Mt. Rushmore depicts four American presidents of historical significance: Washington, Jefferson, T. Roosevelt, and Lincoln.
The sculpture was a massive undertaking; there is nothing else like it in the world.
To use circular logic, it is important because people go there to see it.
Mount Rushmore is not a landmark for North Dakota because Mount Rushmore is located in South Dakota.
Who built Mt. Rushmore
Mount Rushmore
Because they ran out of mountain to continue carving.
Mainly by carving it.
http://www.crazyhorse.org/
The carving of the faces on Mount Rushmore started in 1927, and ended in 1941.
carving of mount Rushmore was complete
Carving began in 1927 and ended in 1941, so it took 14 years to carve Mount Rushmore.
Bosun Chair
Carving the figures into Mount Rushmore took 14 years, October 4, 1927 to October 31, 1941.
Mount Rushmore itself is not a museum. There are two museums on the grounds of the memorial. There is a Sculptor's Studio where you can see the tools and techniques used in carving the mountain. The Lincoln Borglum Visitor Center has photos and videos about the history of Mount Rushmore.
Nothing of any great historical importance happened at Mt. Rushmore, unless you consider the carving itself to historical.