According to the American Geosciences institute, a castle, in the geologic sense, is a natural rock formation bearing a fancied resemblance to a castle – sophisticated science, I know! The limestone boulder pictured above, which is from north central Arkansas, is one such castle. Rocks like this one owe their appearance to their solubility in weak acid.
Most rain water is actually slightly acidic, due to the CO2 it absorbs from the atmosphere and soil it passes through. Over time, this acidic water is capable of dissolving limestone bedrock into features such as caves, sinkholes, and, in this case, castles. The boulder pictured here has been flipped over by the creek’s current; they typically form with the castle side down.
below is an example of a castle that is still forming. The base of the rock dissolves faster than the upper part, because it is under the water more often. This differential weathering is what gives the boulder its characteristic castle shape.
Photos by Richard Hutto
Office of the State Geologist Blog 