https://www.alloschool.com/assets/documents/course-401/les-principes-stratigraphiques-et-l-etablissement-de-l-echelle-stratigraphique-cours-1-2.pdf
https://www.alloschool.com/assets/documents/course-401/les-principes-stratigraphiques-et-l-etablissement-de-l-echelle-stratigraphique-cours-1-2.pdf
