Two types of eroded oscillation ripple marks, 1) asymmetric and 2) flat-topped, occur in Cretaceous Newcastle Sandstone outcrops in Wyoming and South Dakota. The asymmetric type resulted from a shift of the ripple form just prior to burial; the steeper or eroded side faced seaward. The flat-top forms resulted from erosion of crests of regular oscillation ripple marks. Both types occur together in delta distributary sands and probably record the effects of incoming and outgoing tides.

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