Isn't a 15% slope really noticeable? Like it doesn't look like a mountain peak but it would be obvious which way was up
Apparently grade limit on mountain public roads is 7%, and that is definitely felt.
It's a bit steeper, much of the length of the canyon the incline is 35%-ish.
Google Earth also has Mars. You can measure the length of the banks. It's roughly 30 km, and the maximum elevation change is 7–8 km. That's the maximum, not the average. It gives a slope between 10% and 15%.
But visually, it's even less, because Mars is smaller than earth, so its curvature matters.
On Nasas map I get 10km over 25km.
That's wild to think about. You could be standing inside the largest canyon in the solar system and have absolutely no idea