Strange Patterns

I am a big fan of patterns. Today was bridge day and six of us met for lunch at the Greek restaurant where we often meet. Two of us played Cribbage while the other four played bridge. Later we joined the bridge game when two gals left early. But none of that explains this pattern. The sun direction was just right to create this. I really doubt anyone will guess what was causing the pattern. Give me your ideas and I’ll tell you what it is tomorrow.