ripple marks - teaching hand specimen from the Jurassic Moenkopi Formation

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Teaching hand specimen of oscillation ripple marks from the Jurassic Moenkopi Formation
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.
The pencil is 5.5" long, for scale
.
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