What caused the earth's crust to break into a series of valleys and mountain ranges in the Sierra Nevadas

What caused the earth's crust to break into a series of valleys and mountain ranges in the area that we call the Sierra Nevadas? The continental crust east of the Sierra Nevadas began to stretch in an east-west direction, causing the crust to break into a series of valleys and mountain ranges.

Explanation:

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a big wall across the mexican border so no drugs, terrorist, or illegal immigrants can come through : )    

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