The 20-year-old worker was loading the granite when two pallets broke, and heavy slabs came crashing down, trapping his legs but missing his chest.
Firefighters used air bags to stabilize the granite. They also cut a hole in the rear of the container and brought the man out.
He was conscious and alert, and was airlifted to a hospital for treatment of leg injuries.
Investigators say the man was unloading the truck on ground that was not level.