This image, taken by the microscopic imager onboard the Mars Exploration Rover Opportunity, shows a close-up of the region dubbed "El Capitan," which lies within the rock outcrop at Meridiani Planum, Mars. In the lower left, a spherule, or sphere-shaped grain, can be seen penetrating the interior of a small cavity called a vug. This "cross-cutting" relationship allows the relative timing of separate events to be established. In this case, the spherule appears to "invade" the vug, and therefore likely post-dates the vug. This suggests that the spherules may have been one of the last features to form within the outcrop.
Image credit: NASA/JPL/Cornell/US Geological Survey