When Daenerys makes landfall at Dragonstone for the first time in season 7, she does it on Itzurun Beach in Zumaia. There are few places in the world like this beach and that’s because of the flysch.
It was formed over a period of 100 million years. The collision between the Iberian and European tectonic plates exposed these sediments about 50 million years ago. The layers lean west to east and provide valuable information of the earth’s history between 100 and 50 million years ago.