The Joy fault marks the structural and topographic wall of the southwestern part of the Thomas caldera and separates thick intracaldera deposits of Joy Tuff from thin deposits of the same formation to the west, outside the caldera.

SLIDE 20 is an aerial view of the northwestern part of the Joy fault. In the view, the fault extends clockwise from the bottom through the shadows in the left middle distance and passes to the right of the ridge of Paleozoic rocks in left center. Large hills to right of center are intracaldera Joy Tuff. The view is to the northwest, from a point approximately over Lady Laird Peak in the Drum Mountains.