Mount Rushmore National Memorial is located along the northeast edge of what is known as the Harney Peak Granite Batholith in the Black Hills of South Dakota. A batholith is a geologic feature that formed by the cooling of a large igneous body of magma below the earth's surface; if a similar igneous body reaches the earth's surface, it would form a volcanic feature such as a lava flow. The Black Hills magma was emplaced into the older "host" mica schist rocks during Precambrian time, approximately 1.7 billion years ago!
The mica schist originated from the metamorphism (alteration by heat and pressure) of muds and sands from an ancient sea floor sometime prior to the emplacement of the Harney Peak Granite. Metamorphism of this original material produced the "slabby" appearance in the mica schist that now contains minerals such as muscovite, biotite and quartz.
The Harney Peak Granite (of which Mount Rushmore is carved) consists of fine-grained minerals including quartz, feldspar, muscovite and biotite. It is believed that these minerals formed approximately 8 miles below the earth's surface from molten magma. Some cracks developed as a result of the cooling of the magma and were later "patched" with molten magma. The result was the emplacement of pegmatite dikes that filled the fractures and zones of weakness in the granite. Today these pegmatite dikes are expressed as white streaks on the foreheads of Presidents Washington and Lincoln. Elsewhere in the Black Hills, economically significant mineral deposits are found associated with these pegmatite bodies.
The Harney Peak granite was likely exposed at the surface prior to Cambrian time, but was covered by sandy sediment when the Cambrian seas invaded the Black Hills some 550 million years ago. Today, these sands are part of the Deadwood Formation sandstones that contain grains derived from the ancient Harney Peak granite and the exposed Precambrian surface. The granite core of the Black Hills continued to be further buried during the rest of the Paleozoic and Mesozoic eras of geologic time and wasn't exposed to surface processes again until some 50 million years ago when today's Black Hills began to take on their present form.
Weathering and erosion have been carving these rocks since then, but the most noticeable "carving" occurred in the 20th century when Gutzon Borglum oversaw the project to construct Mount Rushmore as a "shrine to democracy".
The carvings occur within a granite sheet several hundred feet thick that has intruded the older schists. The irregular nature of the granite intrusion is noticeable just below the bust of George Washington, where the lighter colored granite sharply comes in contact with the darker schist.Shrine of Democracy
Sixty million years ago this land was in turmoil. Hills and mountains were being thrust up and gradually eroded. On the nearly indestructible granite face of one of these peaks, Mount Rushmore, the heads of four American Presidents have been carved in bold relief. These figures symbolize the birth and trials of the first 150 years of the United States. Individually they represent the ideals of the Nation.
- George Washington signifies the struggle for independence and the birth of the Republic,
- Thomas Jefferson the idea of representative government,
- Abraham Lincoln the permanent union of the States and equality for all citizens, and
- Theodore Roosevelt the 20th-century role of the United States in world affairs.
The Memorial Idea
In 1923, Doane Robinson, the South Dakota State historian, conceived the idea of carving colossal statues of romantic western heroes such as Jim Bridger, John Colter, and Kit Carson on the granite formations known as "the Needles" in the Black Hills. The proposal had only moderate public acceptance, and at times criticism of the project was severe. But Robinson was able to gain the influential support of South Dakota Senator Peter Norbeck and Representative William Williamson. Slowly public opinion changed, the memorial was authorized, and some funds were obtained to begin the work. Robinson invited the sculptor Gutzon Borglum to the Black Hills in the autumn of 1924 to study the proposal.
John Gutzon de la Mothe Borglum was born in St. Charles, Idaho, on March 25, 1867. He began painting at an early age, and in his early twenties sales of his works enabled him to study art in France for several years. It was there, in 1890, that he began to sculpt. His final paintings were completed in 1903, and from that time on he worked only as a sculptor. His fame grew, as did the size of his works. In 1915 he was asked by the United Daughters of the Confederacy to carve a head of Gen. Robert E. Lee on Stone Mountain in Georgia. Work did not begin until 1923, but some demands made by Borglum soon led to his dismissal. The invitation to the Black Hills presented him with an opportunity to create a monument whose dimensions would be "determined by the importance to civilization of the events commemorated". For this purpose a location other than the Needles was needed. After much searching Borglum selected Mount Rushmore because:
- it was smooth-grained granite,
- its 5,725-foot height dominated the surrounding terrain, and
- it faced the sun most of the day.
Carving the Monument
Work on the mountain began August 10, 1927, the same day President Calvin Coolidge officially dedicated Mount Rushmore as a national memorial. Fourteen years were needed to bring the sculpture to its present appearance, but because of delays caused by lack of funds and bad weather only 6½ years were actually spent in carving.
In the early years private donations supported the project, but when more funds were required the Federal Government assumed full financial responsibility. Federal appropriations accounted for $836,000 of the $990,000 spent on the memorial between 1927 and 1941. In March of the latter year Gutzon Borglum died. His son Lincoln, who had worked closely with his father on the monument, continued the project until funds ran out later the same year. Since then no additional carving has been done, nor is any further work on the memorial planned.
To say that Mount Rushmore was "carved" is to use a convenient figure of speech. Very few conventional sculpturing methods were employed in what was actually "a unique engineering accomplishment". Gutzon Borglum used the engineering techniques at Mount Rushmore that he had developed during his work on Stone Mountain. He first designed a grouping of the four Presidents to conform to the mountain's granite cap, but deep cracks and fissures' later discovered in the rock, required nine changes in the design. Five-foot models of each figure guided the workmen on the mountain. Measurements were taken from the models with a horizontal bar and plumb bob, enlarged 12 times, and transferred to the mountain. After a reference point, such as the tip of a nose, was located, excess rock could be removed with dynamite, often to within three or four inches of the finished surface. Some 450,000 tons of rock were removed in this manner.
Drillers, suspended over the face of the mountain in "swing seats" used jack hammers to honeycomb the surface with shallow holes at intervals of about three inches. The remaining rock was wedged off with a small drill, or a hammer and wedging tool. Finally the sculpture was smoothed with a small air hammer in a process known as "bumping."
The General park map handed out at the visitor center is available on the park's map webpage.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A geology photo album for this park can be found here.For information on other photo collections featuring National Park geology, please see the Image Sources page.
Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.
For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.
The NPS maintains a searchable data base of research needs that have been identified by parks.
A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.
NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
General information about the park's education and intrepretive programs is available on the park's education webpage.For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.