What kept the Graboids from diving deep below the floor of Perfection Valley and emerging at the bottom of the “Cliffs to the North”?

As you go down the face of those cliffs, you encounter older geologic layers.  Before you reach the bottom, you come to hard sandstone, which the Graboids can’t penetrate.

What kept the Graboids from diving deep below the floor of Perfection Valley and emerging at the bottom of the “Cliffs to the North”? was last modified: by

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