The Mount Rushmore Memorial was carved from a type of granite locally known as Harney Peak granite. This granite is fine-grained and has veins running through it, making it susceptible to cracking. After the carving of the sculpture was complete, sculptor Gutzon Borglum devised a special sealant to fill in the cracks that were already apparent.

In 1989, the National Park Service and the Mount Rushmore Memorial Society began studies to understand the structure of the mountain and the effectiveness of Borglum’s original sealant. Through these studies, the original sealant has been found to be ineffective and a modern sealant has been used to replace it. Furthermore, the major fractures and blocks of granite that make up the mountain have been identified and mapped. A special monitoring system has been installed to detect the slightest shifts in the sculpture’s granite.