The elevation readings that I trust the most are the lake shoreline at a known date, which I can cross-check with the official water elevation. People think I'm crazy for taking pictures of my foot at the bottom of the concrete at the Bullfrog main ramp, but I know within inches what that elevation is. Back to the Ft. Moki elevations, the only reasonable topo map of that location that I have found is the 1954 White Canyon 2 SW, UT map. The contour lines on those are usually drawn by hand based on stereoscopic aerial photography and reference elevations on the ground (benchmarks). The newer maps just show lake in that area.Remember Contour lines are usually only accurate to a half a contour interval...In this case plus or minus 20 feet. Time to let it rest!
I can imagine a few feet of sediment piling up by the walls of the structure, as seen in the 2005 Tom McCourt photo, followed by the meander of the Colorado River washing away all of the sediment and stones, leaving behind the bare sandstone.
For Bob, you're welcome to come out and take some readings, and see some terrain that is vastly different from England. It's hard to beat boots on the ground for reliable information. I still need to get a day or so to traipse around the Wahweap area to see the Wahweap Auxiliary Ramp in person and to see what the strange round structure to the north of the Stateline Marina parking lot is. Despite boots on the ground at Bullfrog I have not been able to figure out the phantom boat ramp a little to the south of the main ramp there.