Friday, October 21, 2016

Ancient wave-formed ripple marks

I have always enjoyed finding ancient examples of sedimentary structures that help a geologist determine depositional environments. The example I show here are ripple marks made by waves in a middle Miocene (about 13 million years old) lacustrine (lake) deposit in the Mint Canyon Formation, northern Los Angeles County, southern California.

The above picture is a vertical look at the ripple marks. The rock is siltstone, and these ripple marks were found in a section of muddy rocks containing very small freshwater gastropods.

This picture is a side view of the rock slab shown above. The up-current side of each ripple mark is low angle, whereas the down-current side is high angle. So, as viewed in this photograph, the wave current moved from right to left.

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