(CNN)Here's some background information about the Mount Rushmore National Memorial, composed of sculptures of the faces of former Presidents George Washington, Thomas Jefferson, Abraham Lincoln and Theodore Roosevelt.
Mount Rushmore National Memorial Fast Facts
The monument is located on Mount Rushmore in the Black Hills of South Dakota.
Sculptor Gutzon Borglum selected the four presidents to represent major events in the history of the United States: Washington represents the birth, Jefferson symbolizes the growth or expansion, Lincoln illustrates the preservation and Roosevelt embodies the development.
It is carved in Harney Peak granite and each head is about 60 feet tall.
The sculpture's rate of erosion is estimated at one inch per 10,000 years.
It stands 5,725 feet above sea level.
Approximately two million tourists visit each year. In 2019, 1,963,540 people visited the monument.
Following 9/11, the Parks Service made various security enhancements, such as installing security fencing, lighting and gates; improving existing mechanical systems and purchasing all-terrain vehicles for use in patrols and at special events.
1923 - The idea is suggested by Jonah "Doane" Robinson, secretary of the South Dakota State Historical Society.
October 1, 1925 - Mount Rushmore is dedicated as a national monument.
October 4, 1927 - Work begins on the monument. About four hundred workers sculpt the monument using dynamite and drills. Workers use much smaller models (about one-twelfth the size) to envision the final product.
July 4, 1930 - The section of George Washington is dedicated.
June 10, 1933 - Franklin D. Roosevelt issues Executive Order 6166, making Mount Rushmore part of the National Park Service.
August 30, 1936 - Thomas Jefferson's section is dedicated.
September 17, 1937 - Abraham Lincoln's section is dedicated.
July 2, 1939 - The Theodore Roosevelt section is dedicated.
March 6, 1941 - Gutzon Borglum dies. His son, Lincoln Borglum, completes work on Mount Rushmore.
October 31, 1941 - Drilling on the monument is complete. It only takes six and a half years of actual work on the monument, but the Great Depression slowed down its completion due to lack of federal funds.