Thursday, February 26, 2026

Geology in the Wild - Crazy Horse Memorial

During our travels from Utah to New York, we had been visiting numerous National Parks along the way. (You can read more about those in my Geology of the National Parks Through Pictures series.) We had also hit up some other sites. The first non-National Park geological site that we visited was the Crazy Horse Memorial in Crazy Horse, South Dakota. 

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View of the Crazy Horse Memorial on May 30th, 2021

The Crazy Horse Memorial is currently the largest mountain carving project in progress in the world. The work honors not only the Lakota leader Crazy Horse, but all of Native American culture. The planning for the Crazy Horse Memorial started in earnest in 1939 "when Lakota Chief Henry Standing Bear asked sculptor Korczak Ziolkowski to carve a memorial to the spirit of Lakota leader Chief Crazy Horse and his culture. 'The red man has great heroes, also,' Chief Standing Bear said." Deadwood.com. This is two years before the completion of nearby Mount Rushmore. The actual blasting and carving started on June 3rd, 1948, and continues to this day. 

Geological map of the Black Hills region with the Crazy Horse Memorial highlighted. Image courtesy of the NPS.

The geology of the Crazy Horse Memorial is nearly identical to Mount Rushmore, with both monuments being carved out of the Harney Peak Granite (so I apologize if some of the geological text is repeated here). The Harney Peak Granite is the central rock unit of the Black Hill Dome. The large geologic dome is a region where all of the land is bowed upwards, like an overturned bowl. After erosion, the result is a bullseye pattern of rocks, where the oldest rocks are in the center of the bullseye and progressively get younger towards the outside. 


The Harney Peak Granite started to form about 1.6 billion years ago, when, during the Proterozoic, magma began to work its way up through the rocks in this area. While still well below the surface of the earth, that magma cooled slowly to form the granite that makes up the carving stone for the Crazy Horse Memorial. The great thing about the Crazy Horse Memorial, from a geologists perspective, is that you get to take a piece of the mountain home. While the Harney Peak Granite magma was cooling, it cooled unevenly. This caused some portions of the rock to cool quickly, producing smaller, fine-grained, crystals, while other parts of the granite cooled more slowly with very large grained crystals. These large grained crystal granites are known as pegmatites. The granite with the finer grained crystals are easier to carve and are what comprises most of the Crazy Horse Memorial mountain. 

Cross section of the Black Hills. Image courtesy of A Textbook of Geology.

Between the formation of the granite 1.6 billion years ago and 500 million years ago, new rocks were deposited and eroded on top of the Harney Peak Granite batholith. However, due to the extreme hardness of the granite, the Harney Peak Granite remained behind while these other rocks had been lost to erosion and time. After this period of time, between 500 and 100 million years ago, there were some rocks deposited from which we do have remains of. Immediately on top of the granite is the green rock seen in the geological map above. This green rock, titled the "limestone plateau" on the map, can be seen surrounding the central granite bullseye. The "limestone plateau" is made up of several different rock layers and will be discussed in more detail in the Wind Cave National Park post (since that is where Wind Cave is located). After deposition of these rocks, the whole region started to be uplifted around 70 million years ago. This uplift is related to the uplifts seen across the Rocky Mountains at the same time. 

View of the Crazy Horse Memorial with the model for the final carving. Picture taken May 30th, 2021.

In geological terms, a dome is an anticlinal structure where the rocks dip gently away from the center in all directions. After folding, fracturing, and faulting, this causes the overlying rocks to break apart in the middle, allowing for easier erosion of the them. Once these younger rocks have eroded away, the older rocks are exposed with the oldest rocks exposed in the center. As before, due to the extreme hardness of the Harney Peak Granite, it withstood erosion and remained around much longer. The hardness of the Harney Peak Granite is also why the Black Hills have this large core of granitic mountain peaks that have not eroded away.

For comparison, here is my photo of the carving from when I visited the Memorial back in 1996. 

Although it is a bit fuzzy, I had visited the Crazy Horse Memorial back in 1996 during a cross country trip with my father. You can kind of see the differences between the two carvings, separated by 25 years. Progress is coming along slowly. Mostly it appears that in this time the finer details of the upper portions of the Memorial have been carved. 

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Monday, February 09, 2026

Geology of the National Parks in Pictures - Mount Rushmore National Memorial

My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 7th National Park along the way.


You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Obligatory entrance sign photo.

Geological map of the Black Hills, including Mount Rushmore (noted just below the slice through the map). Image courtesy of the NPS. 

Mount Rushmore is set within a large geologic dome. This is a region where all of the land is bowed upwards, like an overturned bowl. After erosion, the result is a bullseye pattern of rocks, where the oldest rocks are in the center of the bullseye and progressively get younger towards the outside. 

Tȟuŋkášila Šákpe, AKA Mount Rushmore, before being carved. Image courtesy of nativehope.org.

Although this could be considered outside the normal realm of geology, I do want to note that before being known as Mount Rushmore, the Lakota referred to the mountain as Tȟuŋkášila Šákpe, Lakota for The Six Grandfathers. The mountain named by Lakota medicine man Nicolas Black Elk after seeing a vision “... of the six sacred directions: west, east, north, south, above, and below. The directions were said to represent kindness and love, full of years and wisdom, like human grandfathers.”


As you drive up to the main entrance looking northeast, you first pass the profile view of Washington. From this direction you get a good view of the mountain sans most of the demolition work that was done with the carvings. You can see what would be the normally weathered surface of the granite before the carving brings out the fresh surface. This surface is also more reminiscent of the uncarved mountain seen in the image above. About 1.6 billion years ago, during the Proterozoic, magma began to work its way up through the rocks in this area. While still well below the surface of the earth, that magma cooled slowly to form the granite that makes up the carving stone for Mount Rushmore. This rock unit is known as the Harney Peak Granite. 


While the magma was cooling, it cooled unevenly. This caused some portions of the rock to cool quickly, producing smaller, fine-grained, crystals, while other parts of the granite cooled more slowly with very large grained crystals. These large grained crystal granites are known as pegmatites. The upper portion of Mount Rushmore is comprised mostly of the fine grained crystal variety, which is an easier rock to carve from. This large body of magma is known as a batholith. 


Between the formation of the granite 1.6 billion years ago and 500 million years ago, new rocks were deposited and eroded on top of the Harney Peak Granite batholith. However, due to the extreme hardness of the granite, the Harney Peak Granite remained behind while these other rocks had been lost to erosion and time. After this period of time, between 500 and 100 million years ago, there were some rocks deposited from which we do have remains of. Immediately on top of the granite is the green rock seen in the geological map above. This green rock, titled the "limestone plateau" on the map, can be seen surrounding the central granite bullseye. The "limestone plateau" is made up of several different rock layers and will be discussed in more detail in the Wind Cave National Park post (since that is where Wind Cave is located). After deposition of these rocks, the whole region started to be uplifted around 70 million years ago. This uplift is related to the uplifts seen across the Rocky Mountains at the same time. 

Google Earth image of the Black Hill dome. 

The uplift formed the dome that we had discussed above. This dome is easily noticeable in the aerial image of the region as well, as seen in the Google Earth Image above. This dome structure stretches across South Dakota and Wyoming, even up to the area in which Devils Tower is located. 


Cross section of the Black Hills. Image courtesy of A Textbook of Geology.

In geological terms, a dome is an anticlinal structure where the rocks dip gently away from the center in all directions. After folding, fracturing, and faulting, this causes the overlying rocks to break apart in the middle, allowing for easier erosion of the them. Once these younger rocks have eroded away, the older rocks are exposed with the oldest rocks exposed in the center. As before, due to the extreme hardness of the Harney Peak Granite, they withstood erosion and remained around much longer. Their hardness is also why the Black Hills have these granitic mountain peaks that have not eroded away. 


As you walk up to the main entrance to the main viewing platform, you come across a rather more grand entrance sign than the wooden one at the park entrance. Another interesting decision that was made for the refurbishment of the memorial in the 1980's and 1990's was the inclusion of several areas encased with granitic blocks. These granitic blocks are found within the Visitor’s & Interpretive Center, the Avenue of Flags, and Grand View Terrace, which, while they are granite, are not the Harney Peak Granite of the mountain. These granitic blocks were trucked in from elsewhere.


Here you can see the granitic blocks in the framing of the memorial at the distal end of the Avenue of Flags. In construction terms, these granitic rocks are known as "Rockville Beige granite" and are from the quarry company Coldspring. Quarried from Rockville, MN, this granite is more commonly known in geology as the Rockville Granite. 

Closeup of the Rockville Granite at Mount Rushmore

Known for its very large mineral crystals and minimal amount of metamorphism, the Rockville Granite presents as a very nice, consistent granite comprised mainly of quartz, feldspar, biotite, and hornblende with other accessory and background minerals. Minnesota has several granitic bodies that all date to around the same age. The Rockville Granite in particular is dated to being 1.812 billion years old (Ga), which is considered the Late Penokean of the Paleoproterozoic Era. When compared to the varying cooling rates of the Harney Peak Granite producing varying textures with metamorphic inclusions throughout, it is no wonder whey they decided to use a more picturesque granitic rock for the entrance and viewing terrace.

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