When fall arrives, we get a riot of color in the Pacific Northwest before the leaves fall.  When they do fall, we get a lot of them on our street and some of the leaves, when they get damp, leach out some chemicals on to the concrete of the sidewalk.  They leave a chemical shadow of their shape on the ground as they dry out and get blown away.  Over time, these marks get washed away but, for a while, we can see where the leaves have been.