Near the bottom of the slope, the skateboarder reaches a patch of gravel and quickly comes to rest... why does this happen

Maybe... because his gravity was halted... and then he does a header because it was an abrupt stop resulting in a broken neck, broken leg, whatever. Then he learns not to do that again by association.:eek:

Tick

near the bottom of the slope, the skateboarder reaches a patch of gravel and quickly comes to rest.......why does this happen

It's not really friction, but energy is very quickly transferred from the skateboarder into the gravel because of the rough surface, and the fact that the gravel is loose. So the skateboarder stops quickly.