The Ancient Greece on Mars... - Sol 1998

This image taken by the Front HazCam on NASA's MER Spirit looks toward the North/West and shows some of the targets examined by Spirit after the Rover became embedded at this site.

"Ulysses" is the area where Spirit's left wheels broke through a crust and stirred up poorly sorted, weakly cohesive sands.
"Sandals" are two small rocks to the North/West of Ulysses.
"Cyclops Eye" and "Polyphemus Eye" are two locations in which Spirit's RAT (Rock Abrasion Tool) was used to bore into the subsurface for detailed textural, compositional and mineralogical measurements.

Ulysses has the highest Sulfate content measured by either Spirit or Opportunity.
Cyclops Eye also has sulfate minerals beneath the surface whereas Polyphemus Eye does not.
Thus Spirit must be sitting over a geologic boundary where materials are different to the West as opposed to the East.