There are coal bits hidden in the sand under Vatlestraumen. These remains from a geological layer from the Jurassic Period were discovered when the undersea Bjorøy Tunnel was built in 1994. Oil- and gas reservoirs in the Troll Field in the sea west of Hordaland are from the same time. It is, nonetheless, quite surprising to find bedrock from dinosaur time inside of the outer islands of western Norway. On the Scandinavian mainland north of Denmark, there are only a very few places where one f inds rock from this time in earth history.
The Bergen Arcs have an unusually sharp boundary to the bedrock in the east. Geologists think that this was caused by movements in the earth's crust during the Devonian Period. Then, the Bergen Arcs on the Lindås peninsula sank a whole 10 kilometres in relation to the Precambrian basement gneisses on the east side of Fens Fjord and Aust Fjord.