Courtesy of Sara Underwood and Steve Bitanga (who I mentioned a couple of posts ago), I just found out about the existence of Morning Glory Arch, or Bridge -- I've seen both names. Bridges go over water and arches don't, and I'm not sure in this case which it is.
Anyway, it's outside Moab, Utah, and it isn't in either a national park, monument, or forest, which I found surprising. And given the way it is oriented to the surroundings, it isn't really easy to see. I recommend searching for more images of Morning Glory Arch to see what I mean.