Fulufjället is made up of sandstone with components of partially weathered and fertile diabase. The sandstone formed when the area lay at the bottom of a warm sea south of the equator 1,200 million years ago. Here and there you can see marks made by waves that have been preserved on the boulders.
For millions of years sun, wind, water, cold and glaciers have weathered and sculpted Fulufjället into its current shape. The fields of boulders have been caused by frost erosion; the rock precipices and ravines were formed by brooks and rivers from the melting ice sheet. The sculpting is still going on, especially by Njupeskär, where the canyon is becoming deeper and deeper.
Where brooks and rivers have weathered the bedrock, you can find stones made of greenish Särnatin guanite, a type of rock found only in north-western Dalarna.