oscillation ripple marks in siltstone from the Moenkopi Formation - large display specimen
Oscillation ripple marks are caused by the back and forth swash of waves in a shoreline environment. The oscillation forms symmetrical ripples, unlike the asymmetrical ripples formed by a water or wind current. For comparison, the field photo shows asymmetrical ripples forming on the surface of a dune, where sand is pushed in one direction by the wind and where the downwind side is steeper than the other.
The mudstones, siltstones and sandstones of the early to middle Triassic Moenkopi Formation were deposited nearshore and on the floodplains of a broad coastal plain, in tidal flats and slow-moving rivers along the western edge of North America and are widely distributed in the southwestern United States, from Colorado to New Mexico, southern Utah and northern Arizona. This excellent textbook example was collected in Emery County, Utah.
Here's an interesting article on ripples from MIT News, with an embedded video that shows ripples forming and adjusting to differing conditions: https://news.mit.edu/2018/beach-sand-ripples-ancient-weather-0928
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