the marl or lutite are softer. Nevertheless, in some cases the deposits are formed from enormous underwater sediment avalanches usually caused by the ocean floor sinking, which occurred when the Bay of Biscay opened and created turbiditic layers.

Now we've got all the layers for the Flysch of Biscay, but of course they're still deep underwater on the ocean floor. But, how did the whimsical formations that we can see today on our cliffs rise and emerge?

It happened primarily during the so called Grand Coupure between the Cretaceous and Oligocene periods. At that time, the small Iberian plate collided with the large European plate and all the sediment that had built up over millions of years was folded, fractured and raised to create most of the folds and high ground in Euskadi and large mountain ranges like the Pyrenees and the Himalayas.

But, to see it first hand and discover all its mysteries you'll have to come visit and sign up for some of the activities listed on this website.

Basic flysch geology dictionary

Aquifer A permeable geologic formation that can store and produce groundwater.

Abrasion A destructive mechanical process occurring on the Earth's surface created by friction between particles in movement.

Cliff A steep hill on the coast that usually has sea erosion at the base.

Outcropping The total area where a rocky unit or structure appears on the surface of the Earth or just below surface sediment, which may or may not be visible.

Clay Sediment or silty rock made up of an aggregate of hydro-aluminium silicates that may be present with other minerals.

Bank A mass of sedimentary rock that forms a stratum or layer.

Bar A rocky crest belonging to a hard almost vertical or very steep layer. 2 Underwater sandy relief with a linear motif that is produced by sea currents. 3 A sand or gravel deposit with a generally elongated shape located in the course of a current.

Coastal bar Sandy sediment shoals created near the coastline by interactions between waves, currents and tides.

Basalt An igneous rock ranging from glass to fine grains that is generally extrusive (lava) and dark or brown, locally intrusive (like a dyke or sill) it is the extrusive equivalent of gabbro. Silica content 45 - 62%. It is rich in ferromagnesian minerals (> 60% by volume) and labradorite.

Limestone A sedimentary limestone fundamentally consisting of calcium carbonate in the form of calcite and occasionally aragonite in recent deposits.

Karst A calcareous solid affected by karstic modelling considered to be a kind of feature created by rocks dissolved by meteoric water loaded with carbonic gas.

Stratum A usually slab shaped Layer of sedimentary rock that has certain properties or attributes that distinguishes it from adjacent layers in visible stratigraphic bands/surfaces created by changes in sedimentation, pauses in sedimentation, or both factors at once.

Stratotype The original reference or designation of a stratigraphic unit or stratigraphic boundary that is identified as a specific interval or point in a succession of strata that forms the pattern or model for defining and naming the unit or boundary.

Vein A slab shaped mineral deposit in a crack.

Flysch The name given to powerful massive sedimentary rock formations (tectofacies) made up of lutite and turbidite sandstone rhythmite deposited on continental boundaries in orogenic regions before they become deformed.

Fossil The remains or natural mould of an organism preserved in sediment.

Pillow lava A kind of lava whose surface is made up of semi-detached or interlocking spherical bulges that look like pillows. It is created on the floor of a body of water, usually a seabed, by volcanic eruptions that cool quickly.

Magma Melted material created inside the Earth when materials melt at a temperature greater than 600ºC. Magmatic/igneous rock is created when it cools and solidifies. It has dissolved gasses and solid silicates in suspension.

Ophite A basic crystalline (granular) diabasic rock that has been altered so much that very few of its original minerals remain. It retains its ophitic structure.

Fold A deformation caused by rocks twisting or bending. Except for special cases, it involves making the space it originally occupied smaller.

Chevron fold A fold with flat flanks, a pointed hinge and low or non-existent asymmetry. Var. Accordion fold [from the French chevron (male goat)]

Rock Solid material that originates inside the Earth or on its crust as a consequence of endogenous and exogenous processes. In most cases, it is formed by an association of minerals of the same or different species. Exceptionally, some volcanic rocks can be formed by minerals and glass or just glass.

Syncline A fold that is concave towards the top and that contains stratigraphically younger rocks in its core. The opposite of anticline. When the age relationships of the rocks are unknown the fold is called synform.

Sediment A natural product in the form of non-solidified rock that is the result of chemical precipitation or accumulation, either by dragging or gravity, of particles from other rocks that have decomposed, regardless of their size. Sedimentation may be continental, coastal or oceanic. Syn. Deposit