Physics Photo of the Week

October 30, 2008

Ripples Old and New
Ripples cast in stone are not too unusual on the Earth.  The ripples pictured here are part of the Grinnell formation in Glacier National Park, Montana.  Dr. Tony Irving, University of Washington, is pointing out the petrified ripples.  These ripples were once the fresh loose sediment (mud, sand) in a creek, river bottom, or tidal flats from an ocean shoreline millions of years ago.  The flow of the water tends to generate the ripples as shown by recent ripples in the sandy bottom of the Swannanoa River in the picture below.

If the sedimentation was quick enough, the sediments rapidly
built up to thicker and thicker layers enabling the ripple marks to be preserved.  Later tectonic (mountain building) processes compressed and heated these sediments under several kilometers of earth for millions of years.  Eventually the high pressure and temperature caused a transformation: the mud and sands become fused together to form hard sedimentary rocks.  This process generates "metamorphic" rocks - rocks that have been formed from sediments, but fused together from the tremendous pressure and temperature to become very hard rocks.  Some of the layers were very highly metamorphosed into "quartzite" shown in the photo at bottom left.  In this photo, one can distinctly see the ripple-like rocks sandwiched between the much more metamorphosed quartzite rocks.

The pictures of the rock ripples were obtained during a Chautauqua course sponsored by the University of Washington on the "Tectonics of the Northern Rocky Mountains" during the summer of 2008, led by Tony Irving.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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