Thanks for the question!

This photo is taken from a weird angle and gives the appearance of a tight exit through the rocks. Though they are actually about 5' above the slide..no scary better tuck your head or else ride here.. please see attached image taken from the exit view point.

No, the rock at the bottom is about 5' above the slide..the photo above this reply shows the view point from a different angle. Thanks for looking and asking

Dan, what color is this pebble?

This is plaster, and its medium gray.