White picket cuneiform

Some natural force or little critter has been drawing on our fence out back.  The patterns look like they could be created by some weird, rain-induced erosion of the mossy, moldy growth sitting on the fence surface.

It's some great drawing via erasure.  Upon close examination, these erasures are actually made up of miniature triangular forms, which it seems must have been made by some strange meandering mechanic process.  I don't know if it's insect feets scrambling up the surface or some characteristic of the fencing material that making the marks.