ASBOG Fundamentals Of Geology (FG) Practice Exam

Unidirectional flow carves asymmetrical ripple marks. As sand grains are pushed forward by a steady current, they climb the upstream face (the gentler slope, the stoss side) and then avalanche down the downstream face, creating a steeper slope on the lee side. This makes the ripple have a longer, gentler upstream flank and a shorter, steeper downstream flank. The orientation of these slopes tells you the flow direction: the downcurrent, steeper side points in the direction the current moved. In this case, the west side is the long, gentle flank and the east side is the short, steep flank, so the current was traveling from west to east. Tidal cycles or alternating directions would tend to produce more symmetric ripples, and a wind-blown pattern would reflect wind direction but the described asymmetry matches a steady west-to-east current.