Throughout the province of Guipúzcoa, secondary terrain predominates with its Triassic, Jurassic and Cretaceous formations, which is the most developed in the province. The entire Deva basin belongs to the Cretaceous. The Cretaceous is divided into two: the Infra-Cretaceous and the Cretaceous proper. One of the forms of the infracretaceous is in the form of detrital rocks. This is the morphology in which it occurs in this valley. In these rocks there is an abundance of sandstones, and in others, slates and marls.