ripple marks

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Andreas Feininger
Untitled / Aeolian (wind-formed) ripple marks give texture to these sand dunes in California’s Death Valley, n.d. [rippled dunes, shadow at left, mountains in distance]
[rippled dunes, dark rocks and slopes in distance]
Gelatin silver prints.

Which way is up?

No, your eyes aren’t deceiving you, these rock layers are nearly vertical! 

Although they were once horizontal, these Paleozoic sedimentary units have been tilted so that they now dip nearly 90 degrees.

These rocks are located in northeast Tennessee and are part of the Appalachian foreland fold-thrust belt. Though built by multiple orogenies, the Alleghenian Orogeny (325-260 Ma) was the final stage in forming the Appalachian Mountains and resulted in a series of imbricated reverse faults.

Consequently, rocks got compressed, thrusted and tilted. In order to determine whether these rocks were rotated clockwise or counter-clockwise from their original horizontality, we have to look for up-direction indicators (i.e. indicators that will tell us which direction originally faced upwards). These may include: graded bedding, cross-bedding, sole markings, ripple marks, etc.

Image by author | Outcrop along Hwy 70 near Rogersville, Tennessee

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These ripples are preserved in ancient Triassic sediments of the Sulphur Mountain Siltstone that were once on the bottom of a shallow sea. Currently sitting high in the mountain range of Western Canada, these rocks can be viewed along highway 40 in the Highwood Pass in Kananaskis, Alberta, Canada.