Physics Photo of the Week

September 26, 2014

"Bear Paw" Granite
This granite outcrop on a popular hiking trail on Flying Mountain in Acadia National Park, Maine, reminds me of the large toes on a bear's paw - at least the pastry version of a bear paw.  When the granite - a mixture of silica, feldspar, and mica - formed from intruding magma hundreds of millions of years ago, the gradual cooling "shock" caused fractures to appear in a roughly rectangular 3-dimensional grid.  Thus the "bear paws' toes" are granite cubes roughly 16 inches across making natural stair steps on the trail.

Another outcrop in the same area is shown in the picture at right.  The flat surface of the granite has been worn down by the eroding power of glaciers - the last of which receded only 10, 000 years ago.  The cracks were formed very early after the magma solidified and provided narrow channels for water seepage, root penetration, and more rapid erosion.  On the shoulder of a mountain, these cubes would become chipped away by the passage of the glaciers.  Click on the right-hand photo for a larger image.

The stress cracks in these granitic rocks are rectangular.  Polygonal columns are also reasonably common - especially in solidified basaltic lava deposits.  Lava columns are shown in the "Sheepeater Cliffs" of Yellowstone National Park (below).  A very readable article on geologic cracks and their significance by Harmon D. Maher can be found in in the following link: http://maps.unomaha.edu/maher/STEP07/supportinfo/cracks.html.


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 dcollins@warren-wilson.edu.

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