"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
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.
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