Could be formed by viscous fingering.
https://www.youtube.com/watch?v=VdwSMytC7y4
https://en.wikipedia.org/wiki/Saffman%E2%80%93Taylor_instabi...
https://www.youtube.com/watch?v=VdwSMytC7y4
https://en.wikipedia.org/wiki/Saffman%E2%80%93Taylor_instabi...
Just some images of what I am talking about: https://duckduckgo.com/?q=dindrite+rock&t=ffab&ia=images&iax...