Erratic boulders of Finland

A stone that doesn't belong here

You can spot an erratic boulder because it's in the wrong place. It may be a different rock type from the bedrock beneath it, it may stand alone in the middle of a sandy heath with no other stones around, or it may rest on a rock on such a small point of contact that it looks like it shouldn't stay put. All of this comes down to the same cause: the continental ice sheet tore the stone from its bedrock, carried it tens or hundreds of kilometres, and set it down when the ice melted around 10,000 years ago.

Erratic boulders are found all over Finland, but giant ones are rare. In folklore they are the throwing-stones of giants and trolls, obstacles to church-builders, and boundary markers carved with crowns. Many have earned a name, and many are protected as natural monuments.

How the boulders formed

During the last ice age, the ice sheet over Finland was, in places, three kilometres thick. Ice does not stay still: it flows slowly under its own weight, and its underside tears loose rock material along with it. Whole boulders broke free from weakened, fractured zones in the bedrock, froze into the ice, and travelled with it. As the climate warmed and the ice margin retreated north, the boulders dropped to the ground wherever they happened to be.

In Finland, the direction of transport was mostly from northwest to southeast. That lets geologists trace a boulder back to its point of origin: if the rock type is unusual, its source lies to the northwest. The same method was used in mineral prospecting – the Outokumpu copper ore deposit was found in the early 1900s by tracing ore boulders back to where they came from.

Balancing positions, like Kummakivi in Ruokolahti, are no accident. As the ice around the boulder melted slowly, the stone settled gently onto its base instead of rolling. Since then, only frost and lichen have worn at its position – and in ten thousand years, even that has amounted to little.

Where the giant boulders are

The largest known boulders are in Ostrobothnia and the Lakeland region. Ilveskivi in Pedersöre rises 16 metres, and Isokivi at Kankari in Muhos weighed over 500 tonnes before it split. Kummakivi in Ruokolahti is the best known for its balancing act. In southwest Finland, boulders often served as boundary markers and assembly stones; in Lapland, as sacred Sámi seids.

The map also has smaller named stones with stories of their own: a dressing stone for churchgoers in Paimio, the stone from Topelius's fairy tale in Ruovesi, and stones that once sheltered people during wartime. Each site's page has a description, a map and driving directions.

Before you go

• Many boulders stand in the forest without signage. Save the site's coordinates on your phone beforehand; the site page's map gives you a route.
• Nothing may be attached to a protected stone, and it must not be damaged. Tearing off moss and lichen counts as damage.
• Climbing is at your own risk. Mossy granite is as slippery wet as ice. Ilveskivi and a few others have steps.
• Boulders often stand on private land. Everyman's right permits a visit, but not passing through a yard or lighting a fire.
• The best time is spring and autumn, when leaves don't hide the stone and the lichen is damp and colourful.