oscillation ripple marks in Moenkopi Formation siltstone - teaching hand/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.
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.
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
The specimen with just a black pencil in the photo is inverted, so instead of ripple peaks, the curved valleys are facing up, If a geologist found them in this position in place in the field, they would realize that the beds had been overturned. This specimen is useful for teaching that concept. The other side doesn't show ripples.
Select a specimen: If there is more than one specimen shown, you can select a particular specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.
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