The name Normandie (Normandy) derives from northman, as it was settled by Vikings during the 9th century, and this was confirmed by treaty in the 10th century between King Charles III of France and the Viking jarl Rollo. Formerly split into two regions, it was reunited in 2015, probably the most obvious merger. Located at the English Channel, the region is quite low and flat. A basin of Mesozoic sedimentary rocks, covered by moraine from the glaciers of the ice age. There are not many caves, not because of the lack of karstifiable limestone, but due to its low elevation, which does not offer the necessary gradient for underground drainage. Nevertheless, there is a single show cave, but it is not very spectacular and mostly interesting for its archaeological remains. Also there are some karst areas, for example along the Risle river between Ajou and Grosley-sur-Risle. But the karst is still draining on the surface and the caves are water-filled. The sedimentary rocks contain coal layers which were once mined underground, one of the mines is now a show mine.