Above is Kevin Gill’s assessment of how the western hemisphere of Mars would look either terraformed by human hands or returned to its ancient state. You can see Olympus Mons off to the left, the volcano three times the height of Everest reaching towards the sky. The open maw of the Valles Marineris canyons dominate the center of the concept art.
Gill also mapped the Southern hemisphere (above). Both images were painted using GIMP after being created using jDem846 software. Extrapolating terrain based on Earth ecosystems, Gill put desert around the active Olympus Mons with lush greenery taking hold further from the equator before becoming tundra.
This project was done as a hobby, as Gill is not a geologist. On his page he says,
This wasn’t intended as an exhaustive scientific scenario as I’m sure (and expect) some of my assumptions will prove incorrect. I’m hoping at least to trigger the imagination, so please enjoy!”
You can read his whole process over here.