The first record of Sanctuary Arch is in notes from a reconnaissance survey party for Northern Pacific RR in the 1880s. Unknown whether they reached it, though. A Packwood Ranger District employee, John Klinkham, was contacted by a niece of one of that survey party in the late 1930s and wanted to see it. Don't think that happened, either. Early travellers along the old Johnson Creek Trail probably saw the hole in the ridge from near where you can see it now on Road 2140.
Paul Hammond, a well-known Northwest geologist from Portland State University, told me the arch is really a "keyhole" formation, caused by the ridge beneath the arch failing. Hence the steep rock face below the arch to the NE.
Buddy