Here on the "outermost naked island", there are rocks from Devonian times. With the exception of Bjorøylaga, the conglomerates on Holmengrå are the youngest sedimentary deposits in Hordaland county. Geologically, they belong more naturally to Sogn and Fjordane, where there are much larger occurrences of conglomerate rocks from this time. From Holmengrå, one gets a good view over to the characteristic mountain formations in Solund, which are almost exclusively built up of Devonian conglomerate.
Conglomerates were formed when the high mountains that were created under the Caledonian collision with Greenland, got eroded down. Large blocks and smaller stones crashed down into the deep valleys or basins between the mountains. Some of the landslides must have been dramatic, since we have found blocks as big as houses near the islands further north. Along the coast of western Norway and Trøndalag (area around Trondheim), there were a series of basins during this period that gradually got filled up with layer after layer of sand, stone and gravel, totalling several kilometres thick. Holmengrå was believed to have been near the bottom of just such a basin.
We believe that its base, the contact surface for sedimentary rock deposits, is intact on Holmengrå. The conglomerates rest namely on a quartz-mica schist that bears clear signs of having been exposed to mountain-building before the deposition of the conglomerate.
The conglomerate itself is composed of cemented stones of red jasper, greenstone, different slates, granitic gneisses and quartzite. Holmengrå (“grå” = “grey” in English) is therefore anything but just plain grey rock. None of the characteristic rock types in The Bergen Arcs have been found here. This suggests that the rock types in the Bergen Arcs had not yet risen to the surface when the conglomerate-stones plummeted down from the high mountains. Conglomerate from Devonian time is also found in Great Britain and on Greenland, which at that time lay much nearer to each other.
