Nice pictures and report! Theres JUST enough routefinding on that cottonwood/marble loop to keep it spicy as a solo trip. Years ago, I ran into a group that had missed the final jog over ridge and into marble canyon. Got turned around in those canyons and spent 3 days finding their way out.
Cheers!
That road is a little rough, but the petroglyphs are worth it! I did it on my DRZ-400 with a buddy (who had never ridden in sand-well he has now!) and it was challenging but fun. I talked to a Ranger who said that they don't put up signs for Marble Canyon because they had to pull too many stuck vehicles out...Neat overnighter.
If someone wants to do that Cottonwood/Marble Canyon hike, the drive to the "trailhead" gets pretty rough by the end; you're basically driving down a dry wash. You start hiking wherever you decide to stop, which in this case, was next to a significant wash-out.
If you're willing to walk a few more miles, it can be done in a car you don't care about very much.