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.
If you've looked at the FlashEarth location for the last couple of days, you can see the promontory at Arch Rock has a significant north-south extent. This and the last two photos were taken at a viewpoint near the south end of the headland. This is supposedly the largest arch on the Oregon coast, and I know I haven't seen a larger one anywhere. If you look carefully, you can see the shearing and faulting that weakened the edges of the arch.
"Since recrystallization hardens rocks, and shearing and fracturing weaken them, the combined effects produce extremes in rock durability. This largely accounts for the differential erosion of bedrock along the coast of Curry County and the resulting ruggedly beautiful scenery." (Paragraph 2, under "Geologic Setting," Landforms along the coast of Curry County Oregon, by Ernest H. Lund in The Ore Bin, Volume 37,No. 4, April 1975- PDF)
I was scanning over this document yesterday, and that line popped out at me. I have decided, over the 30-plus years I've spent exploring Oregon, that the three parks near Charleston (Sunset Bay, 2/11-2/17, Shore Acres, 7/22-8/11, and Cape Arago, 2/11 & 8/8, but not really covered) comprise my favorite coastal location, but generally speaking, this section from Bandon south to the California border is the most spectacular visually. The geology is spectacular, too, just so spectacularly complex that it makes me uncomfortable to try to say anything about it.
(Size options here) Looking southward from the promontory toward Arch Rock, in Samuel H. Boardman State Park. It looks as if there's a fault cutting through the folded sediments to the left, under the small stream that runs down the cliff and across the beach. The sediments themselves look as if they're likely turbidites. Page 15 (as paginated in the PDF; 71 in original) of this document says that this is once again Otter Point Formation, the same as the Bandon Blueschist, and the bedrock below the Miocene/Pliocene terrace gravels at Cape Blanco. Note that the date of the publication is 1975, well before the finer details of flake tectonics/terrane accretion were developed. I find older things like this useful for getting myself oriented, but try to keep in mind that nomenclature, and certainly broad relationships (i.e. beyond outcrop scale) have quite possibly been largely revised.
Panorama stiched in Hugin, converted to .jpg and run through autolevels routine in Paint.net. May 8, 2013. FlashEarth location.
(size options here) As you can see, May 8 started, at the coast, the same as May 7: cloudy, foggy and windy. What you can't really see is that it was also just as cold. Lockwood was not a happy camper. However, we were only about 30 miles from a turn inland, and an enormous change in the weather. This was our first stop, at a pull-out just south of Cape Sebastian, the headland on the right. Despite the lousy weather, it's hard not to admire all the stacks. But due to the lousy weather, and an extremely crowded itinerary with some mind-blowing geology (Yeah, we're gonna drive through the mantle. No biggie.), I can't really say anything about the rocks here, because we didn't go look closely at them.