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By Lee J. Nelson
I always have been fascinated by the Mount Rushmore National Memorial (Keystone, S.D.); ever since noted New York Times Magazine photographer Sam Falk gave me an image he took of Gutzon Borglum’s sculpture. When Falk presented me with his photograph in the 1960s, little did anyone realize that the effects of erosion—although not yet readily apparent—already had taken hold.
Carving of the monument actually began in 1927 and finally was completed in 1941. Although jurisdiction over Mount Rushmore was ceded to the Department of the Interior’s National Park Service in 1933, nearly 50 years passed before general acknowledgment of the necessity for preservative action.
Weathering breaks down rock. While granite is fairly resistant, outcroppings constantly are subjected to the elements; causing gradual chemical and physical changes. Carbon dioxide and oxygen from runoff attrite substance. Physical erosion occurs when water, lodged in cracks, turns to ice and expands. Annual freeze/thaw cycles additionally force apart fractured edges. Tree roots produce a similar result which, ultimately, causes rock to disintegrate into clay and sand.
To safeguard the statues, experts would need to predict which of the granite blocks were most prone to shifting. In 1989, Respec Engineering, Inc. (Rapid City, S.D.) conducted a high-tech checkup that included photogrammetry and three-dimensional AutoCAD (AutoDesk, Inc., San Rafael, Calif.) modeling. Using precision cameras mounted aboard fixed- and rotary-wing aircraft, the survey comprised some 300 photographs. From those images, a computer created 3D projections of the monument’s internal fracture system. It was part of a sprucing up for the Memorial’s impending 50th anniversary. Apparently, in the 1930s, sculptors had noticed some crazing of the granite; but, the longer-term impact was unknown. Each fall, workers sealed cracks and fissures with a mixture of granite dust, linseed oil and white lead to reduce damage when cooler weather causes rock to contract. After the Respec structural inspection, preservationists were able to estimate the damage potential. They distinguished stable fractures versus those that should be filled. Eventually, the ones most in jeopardy would have to be reinforced with steel pins to arrest further movement.
Even though Rushmore looks like a single, solid chunk, it actually is a series of more than two dozen granite blocks. From 1999 to 2003, the National Park Service contracted Respec Engineering to install LVDTs (linear variable displacement transducers) at strategic locations. The apparatus—grouped in small boxes and attached to the rock—automatically took measurements every four hours and transmitted that information to a computer for subsequent analysis. Other sensors measured rock temperature. LVDT data show that rock typically moves in increments which range from five one-thousandths to five one-hundredths of an inch (127 μm - 1.27 mm). All motion during the four years of monitoring correlated with temperature fluctuation.