The rock surface in the stone forest is fully covered by channels and flutes, which are called karren. Karren was formed by water erosion on the rock. The evolution of the Stone Forest in Yunnan underwent two stages: subsoil erosion and subaerial erosion that created two sets of karrens with different directions on the rock pillars. The horizontal karrens on the lower segment of the rock pillars were carved out by underground water whereas the vertical ones on the upper segment by rainwater.