What biological specimen is this? Seed? Excrement? Fungal form?
All right, I'll say it if nobody else will.  Insect feces.  The "wood chips" you saw are bits of your oak (or some other over-hanging tree) that have graduated from the arthropod digestive tract, so to speak.  The pellets soak up water because the bugs are on a tight water budget and extract all the water before making a deposit.  Never heard of a gall infestation that produced feces like this, but who knows?  Look for dieing, deformed, or discolored leaves.  Look for bugs on the tree and on fallen leaves.  On the 2nd page of the document linked below is a larger specimen of what you are getting.  http://collier.ifas.ufl.edu/CommHort/CommHortPubs/Stained_sidewalks_under_Oaks.pdf