Asymmetric Ripples

If the ripples are asymmetric, the flows moved in one direction only (such flows are called unidirectional flows), like currents in a river. Significantly, the direction of asymmetry indicates the direction of the current that formed the ripple. The sand grains move up the gentle side of the ripple to the crest. The grains accumulate on the crest until they can no longer be supported, and then they avalanche down from the crest, forming the steeper face of the ripple. In many respects the process is similar to a snow avalanche.   When these ripples aggrade, they typically produce trough cross stratification.  

Think about how these ripples would look if you flipped them upside down:

        Turns out that they look the same.  Therefore, asymmetric ripples cannot be used as indicators of stratigraphic top.  But, asymmetric ripples are a great indicator of paleocurrent direction.