A dominant feature of the hilly terrain is the narrow mountain ridges, which consist of banded gneiss of the greywacke type.
Gneiss is present in a large vein that usually is called the Stora Le – Mars transformation. It probably formed more than 1,650 million years ago, making it one of the oldest types of rock in western Sweden. There also are a few thin dikes of diabase and pegmatite. A long time ago, pegmatite was mined in the area along with the magnetite found in some places. Traces of magnetite mining remain at Orshöjden and at Sminäset.