The edges of the isolated region consist of solid granite mountains to east and west, cliffs to the north and a rock ridge to the south. In theory, these factors should keep Graboids from leaving the area. what kept the Graboids from diving under these rock layers were even older geologic layers. Hard sandstone, which the Graboids can’t penetrate would have greeted and killed them.It also keeps the Mixmaster compound from spreading. Thanks to Melvin Plugg it increasingly faces the threat of further development by man a fact that a group of protesters tried to do something about.