J. Grotzinger

6) Ripple Cross-stratification.  This image of the rock nicknamed
    "Last Chance" shows evidence of ripple cross-stratification.  At
    the base of the rock, layers can be seen dipping downward, to the
    right.  The bedding which contains these dipping layers is only a
    one-two cm thick.  In the upper right corner of the rock, layers
    also dip to the right, but also have a weak concave-up geometry.
    These two features -- the thin bedding which is cross-stratified,
    combined with the possible concave geometry -- suggest small-scale
    ripples with sinuous crest lines.  Although wind can produce
    ripples, they rarely have sinuous crest lines, and never form
    steeply dipping layers at this small scale.  Thus, it is possible
    that the ripples were formed in the presence of moving water.