taken from the on-site information board:
The Horster Sonnenstein
During the Bronze Age around 1000 BC, the life-giving power of the sun was probably worshipped, and 17 regular concentric circles were carved into this grey-red granite porphyry slab as a symbol of the sun.
There are only two other sun stones in northern Germany (in Beckstedt with 11 circles and in Harpstedt with 12 circles). Such concentric circles on stone blocks are known from Ireland and on rocks
in Scandinavia.
The front side probably faced east towards the rising sun or south towards the highest point of the sun. Where this Horster sun stone stood 3000 years ago and what significance it had will probably remain a mystery.