The geology of the Basin and Range is created by normal faulting.  An outward pressure that represents horizontal lengthening is caused by plate tectonics.  Plate tectonics is the theory of how the surface of the earth sets on plates that move relative to one another.  The horizontal lengthening, known as normal faulting, causes a crack or fault in the earth with the hanging wall (the block above the fault plane) dropping and the footwall (the block below the the fault plane) rising. A reverse fault is caused by compression of the earth, which also results from plate tectonics.  In this case the hanging wall rises and the footwall drops. 

                Normal Fault Diagram                                               Reverse Fault Diagram 

    Multiple normal faults will create a series of mountains and valleys or basins.  The mountains formed in this way are called horsts and the basins are called grabens.  The horsts erode filling the grabens with sediment and changing the shape of the horsts.

   Basin and Range (Horst and Graben) Diagram                       Horst and Graben Geology