I was out in Swinley Forest in Bracknell(United Kingdom) last weekend. The area is a large pine forest but the ground is full of flint and is very sandy.
As you can see on the right edge of the rock there is an indentation with some vertical lines and some circular patterning. I've included a zoomed in image below.
My question is whether this could be plant fossil or even part of the fin of a fish with scales attached.