A grotto stone is gypsum that has been leached out by water, which leads to the formation of visually striking surface shapes. This specimen shown in the local historical collection of Walkenried was found at the outlet of a fishing pond (Burglöcher) near the Sachsenburg. The local outflow into a karst channel became active in 1973 by raising the pond.
<a href="//commons.wikimedia.org/wiki/User:C.Reinboth" title="User:C.Reinboth">Christian Reinboth</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>