Sten – Halland

We are two round loose boulders of gneiss granite from Rydöbruk northeast of Halmstad in Halland, I am fairly light and fine-grained, my friend is a little coarser and redder in color and very round. Actually, we have come from somewhere further north, with the inland ice and the ice river that followed the present Nissan valley. There we tumbled around and were ground round and fine by the masses of water and by other stones and gravel and sand that came with the ice river when the ice melted. Then we landed at Rydöbruk when the ice river opened into the sea. If you look at a geological soil map, you will see that there is a lot of such glacial material - green on the map - around Rydöbruk and further upstream and downstream along Nissan.

Since we are loose stones, it is uncertain where we actually come from in the solid bedrock. Maybe we are from Halland, or we have come from western Småland or even further away from Västergötland. Most of the bedrock in this area is roughly the same, and is dominated by gneissic granites that have an age of about 1700 million years. Sweden and Baltica, i.e. what is now northeastern Europe, were included at that time, between 1800 and 1250 million years ago, in a supercontinent usually called Columbia (or Nuna), where most of the continents of the time were gathered. We don't really know what Columbia looked like, but southwestern Sweden must have been close to its edge facing the ocean, because there was a lot of geological activity here throughout this time: volcanic eruptions, and magma that intruded at depth and solidified as granite or gabbro or something in between, and recurring transformations and deformation of the bedrock. It was like being at the edge of the Pacific Ocean today!

So most of the gneisses in Halland and adjacent parts of northern Skåne, western Småland and southern Västergötland, the southern part of what the geologists call the "Eastern segment of the Sveconorvegian province" (I know, a long and complicated name, geologists like that) are about 1700 million years old, and were originally granites or related rocks that intruded into the Earth's crust as hot glowing magmas—rock melts—that solidified. Some of the gneisses may also have been volcanic or sedimentary rocks deposited on the surface at about the same time. A gneiss is a striped or banded metamorphic rock, i.e. a rock that has been transformed at high pressure and high temperature deep down in the earth's crust. Often, geologists can have a hard time determining what the gneiss was originally, before it was transformed into gneiss. It may have been a granite or a similar intrusive rock, or a volcanic rock, or a sedimentary rock, it is not always so clearly visible, and may require detailed investigations to determine. We gneisses are a little secretive of us. My smaller friend next to me probably looks more granitic than I do, with his content of coarser dark minerals; I myself am quite light and fine-grained and anonymous in appearance. But nowadays the geologists believe that most of the gneisses in this area have been granites, so then we can call ourselves gneiss granites or granitic gneisses.

We have been exposed to transformation - metamorphism - and deformation deep down in the earth's crust several times, both about 1450 and about 1000 million years ago, when the Sveconorvegian mountain chain was formed. So we have a very long and complicated history behind us. After the Sveconorvegian mountain range was eroded away, maybe 600 million years ago, we came up close to the earth's surface and could exhale and breathe some fresh air (just kidding, we rocks don't need to breathe any air at all, we're not like animals and humans) . As with the Bohus granite further north, Halland gneiss is mined along the coast and sold as building stone for various purposes. The beautiful veined gneiss can be worked in different ways – cross-hammered, flamed, ground, polished – giving different looks.

Geologist Åke Johansson
Swedish Museum of Natural History