Trees? Cave Paintings?

At first glance I can see trees, but in close-up I see lots of different thing including cave paintings.  I first noticed this a few weeks ago but have not been able to replicate it till today.

If you use ground coffee bags then you may have seen this before.  The very finest grains  escape the bag, and then they settle to the bottom of the cup. When I drank the last few mouthfuls the grains formed into tracks, patterns, and shapes.

(You can see the last of the coffee bottom left.)