As the continental plate gets shoved and heaved, landslides deposit layers of sediments.
Due to the different sedimentation speed of grains with different size, a gradation takes place.
Bigger particles sink faster and build up the ground layer and are overlain by finer grains.
At some point, structural deformation sparked by colliding tectonic plates tilts the sedimentary beds to near vertical.
Over time, the less resistant layers weather out more quickly creating long parallel grooves in the rock.
These rocks are also the longest set of continuous rock strata in the world.
The flysch in Zumaia were formed over a period of 100 million years.
The collision between the Iberian and European tectonic plates exposed these sediments about 50 million years ago.
These layers provide valuable information of the earths history between 100 and 50 million years ago.
Sources:Tidal zone between Deva and Zumaya (Wikipedia) /Flysch (Wikipedia)/Geocaching
