URDAIBAI is an exceptional example of how, over millions of years, the earth’s crust has been folded and fractured by the molten magma that still remains active in its core. Erosion caused by atmospheric effects and other surface phenomena have also helped form the current landscape of the reserve. The geological history of URDAIBAI is rich and complex. The area contains the only Jurassic rock at surface level anywhere in Bizkaia.