Sten – Hällekis

I was deposited as calcareous mud in a shallow sea, much like today's Baltic or North Sea, which covered large parts of Scandinavia between 540 and 400 million years ago, during the Cambrian, Ordovician and Silurian geological periods. I myself was formed during the Middle Ordovician, about 470 million years ago. You might think that sounds like an awful lot, and that I'm terribly old, but the Earth is actually almost ten times older, 4550 million years, and I'm actually one of the youngest rocks here. Only the limestone from Gotland is younger.

Below me at Kinnekulle in Västergötland, where I come from, there is first the really old Precambrian gneiss, and on top of that lies Cambrian sandstone and alum shale. Then I come, the Ordovician red limestone, and on top of me there is shale and more limestone, from the Silurian period. All these overlying layers compressed me into a hard rock called limestone. At the top of Kinnekulle is a small cap of hard black diabase, which intruded into the Silurian strata about 300 million years ago (around the Carboniferous-Permian boundary), and has protected me and the other underlying strata from erosion. On several of the other plateau mountains in Västergötland, such as Billingen and Halle- and Hunneberg, the diabase forms a thicker and more extensive protective cover on top. But the Silurian layers that probably existed above the diabase cover have been completely eroded away.

When I was formed, Sweden and Scandinavia and the entire continent called Baltica, which includes the northeastern half of today's Europe, were in the southern hemisphere, approximately at 30-40 degrees south latitude. The climate was warm, but not quite tropical. At that time there was hardly any life on land, but the seas were full of various invertebrates, such as clams, trilobites (which looked like large wood-lice), and squids with elongated shells called orthoceratites. Much of the lime mud that I am made up of was probably crushed pieces of lime shells from these animals. You can also see whole shells preserved as fossils in the limestone; that's why scientists know so much about which animals lived at that time. The elongated squid shells, the orthoceratites, are common in my type of limestone, so common that it is sometimes called orthocer limestone. Perhaps you have seen such squid shells that look like white narrow cones in floors, stairs or garden tiles made of such limestone. But in my plate, no orthoceratite shell is visible, at least not on the upper side.

If you are wondering about the red color, it is due to small amounts of iron in the limestone that has oxidized to red iron oxide; unless the iron oxidizes in this way, the limestone remains gray, which is also common. The younger Silurian limestones on Gotland are grey, for example.

The red orthocer limestone, and the other Cambrian, Ordovician and Silurian sedimentary rocks originally covered a large part of southern and central Sweden, as well as the areas where the Scandinavian mountains now are located. But since then, as these areas became land again, erosion has caused much of this cover of sedimentary rock to disappear. We remain in parts of Scania, on Öland and Gotland and on the bottom of the Baltic Sea and parts of the Bothnian Sea. We are also still present on the Östgöta and Närkeslätten, around Siljan and in parts of the mountain range. And so, as I said, we are still in the Västgöta plateau mountains, where the hard diabase on the top protects us from continued erosion. These plateau mountains are so remarkable that they have recently been named a Global Geopark by UNESCO, Platåbergens Geopark (https://www.platabergensgeopark.se/).https://www.platabergensgeopark.se/Guess if we were happy!

In Hällekis on the northern tip of Kinnekulle, where I come from, there is a large quarry, although it is now abandoned. Limestone was quarried here for a long time for the manufacture of cement, but also for limestone tiles for floors and stairs indoors and garden tiles outdoors. Because I'm so nicely horizontally layered, it's easy to split me up into thin slices. If you want to use them as a floor covering indoors, you have to polish them so that they become smooth and even, but if you want to use them outdoors, you can keep the slightly uneven bumpy surface, just like on me. It is very fitting to have me as a welcome stone that you can step on on your way into the stone circle! The quarry in Hällekis is closed, but feel free to visit it and admire the 40 meter high limestone walls, if you pass by and visit Platåbergens Geopark. The scientists say it took 1000 years just to form 1 millimeter of limestone, and then maybe you (or your parents) can work out how long it took to form 40 meters of limestone. But elsewhere on Kinnekulle, limestone is still mined, and on Öland. There, the red orthocer limestone is very common, so common that it is often called Öland limestone. It is the one that forms the flat upper surface of Stora Alvaret in southern Öland.

Geologist Åke Johansson
Swedish Museum of Natural History