Looking down to Arch Rock from a viewpoint near the northwest corner of the headland, we can now see right through the keyhole. If you look in the area at and around the arch, you can pick out a number of faults, including, I think, the ones responsible for shaping the arch itself. Below is my interpretation- photo crop heavily processed to increase clarity and contrast. This looks to me like what are called conjugate faults, which would mean sigma 1, the axis of greatest compression (for this episode of deformation), is roughly horizontal and parallel to the face of the block. I'm right at the edge of my confidence here, so other geo-people, please chime in if I've bungled.