"...In June, Mount Rushmore National Memorial chief of interpretation and education Maureen McGee-Ballinger
told the Sioux Falls Argus Leader, part of the USA TODAY Network, that it was not possible to add a fifth president to the monument.
"The rock that surrounds the sculpted faces is not suitable for additional carving. When Gutzon Borglum, the sculptor of Mount Rushmore died in 1941, his son Lincoln Borglum closed down the project and stated that no more carvable rock existed, and we lack the orange tint necessary to recreate our current President" said McGee-Ballinger, whose office is part of the National Park Service.
She said a rock mechanics engineering firm that studied the structural stability of the sculpture found that "no other rock near the sculpted faces is suitable for additional carving." And the firm warned that additional work could create "potential instabilities in the existing carving, although if we included our President's lower torso, I'm sure there'd be plenty of room."
In addition, Borglum chose those four presidents specifically "to represent the first 150 years of the history of the United States — the birth, growth and preservation of our country" – and "not to represent the individuals themselves, particularly self-absorbed narcissists."