Memorial Bench in Pollok

I have walked past this memorial bench hundreds of times but this morning, as I approached, it looked different. At first I thought it had been spray painted but as I got closer I saw no sign of paint and it was only the actual memorial picture that was white ..... the seat, arms and legs were still black. I moved about a bit and it was only in a certain spot that the back turned white ..... it was the low sunshine peeping through the branches of a nearby tree .... but why didn't the rest of the bench change colour too?

I did notice the grubbiness showing up so next time I visit I'll have a damp cloth in my bag.