Inside the mine in Lugnås you can see the gneiss bedrock and the sediment that was laid down on top of it.
The gneiss has been exposed to erosion and conversion of the minerals, making it softer than the bedrock would normally be. This made it possible to quarry the gneiss here and produce fine millstones.
The sediment deposited above the gneiss consists of stones of various sizes (rounded by the action of water), that were compressed to form a type of rock known as conglomerate. On top of the conglomerate is sandstone with pockets of clay. This can be seen today in the roof of the mine. The sandstone was deposited when Sweden lay south of the equator, more than 500 million years ago. It was laid down as sand on the bottom of a sea. If we look closely at the roof of the mine we can see traces of the ancient creatures that lived in the sea at the time, as well as the patterns left by waves in the sand (ripple marks), which show that the sea must have been quite shallow.
The millstone quarry in Minnesfjället is the only quarry of its type in Europe that is open to the public, and it was given Swedish geological heritage status in 2012.