sedimentary, wisconsin

Ripple Marks

If you’ve ever looked through clear waters at a beach or gentle flowing river to the sand below, you’ve likely seen ripple marks formed by the moving water.

The image above actually shows preserved “fossil” ripple marks in the 1.7 billion-year-old Baraboo (Wisconsin) quartzite. These ripple marks were formed when the marine sandstone was initially deposited and then were later metamorphosed. However, these ripple marks lie along a vertical rock face, meaning that these rock layers have nearly been overturned from their original horizontal position!

Ripple marks can be useful as paleocurrent indicators—sedimentary structures that help to determine the direction in which water flowed in the geologic past.

Symmetrical ripples are formed by the back and forth motion of waves, like at a beach. Asymmetrical ripples (which have a gentle up-current slope and a steeper down-current slope) can be formed in rivers and also in aeolian environments.

So although we’ll never actually be able to travel back to a Wisconsin beach nearly 2 billion years ago, we can still get pretty close!

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