There are some hollow stone pillars, the hollow interior is narrow in upper part and wider in lower part, resembling an upside-down trumpet, which is one of the unique morphological features of the stone forest. There used to be a thick volcanic layer (basalt) overlying the limestone, the basalt layer concentrated surface water on some depressive locations and the pouring down water made some solution shafts on the underlying limestone. These solution shafts  turned into hollow stone pillars in later stage. At present the overlying basalt no longer exists, this special structure is hard to form any more.