Mushroom shaped rocks are not uncommon in nature,but their origin is often misunderstood because they can be formed by different natural forces.A huge mushroom stone in a stream valley of the Xiqiao Mountain of Guangdong,China is widely believed to be formed through the way of stream water erosion.However,the result of the simulation performed with Flow-3D in this study indicates that the mushroom shape of the rock could not have been sculpted by the flowing water erosion because the simulation result contradicts that of the field investigations:(1) the sediment brought by upstream flowing water tends to be deposited on the leeward side of the flowing water preventing the rock on this side form being eroded,but the narrowest part of the rock bottom is exactly located on this side of the rock and (2) the stream flow should erode the bed sediment and produce high abrasion on the rock surface on the upstream and lateral sides of the rock,respectively,but no abrasion marks are found on the same sides of the rock.Subsequent geological analyses in this study provide evidence that the narrow bottom of the mushroom stone was resulted from chemical and physical weathering happening within the sediment that used to deposit on the lee side of the stone.