In the summer of 1969 came Woodstock, building on the 1967 Monterey Pop Festival and ushering in a new age of multi-act, multi-day musical festivals. This new age didn’t last more than a few months. In Florida, I attended two that year, in West Palm Beach and Miami – in the former, the closing three acts on the final night were, in order, Jefferson Airplane, The Who, and the Rolling Stones. Not too shabby. But then, before 1969 was out, came Altamont, where the Rolling Stones, one of several acts playing or scheduled to play there, sang and played as the Hell’s Angels security force stabbed an audience member to death. This 1970 film, by David and Albert Maysles, presents it all. The music is powerful, the mood satanic, the end of an era undeniable. And a few years later, the Maysles brothers would film another eerie, unforgettable documentary: Grey Gardens.