Just to clarify - I don't believe this was accidental. Given how much yelling we did after the first big rock hit the tree, I am certain the people responsible continued kicking rocks intentionally. The final rock couldn't have been aimed any better to kill my wife, it landed exactly where she was sitting minutes before. Nor were these pebbles or even fist sized rocks, I'd guess these stones were twenty pounds or more. The last, which you can probably still see at the base (I'll post a pic when we get home), was easily thirty or forty pounds.
I've reported this incident to the rangers and will make a police report when I figure out who to do it with. Valpine, I encourage you to do the same, especially as you may have a decent description of them.
Someone will be killed if these incidents continue and, all due respect Rangerrob, this isn't misdemeanor territory.
Sorry to be such a downer, I'm still furious about this near tragedy. Stay safe, everyone!