Ex-Rolling Stone with Moss
Can you see what it is? It's not an artefact. It's half an upside-down giant's kettle. (Anyone know what the real English term used by quaternary geologists is? I'm just translating the Swedish word.)
Giant's kettles are common along escarpments in areas once covered by the inland ice, such as Sweden. When the ice melted away, water would stream with great force across cliff faces for absurdly long periods of time. This would frequently set boulders spinning in depressions in the rock, and with time, they would drill down into the rock provided the boulder was made of a harder mineral than the surface it was sitting on.
Giant's kettles vary greatly in size from baking bowls to swimming pools. If you manage to find one that hasn't been emptied, there's often a stone ball at its bottom, the remains of the spinning boulder. Such a stone is without doubt among the dizziest members of the mineral kingdom.
The rock shown in the photo has been part of a kettle somewhere up the mountainside. Half of it has broken off and tumbled down to the foot of the mountain, landing upside-down. I'm thinking it should be possible to find the other half as well.
[More blog entries about quaternary, geology, Sweden; jättegryta, kvartärgeologi, geologi, Tyresö.]