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asymmetrical ripple marks in siltstone from the Moenkopi Formation - teaching hand specimen

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Asymmetrical ripple marks are caused by a current in wind or water. Unlike  oscillation ripples with symmetrical ripples, asymmetrical ripples have one side steeper than the other, though this may be subtle. Careful observation by your students will let them discover that the distance from the bottom of a "valley" to the top of the ripple is greater on one side than on the other. The field photo shows obviously asymmetrical ripples forming on the surface of a dune, where sand is being pushed in one direction by the wind. In these ripples, the steeper side is downstream.

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. 

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

These ripples are subtly asymmetrical, with one side steeper than the other. A little tricky, since weathering has removed the ripple tops. You can challenge your students or just use this specimen as an example of ripples in general.

 


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