re: That works for Earth, not Mars.
If I remember correctly it was hypothesized that torrential out flows of water possibly created the valley that Spirit landed in.
One can certainly see how much a flash flood can carve up a landscape.
Would this hydraulic force be even more effective on a lower gravity planet? Hmm thats an interesting idea.
Then theres glacier movement carving things up.
All it really takes is a softer subsurface bedrock with a much harder boulder resting on top.
The water only has to wear the softer material away.
I figure even low pressure sand blasting could accomplish this given enough time.