The Big Rock is a location that only appears in the episode Backpack as the second place.
The Big Rock is large, blue rock. However, when it first appears on-screen, it is smaller. It is in a narrow canyon, and it has a slide on the back and a tree on the top (pictured above). The Icky Sticky Sand is in front of it as well as the Troll Bridge. In the PC game, it can be seen out the window at the Library.