This magnified view from NASA's Mars Exploration Rover Opportunity of a portion of a martian rock called "Upper Dells" shows fine layers (laminae) that are truncated, discordant and at angles to each other. Interpretive black lines trace cross-lamination that indicates the sediments that formed the rock were laid down in flowing water.
This rock, like another called "Last Chance," preserves evidence for trough cross-lamination, likely produced when flowing water shaped sinuous ripples in underwater sediment and pushed the ripples to migrate in one direction. The direction of the ancient flow would have been toward or away from the viewer. The interpretive blue lines point to boundaries between possible sets of cross-laminae.
Several frames taken with Opportunity's microscopic imager during the rover's 41st sol on Mars (March 5, 2004) are stitched together to make this mosaic view. Eight spherules can be seen embedded in the rock, and one larger pebble sits on the present-day surface of the rock.
Image credit: NASA/JPL/Cornell/USGS