Next to the Gorliz hospital, in Astondo, there are petrified dunes. They were classified as a Geological Point of Interest by the Bizkaia Provincial Council in 1990 and they are also a Special Conservation Zone according to European assessments. These coastal dunes are the single example in the Basque Coast.
What are petrified sand dunes?
Gorliz petrified dunes are accumulations of sand moved by the wind that has settled and dried up. They become solid when the humidity evaporates and, by weight, the sand is compacted creating different layers. Thus, the sandstone is formed. The different layers can be in horizontal position and make curious shapes thanks to the action of the wind.
As regards their colour, it is very similar to the one of the beach sand since this is its origin.
On the other hand, its composition is quite varied. Some of the levels of these formations have found fossils up to 6000 years old. This is why it is estimated that the dunes are about 6000 years old. These fossils are small living beings’ shells and carapaces that were buried under the layers of sand. In addition, the formation is not homogeneous at all. In some areas it is very porous and this is why the water has been seeping through these dunes for centuries until an aquifer is created, from which the Gorliz Sanatorium collects water today.
Discover the dunes through this video.