For a project that I'm running with a student here in Melbourne I've created a couple of images. I thought I'd share them with you folks...

The Grand Canyon on Earth
The Grand Canyon

The Grand Canyon is just left of and above centre as a wiggly blue streak. The grid is 1 degree of Lat and long - about 120 km

At reduced Scale
Mars-scale

Maja Valles on Mars at a similar scale (Grid is now 2 Mars degrees, but essentially the same 120 km)
Maja Valles

Maja Valles extends from the chaos zone at bottom centre up towards the top right. The VERY large channel at top left is part of a different system. Note how puny the Grand Canyon is compared to features on Mars.

Imagine how much erosion is required to make features of this scale. Either Mars surface material is very weak or the floods were immense and long-lasting.

(This is why Blue Mars enthusiasts get so keen on wet Mars. *IF* it were caused by water there would be a lot of water on Mars)

Full-Scale Maja Valles image for those interested in it.
Warning! 1.4 Meg!

- Nick