Ichnofossils on flysch Deba, Geopark of the Basque Coast, Basque Country, Spain. Ichnofossil: Helminthorhaphe - Flysch formed at the end of the Mesozoic era, in the Cretaceous - flyschs are formed by sediment accumulation following repeated submarine avalanches, caused by earthquakes, and producing after compaction and crystallization of very regular layers of sandstone and limestone, here corrected by the Pyrenean orogeny and released by erosion.