Mesa Arch is probably the most famous attraction at Canyonlands. The interesting thing about it is that the other side of the arch is a 500 foot drop down into the canyons.
While there, we noticed a French family taking turns climbing the arch to get a unique picture. Pretty risky, given the severe drop down one side, so we took a pass on climbing up it and opted for the more traditional "in front of famous thing" pose.
Rafael
P.S. 100 degree weather is killer. You should see how much water we've all put away today.