Caledonian folding in Saltstraumen, Bodø, Norway

There are some rock faces that are simply spectacular, including this rock face in Saltstraumen, near Bodø in northern Norway. These fascinating folds have formed under distinct circumstances.

About 490 – 390 million years ago the Caledonian Orogeny happened due to the continents of Laurentica, Avalonia and Baltica colliding. In Scandinavia, the collision involved Laurentica and Baltica only. The collision created an area of metamorphic rock called the Caledonian fold belt [1]Image Credit
Flickr User Battleboy.

This Caledonian Orogeny lasted from 490 Ma – 390 Ma years ago. The formation of the Caledonian fold belt was preceeded by the closure of the Iapetus ocean. The ocean was enclosed by these continents and then subduction under Laurentia started.

This subduction of the oceanic crust under Laurentia shaved off layers of sediment from the subducting plate. This is called an accretionary wedge, which we can also find on the coast of Chile today. This wedge between Laurentia and Baltica mainly consisted of sand and mud, from river run-off. Additionally, some sediments forced underground by the subduction was limestone from calcite banks off the coast.

Reddit user /u/Bbrhuft explains the following three modes of metamorphism [2]Reference
https://www.reddit.com/r/pics/comments/3781n0/this_rock_formation_in_norway/crkvnl6:

Some sediment was metamorphosed under a regime of low temperatures and high pressures. We call this type of metamorphism Blue Schist Facies (after a blue mineral called glaucophane). Other sediment was heated to a higher temperature but experienced a relatively lower pressures, we call this Green Schist Facies. Then there was a third regime where sediment was deeply buried and heated to high temperatures, we call this Amphibolite facies. This level of extreme metamorphism was mostly attained when the continents of Baltica and Laurentica finally collided and subduction ceased, the accretionary prism was squeezed, thickened, stretched, contorted, sheared and then heated.

This continent-continent collision formed a mountain range that was probably somewhere between the Alps and the Andes, however the exact dimensions remain unknown. /u/Bbrhuft further explains:

Possibly due to the thickening of the crust, the thick pile of rock at the collision site (200 – 300 km wide) was heated by its own internal content of radioactive elements. This caused the crust to melt in a process called anatexis. Great batholiths (blobs) of Granite, Granodiorite and Diortite (igneious rock) formed underground. These hot blobs of Magma, some 100s or even a 1000 cubic kilometres in volume caused contact metamorphism. Also, some formally deeply buried and highly metamorphosed rocks were squeezed back towards the surface. This rock went though a process called retrograde metamorphism (reverse metamorphism). This is how highly metamorphosed rock, that experienced high pressures and temperatures, ended up close to the Earth’s surface.

The rock exposed in the picture is most likely gneiss like most rock surfaces in this area in Norway. The third stage is the metamorphosis of the sediments under high temperatures and high pressure (amphibolite). The prominent layering was then caused by erosion of layers that were softer. This is most likely due to dissolution of more soluble Marble-rich layers [3]Reference
McKerrow, W.S., Mac Niocaill, C. & Dewey, J.F. 2000. The Caledonian orogeny redefined. Journal of the Geological society, 157, 1149–1154. http://www.earth.ox.ac.uk/~conallm/Caled.pdf.

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