Most of the breaking was done underground long before the rocks were revealed. This is the soft rock shale. This is the hard rock shale. The Shinarump is made up of mountain sediments that were deposited during the Cenzoic era. The rocks that you see out there have what are called joints.
Joints are vertical, very smooth and even, it determines how a rock erodes.
If you look at the base of the rock, you will notice a slight change of color, that part was eroded away. A mesa does not start out as a mesa, it begins to emerge when a succession of hard and soft layers is cut into by a river or brought up by a fault. The mesa becomes a butte, then becomes a spire, and then it disappears.