Monday, February 20, 2012

Minnesota Geology Monday - Mount Rushmore

Minnesota Geology Monday will move the borders to South Dakota in the spirit of President's Day.  Mount Rushmore National Memorial is managed by the National Park Service ( and brings almost two million people to the area anually.  This Google Earth image shows the immediate area around Mount Rushmore in the Black Hills.

Construction of the monument began in 1927 and ended in 1941.  Original plans called for the presidents to be sculpted from the waist up, but due to lack of funding, the sculpture was limited to their heads.  Each head is roughly 60 feet tall.  George Washington, Thomas Jefferson, Abraham Lincoln and Theodore Roosevelt were chosen for their roles in the United States during the first 150 years of the nation’s existence.

Gutzon Borglum selected Mount Rushmore for several reasons.  First, the granite weathers slowly, allowing sculpting.  Second, Mount Rushmore is the tallest mountain in the area (5,725 above sea level).  And lastly, the mountain faces southeast, which would allow the workers to be in the sunlight most of the day.

Mount Rushmore is composed of granite of the 1.6 - 1.8 billion year old Harney Peak granite batholith.  Pegmatite dikes are also found throughout the region and can be seen as streaks in the Lincoln and Washington heads.

The Harney Peak batholith intruded into earlier mica schists.  During the Paleozoic Era, sedimentary rocks were deposited onto the granites during the Cambrian.  The Black Hills was uplifted 70 million years ago during the late Cretaceous, during the Mesozoic Era.  This uplift allowed erosion to remove the softer sedimentary rocks and exposing the granite.

Originally, the Jefferson sculpture was going to be placed on Washington's right, but the granite turned out to be inappropriate for sculpting.  The work that had been done in the area was later smoothed by dynamite blasts.

Beneath the sculpture, the talus pile remains from construction of the monument.

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