Physics Photo of the Week

September 21, 2012

Glacial Scarps (click on the photo for larger image)

The mountains surrounding Willoughby Lake in Vermont consistently show steep scarps facing the south (to the left in the panoramic photo).  Many people ask why all the steep sides face the same direction.  The answer is explained by the glacial history of the area.  Between 20,000 and 10,000 years ago the northeastern United States was covered by a large ice sheet - more than one kilometer thick - that moved slowly towards the south.  The ice in the glaciers itself does not carve mountains, but the rocks imbedded in the glaciers, under the tremendous weight of the ice above, does extensive scraping and cutting reforming the valleys as well as mountain tops.  These mountains are part of a monolithic granite pluton - perhaps the core of an ancient volcano about 400 million years ago.  As the millennia passed, glacier erosion has smoothed over the tops of the mountains.  The rounded tops is characteristic of all the mountains in the northern Appalachians.  However as a hard rock under the weight of the thick ice scrapes along the top of the pluton, parts of the pluton are chipped away causing the major scarps.  See the diagram below.

The mountain names in the photo are Mt. Hor - the large mountain at the left.  Wheeler Mountain is the just to the right of center.  Mt. Hor used to be called "Whaleback" in the 18th century and early nineteenth centuries.  The whale's blowhole is the dimple in the top and the whale's tale is obvious - unnamed in the official maps.  The "snout" of the whale is labeled Bartlett Mountain.  The photo at right shows a view from the cliffs of Wheeler Mountain showing a south-facing 4 ft scarp in the granite pluton produced by the rock-chipping by glaciers.  The granite is largely coated with green lichens.  A lichen is a symbionic community of algae and fungi.

I learned of the theory of the steep leeward sides of mountains formed by glaciers from a short Chautauqua-style course, Glaciers in Alaska, led by Dr. Kristine J. Crossen, a geology professor at the University of Alaska, Anchorage.  Many thanks to Dr. Crossen for an informative 3 days in June, 2012!

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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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