Wrinkled Rock is the big hump of granite on the right as you walk up the Chopping Block approach trail. This is the hardman crag at Rushmore - just about everything is hard, with the difficulty topping out on Lizzy Beam's Desire, a Todd Skinner 13d/14a.
Wrinkled Rock is named for the dykes lacing the face of this hulk. A Wrinkle in Time in particular follows a beautiful dyke that wanders across an otherwise blank piece of granite.