Remember that scene in Back to the Future III when Marty McFly ends up in the old west? We went there today. Or, more accurately, the place where they shot the exterior scenes, and just about every John Wayne Western ever made.
So what's Monument Valley like? Towering red mesas erupt from scrubland without warning. Every few miles a small shack peopled by members of the Navajo Nation try to sell you jewellery or other trinkets. It's hot, it's awe-inspiring, it's hot, it's amazing, it's hot.
And if you know where to have lunch you can try Navajo Frybread. Imagine a yorkshire pudding crossed with a doughnut and you'll have a good idea what it tastes like.
Then we reached the Grand Canyon, which is, indeed, "grand". But the problem is it's so vast you can't really gauge the scale of the cliffs, mountains, ridges etc. You can stand there and gawp, but that's about all you can do. At Arches you could interact with the landscape - go and stand under an arch, walk round a mesa. Here all you can do is stand and say 'Yes, it's very grand...'