Image: Onion Layers Pattern
The form and the pattern of the stones that carved by the external power of the earth depend of the mineral types of the stones. Stone's mineral which have different types will have different hardness. The hardness of mineral in the stones influenced the form or the pattern of stones. The weakest part will eroded first and left the harder part. Slowly, the harder part of the stones will be eroded too. The duration depends on the intensity of external power the causing the erosion progress.