Where was he headed, that naked guy clambering his way through the crowd? The date was Dec. 6, 1969, and the place was the Altamont Speedway in Northern California.

Though there are many reasons to recall a moment when, as Rolling Stone described it, “everything went perfectly wrong’’ — a day when famous bands didn’t play; when bad acid felled dozens and scores more were injured; and when one of the Hell’s Angels hired to police the event knifed and killed a man during a Rolling Stones set — the image I return to is a peaceable one.

“What the hell was he doing?’’ said Bill Owens, the journalist who took the picture that appears on the cover of “Altamont 1969,” a book of previously unpublished photographs from the concert released in May. “He was just somebody that decided to take off his clothes and walk off into the distance.’’

Maybe he was headed for the future. The thought occurred to me as the 50th anniversary of Woodstock approached — it took place just four months before Altamont — and, with it, an inevitable torrent of images conjuring up a bygone era of peace-and-love.