This false-color image taken by the Mars Exploration Rover Spirit shows a group of darker rocks dubbed "Toltecs," lying to the southeast of the rover's position. The rocks are believed to be basaltic, or volcanic, in composition because their color and spectral properties resemble those of basaltic rocks studied so far at Gusev Crater. Scientists hope to use these presumably unaltered rocks as a geologic standard for comparison to altered rocks in the area, such as "Clovis." This image was taken by the 750-, 530- and 430-nanometer filters of rover's panoramic camera on sol 220 (August 15, 2004).