Years ago I read a short story about a tour group that goes back in time on a nature walk. The group has to travel on an elevated walkway because nothing can be touched on the planet (because of the "butterfly effect"). Inevitably, someone falls off the walkway and the group returns to find their present forever different.

I am pretty sure this is a Ray Bradury story. If anyone knows the name of the story and/or what book it is from, I'd be very grateful

"A Sound of Thunder," again, collected, among other places, in "The Stories of Ray Bradbury."

I'd gone through a couple short story collections and hadn't come upon it. I Guess I'll be stopping at the book store on the way home tonight.

Thanks so much for you help! -N.