Cracks and Fluids in Gusev
For those who've been following the debate about fluids in the subsurface of Mars, here's an intersting photo from Spirit:
Note that the cracks show signs of multiple banding, which on weathering results in a resistant "lip". This is a small-scale version of the "teeth" that Opportunity is seeing along the cracks in the bedrock in the crater wall.
In each case, the likely explanation is that fluids have moved along these cracks and altered the wall rock to something more resistant to erosion (likely mechanically harder or better cemented).