Essentially all of the slate quarries in Hordaland use stone from the bottom of the Caledonian thrust sheet, just over the phyllite, which in turn overlies the basement. All of these slates are metamorphosed sandstone. The Sålesnes-slate is the only exception. This is a strongly flattened granite, a granitic gneiss, which originally contained big crystals of red K- feldspar, together with quartz. But, the centimetre-sized feldspar crystals were very reduced in size by pressure. The remains of some of the crystals can still be seen as red “eyes” in the slate. Otherwise, the feldspar grains have largely been altered to light mica (muscovite). The same altered granitic gneiss re-emerges at Hereiane, where a bit more of the original bedrock is exposed. The metamorphosis was not equally intense at all localities. The ruffians from the Middle Ages knew how to choose the sites with the strongest metamorphism to get the best cuts of slate.
Mica is a flake mineral that has grown in a certain direction at this locality, contributing to the development of the strong slate structure in the bedrock. Such metamorphosis from granite to granitic gneiss is typical in the thrust sheets on Westland. But, seldom to such an extent as in Sålesnes, with so little left of the larger feldspar grains and so much generation of mica. This intense and thorough alteration occurred at great depth. It was the result of the bedrock being pressed down deeply under the influence of the same forces that caused the collision with Greenland. The effect of the pressure on the bedrock varied, and the granite at Jonahornet was more affected than other areas.
The mica gneiss is best suited as large, slightly thicker slabs. Nonetheless, it has been much used in roofing. Best known are the big, rectangular roof plates, which are quite common in Hardanger. But, smaller roof slates have also been produced. In earlier times, millstones were a main product, with baking slabs and drying ovens as additional items. Since then, the Sålesnes slates have been widely used in sidewalks and in building stone, both in Bergen, Haugesund and Stavanger.
