Philadelphia: where graffiti got its start

Riding back from the Big Apple on an Amtrak Keystoner, I watched the sky go from gray to blue as we neared Philadelphia.  It was that time in the afternoon when that glitter thing can happen, and Philly's graffiti was lit up, as if for a show.  Modern graffiti (for of course there were cave painters eons ago) started here, the legends go, in the early 1960s.  Cornbread and Cool Earl left their tracks everywhere.  Embellishment ensued.