Fossil Canyon

Coalition of American Canyoneers say: "Way on the other side of Great Thumb Mesa is a stunning place called Fossil Canyon. Good rock climbers can escape up Fossil canyon from the river, but not without swimming, stemming, and hard climbing. While not completely ‘technical’ in the classic sense, it’s real gem to behold and hikers and canyoneers can use this slot to escape back to the rim of Great Thumb Mesa. An excellent loop is down Royal Arch Creek via the Pt. Huitzil Route with a walk/packraft combination to Fossil Canyon. From there you can escape to the rim of Great Thumb Mesa via the ‘Cave Route’ or walk the Esplanade to exit at the Apache Point route."

Several reports confirm the canyon can be ascended from the Colorado River, unclear at which point you exit the slot. From the top, Great Thumb Mesa, Harvey Butchart says there are several access points, the best might be just north of a checkpoint marked "Cave" on the 7.5 minute topo map, this checkpoint is roughly 36.32165025465648, -112.5637708579818. Presumably this is known as "Caves Route".

This canyon is rumored to be a walkable escape route to the rim from the river or vice versa. no major technical work, just exposed ledge walks, bridging, boulder fields near the river are the only noted obstacles. There is rumored to also be a way to get to Great Thumb Mesa thru the Muave Limestone layer via the "cave route". Satellite imagery shows possible walk out locations to the west of where the canyon levels out onto the mesa, possible where the Cave route is located. Trip reports show this canyon to be "beautiful and something to see" as an added bonus.