Västerbotten County - Hörnsjö
Granite
about 1800 million years (?)
I am a round block of light gray granite from Hörnsjö west of Umeå in Västerbotten. Since I have been transported by the inland ice to the site as a loose block, it is difficult to know where I originally come from in the solid bedrock, whether I am of local origin or have come from far away. The fact that I am so rounded may indicate that I was transported quite a long way by the ice sheet, and rounded off during transport by rubbing against other boulders that were frozen into the ice. Probably also running water, perhaps in an ice river, helped to grind me round. There is a glacial esker formed by an ice river that runs past Hörnsjö on the geological soil map, so I probably come from that esker, just like the round Uppsala stone here in the stone circle comes from the Uppsalaåsen esker, and has been polished round by the water and rubbing against the other stones in the ice river.
Since no one can really tell where in the bedrock I originally came from, it's also hard to say how old I am. A granite is formed when a magma - a hot and glowing rock melt - solidifies deep down in the earth's crust into a hard rock, a granite. In a large part of northern and central Sweden, there are two generations of granites. The oldest type formed between 1910 and 1860 million years ago. These granites were then subjected to pressure and movement in the crust during the subsequent mountainbuilding event, so that they acquired a more or less distinct gneissic structure with elongated mineral grains of various colors. The younger type was formed between 1830 and 1750 million years ago, towards the end or after the mountainbuilding event, and has therefore retained its original dotted (massive) structure in terms of the different colored mineral grains.
Myself, I am light and speckled - massive, as the geologists say - without any visible gneissic structure, so I probably belong to the younger granite type, with an age somewhere around 1800 million years. But one cannot be completely sure, there may be parts of the older granites that survived the deformation quite well and retained most of their original massive structure. So I could come from such a part in an older granite, and in that case be around 1880 million years old instead. I'm old anyway.
Geologist Åke Johansson
Swedish Museum of Natural History
