In Peekskill, in 1949, a stone the size of a softball was hurled through the window of Pete Seeger's car as he drove to a concert, nearly hitting his baby son. There was a riot around the concert grounds, with angry "conservatives" wielding fenceposts and throwing stones at "the communists". (Paul Robeson was the primary target of the attack, but he didn't go to the concert, having been tipped off).

Pete has the softball-sized stone cemented into his fireplace chimney to remind him of the intersection between folk music and conservative politics, according to "How Can I Keep from Singing" By David King Dunaway. A fascinating read, the book can be found here in Google Books; the concert story is on page 13.