During one of my walks through the city streets, I noticed that some basalt blocks that make up the sidewalk have in their surface some very curious natural patterns:
The photos above was taken at the Porto Alegre city, Brazil. (Coordinates: 30°01′59″S 51°13′48″W)
What are these fractal-like patterns? How and when are they formed? Is it some kind of organic formation? Is it living being or some kind of fossil?