Sponge Bob Rock Pile is just a seperate cache from the series on the road. Pull in, jump out, grab it, sign it and get back in the car and on your way! What fun! You are looking for a giant preform wedged in the split of a small tree nearby and covered with pieces of bark! (Please... put it back in same place so others cant see it!)
Congrats to Beetlebub for First to find!