Ripple Marks

When sand-sized particles begin to move, they love to form ripples—the grains arrange themselves into regularly spaced and organized “bumps” that are called ripples. There are two aspects of ripple development that are useful in interpreting the rock record. The shape of the ripples in cross section—whether they are symmetric or asymmetric—tells the direction of the dominant current that formed the ripples. In all cases, the orientation of the crests is perpendicular to the direction of the current that produced the ripples.  The shape of the ripple crests in plan view gives important information about the speed of the flows that formed the ripples.

Cross stratification

When ripples move over the sediment surface, and have time and space to deposit sediment, their passage is recorded by the development of cross stratification. As the ripple advances downstream, sand grains roll up the upstream side and collect on the ripple crest. When too many grains accumulate, they avalanche down the downstream face of the ripple. This motion produces a sediment layer that is oriented at an angle to the sediment bed. The resultant deposit is said to be cross stratified.