Monday, April 06, 2026

Geology of the National Parks in Pictures - Badlands National Park

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 9th 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.

The Badlands are a very geological park, where the geology is the focal point of the beauty of the park. Even the term "badlands" is a geological term meaning a landscape made up of generally soft sedimentary rock that erodes easily and has a lot of streams eroding the landscape with little plant life to inhibit the erosion.  The main road, Badlands Loop Road, that travels through the park, actually does not cover the vast majority of the park, which is much more difficult to get to. However, there are still a ton of geological sites that can be viewed from the main road. Below is a picture from the visitor's center of the overview of the geological stratigraphic column featuring the main park formations. 

Geological sign from the Badlands NP Visitor's Center

The oldest formation, at the base of the park, is the Pierre Shale. The Pierre Shale is Cretaceous in Age (~75 to 69 million years old) and formed when this area was covered in a vast inland sea known as the Western Interior Seaway. The Pierre Shale covers the lower levels of the landscape towards the northwest of the park, away from the main road. There are some places along the main road that you can see the Pierre Shale, such as at the Pinnacle Overlook, near the western entrance to the park. The shale is at the base of the erosion escarpment here. Within the Pierre Shale, several fossils can be found such as ammonites and mosasaurs. These lived at the same time as the dinosaurs, but since this area was covered in water during the Age of the Dinosaurs, dinosaurs were not to be found. 

View from the Pinnacles Overlook

The yellow layer immediately above the Pierre Shale is known as the Yellow Mounds. It is a weathered extension of the Pierre Shale, that was altered when the Western Interior Seaway went away. Within this layer, which can be plainly seen at the Yellow Mounds Overlook pictured below, a rainforest grew up within the Pierre Shale and converted the upper portions of the shale into a soil, or what is now known as a paleosol (an ancient soil). 

View from Yellow Mounds Overlook

Above the Pierre Shale is the Chadron Formation. The Chadron Formation is Eocene in age (~37 to 34.3 million years old) and was formed along an ancient river floodplain. The Chadron Formation is made up of light gray claystone beds and represent an environment similar to the Everglades today. Fossils found here are alligators, early horses, and the large brontothere (AKA titanothere) mammals. In the above image, the Chadron Formation starts just above the red layer and continue upwards until they hit the layer with multiple horizontal layers within it in the left background. That is our next youngest formation, the Brule Formation. 

Conata Basin Overlook

The Brule Formation is Oligocene in age, deposited 34 to 30 million years ago, and is an interbedded siltstone and sandstone deposit with a lot of clay creating a pale-pink deposit with lots of beds. This was formed when the environment started to dry out from the floodplains of the Chadron Formation and became a savannah with rivers periodically meandering across the environment. Fossils include those of oreodonts, animals related to modern bison and camels, early dogs, and nimravids, animals that looked like saber-toothed tigers but were only distantly related. In the picture above, the Brule Formation can be seen in the upper portion of the outcrop, essentially in line with the fencing on the left side of the photo, and stretching across the view. It is also the entirety of the rock formations in the picture below (the Wild River Valley Overlook) and creates the tops of most of the erosional escarpments seen along the Badlands Loop Roads. 

Wild River Valley Overlook

The youngest rock deposit within Badlands National Park (although there are many younger sediments across the park, this is the final layer within the Badlands escarpment) is the Sharps Formation. The Sharps Formation is an Oligocene Age (30 to 28 million years old) sandstone river channel deposit. This formation can be seen in a few places in the park and is present only at the very top of some of the erosional peaks. Below it can be seen along the Fossil Exhibit Trail as the upper half of that big monolith in the middle of the image. The line about halfway up is the Rockyford Ash layer, a deposit created from a distant volcanic eruption 30 million years ago. The Sharps Formation sits directly on top of the Rockyford Ash Layer. 

Fossil Exhibit Trail outcrop view
 
This entire region then began to erode away, producing the pinnacles and peaks that we see today. This erosion started around 500,000 years ago as the Cheyenne and White Rivers eroded their way through the landscape. Because all of these rocks are very, very soft, they are eroding at a rate that is much faster than many other landscapes, especially those further west. It is estimated that 1 inch of rock is eroding away every year and that the entire badlands will be gone in the next 500,000 years. This rate is 10,000 times faster than the granitic rocks forming the center of the Black Hills where Mount Rushmore and the Crazy Horse Memorial are located.  

Geological column with the fossils displayed on the Fossil Exhibit Trail

Towards the western end of the Badlands Loop Road is the Fossil Exhibit Trail. The Fossil Exhibit Trail is a wonderful, 0.2 mile "trail", along a boardwalk with bronze exhibits of fossils with displays explaining the animals to whence they belong. As the trailhead sign states, there are seven types of animals displayed along the trail. As the climate changed in the area, each of these animals had to either move, adapt, or extinct. Since it is a loop, the fossils are laid out chronologically. We are going to go backwards through time, starting with the most recent fossils along the trail, and ending with the oldest fossils. You can see a great layout of the stratigraphic section (geology rock layers) and their associated fossils in the image above that was at one of the trailheads for the Fossil Exhibit Trail.

Oreodont fossil (Leptauchenia) display along Fossil Exhibit Trail 

The Oreodont fossil above, Leptauchenia, was found within the Rockyford Ash layer, and Leptauchenia lived from around 32 to 28 million years ago. An oreodont is what is known as an artiodactyl, which is an even-toed hoofed mammal. These include bison, big horned sheep, and camels, of which the camel is the closest living animal to the Leptauchenia.  

Dying to become a fossil exhibit

Along with the fossils, there are also these great displays which shows additional information beside just the fossil. The display attached to the Oreodont fossil is called "Dying to Become a Fossil" and illustrates some of the processes that an animal takes in order to become fossilized. 

Nimravid (Nimravus) fossil display along the Fossil Exhibit Trail

The next fossil is the Nimravid (Nimravus) fossil, which was found in the Brule Formation and lived around 32 to 30 million years ago. Nimravids are animals that resembled saber-toothed cats, of which these cats are only very distantly related. Nimravids fall under the family Nimravidae, as opposed to cats, which are Felidae, however they all fall under the order Carnivora, along with dogs and bears. Some of the differences between Nimravids and true cats is that the nimravid skulls are much shorter than cat skulls and they walked flat footed, as opposed to cats who walk on their toes.

Fight for Survival

This display, which was paired with the Nimravid skull, displays another aspect of paleontology, taphonomy. This is where paleontologists look at the various aspects of the fossil and try to discern things like how the animal died. In this instance, there is evidence by the hole punctures in the skull of one nimravid, that it was attacked by another one. 
 
Dog (Hesperocyon) fossil exhibit along the Fossil Exhibit Trail

This dog fossil (Hesperocyon) is an early ancestor to modern day dogs and was also found in the Brule Formation. This dog lived around 37 to 28 million years ago. 

It's a Dog's Life

Associated with the Hesperocyon display is the "It's a Dog's Life" display, showing the eventual evolution of the Hesperocyon into modern day dogs as we know them. 

An early horse, Mesohippus, display fossil on the Fossil Exhibit Trail

Next up on the Fossil Exhibit Trail is the Mesohippus, an early ancestor to modern day horses. Unlike modern day horses, who only have one toe, Mesohippus had three toes. Mesohippus lived from 37to 30 million years ago and this fossil was found within the Chadron Formation. 

Off to the races

Alongside the Mesohippus display is "Off to the Races", a paleontological display discussing adaptation. Mesohippus's adaptation was the reduction of digits in it's foot from 5 toes, seen in earlier ancestorial horses, to the three toes in Mesohippus, that will eventually evolve into the one toe of modern-day horses. 

Alligator fossil along the Fossil Exhibit Trail

The next fossil is that of an alligator skull, which lived in the region from 34 to 37 million years ago. This fossil was found within the Chadron Formation. 

See Yea Later, Alligator

The associated sign with the Alligator skull discusses the size disparity between ancient alligators and their modern day descendants. As can be seen with the above skull, modern day alligators are much larger, over twice as long in fact, than this ancestor.

Titanothere (Megacerops) lower jaw fossil exhibit from the Fossil Exhibit Trail

Titanotheres, also known as brontotheres, are very large hoofed mammals that evolved during the Eocene (~50 million years ago) and later went extinct in the Oligocene (~28 million years ago). They lived in Asia and North America and several well known species can be found across rocks of western North America. This Megacerops lower jaw fossil was found within the Chadron Formation, just above the Yellow Mounds layer. They lived in this area from 34 to 37 million years ago. 

Titanic Discovery

The associated sign with the titanothere fossil discusses the process of early field work, where paleontologists go out into the field and dig out the fossils, often with the aid of knowledgeable locals, who initially find the fossils eroding out of the landscape. 

Ammonite (Placenticeras) fossil exhibit along the Fossil Exhibit Trail

The final fossil on the Fossil Exhibit Trail (or the first, if you went in the opposite direction than I did), and the only fossil that isn't of a vertebrate, is the ammonite fossil Placenticeras. Ammonites are ancient relatives to octopi, squids, and nautiloids. This fossil, since it is the only marine fossil, was found in the only marine deposits, the Pierre Shale, located at the base of the rocks within the park. Ammonites lived within this area from 75 to 67 million years ago, before going extinct shortly thereafter 65.5 million years ago. 

Under the Sea

The associated sign with the ammonite fossil is "Under the Sea", that describes that throughout time the environmental conditions have changed. What is now the badlands, was once under water during the Cretaceous within the Western Interior Seaway. 

A display of several fossils found within the Badlands National Park at the Ben Reifel Visitor Center.

At the eastern end of the Badlands Loop Road is the Ben Reifel Visitor Center, and here there are several actual fossil exhibits, as opposed to the "fossils" on the Fossil Exhibit Trail which are clearly casts designed to weather the elements and visitors. The above display shows fossils from the Brule Formation. These include a Subhyracodon skull (top of picture), which was a hornless rhinoceros, a Leptomeryx, a small deer-like animal, (top right), and the Archaeotherium (large fossil on the bottom). The Archaeotherium was a "big pig", that although looked like a pig was only a distant relation to modern day pigs and was more closely related to hippos. 

A display of more fossils found within the Badlands National Park at the Ben Reifel Visitor Center.

The above shows another collection of fossils from within the park. These were found within several of the rock formations. This includes a titanothere femur and skull, an alligator skull, and a tortoise. There are also several more fossil exhibits within the visitor's center and it is a stop well worth taking your time in. 

References
https://www.britannica.com/animal/titanothere
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Friday, March 20, 2026

Dinos in Pop Culture - The Wall Drug, SD

Continuing my travels across country, moving from Utah to New York, we have stopped at our next non-National Park geological stop. Well ... maybe not "geological" per say, but it does have a dinosaur!

Wall Drug, SD


The Wall Drug is in my opinion THE road side attraction. Located just off Interstate 90, the Wall Drug is a massive Drug Store (?) located in the town of Wall, SD. But it is so much more than that. The reason many people visit the Wall Drug is their constant barrage of billboards as you drive down the highway for hundreds of miles in all directions. Per their website they also have signs globally: 
Over the years, Hustead’s Wall Drug signs could be seen in sites as distant as Morocco, Amsterdam, and London, and during World War II, American G.I.s carried the Wall Drug message across the globe, proclaiming how many miles it was back to Wall Drug.
And what started out as a drug store where you could get "free Ice Water", has turned into an ever expanding cavalcade of displays, stores, food, and attractions. But, the one attraction that got my attention was the animatronic Tyrannosaurus rex (AKA a T. rex), located towards the back of the store. 


And like a good animatronic, just taking a picture isn't enough, so here is a video of it in action.


Obviously this display is reminiscent of Jurassic Park, which also included a T. rex under lockdown. To give a general background on the T. rex, the first Tyrannosaurus rex skeleton was discovered in 1902 by Barnum Brown, the then assistant curator for the Department of Vertebrate Paleontology at the American Museum of Natural History (AMNH) in New York. The bones, discovered in the Hell Creek Formation near Hell Creek, Montana, were identified by a local land owner who then told Brown. The skeleton, only 10% complete, took 3 years to excavate and was then transported to the AMNH where the paleontology curator of the museum, Henry Fairfield Osborn, named the fossil in 1905 Tyrannosaurus rex, meaning "Tyrant Lizard King". 

Original mount of the T. rex. Image courtesy of the AMNH

All fossils of the T. rex have since been found in western North America, ranging from southern Canada down into southern New Mexico and Texas. The T. rex was also one of the last living non-avian dinosaurs, having lived during the late Cretaceous period from about 68 to 66 million years ago, which is when the meteorite struck the Earth, wiping out those non-avian dinosaurs. Although the skeleton discovered initially by Brown was only a partial skeleton, an even more complete skeleton was discovered by Brown 6 years later that became the basis for the skeleton seen above, mounted at the AMNH. It is this mount that went on to influence pop-culture for most of the next hundred years, where the T. rex was almost always portrayed in a vertical pose. Unfortunately this pose was set up mostly because of the steel armature could not do a more dynamic pose. Many of the tail vertebrae even needed to be broken in order to get the T. rex to be standing like this. 

And of course, me with the T. rex during the times of Covid.

It wasn't until 1993's Jurassic Park, that movie and tv makers really started to take a look at the advancing science behind the dinosaurs and adjusted their models accordingly. For the movie, the T. rex is presented in a much more accurate model, where the body of the beast is balanced over the legs, with the tail acting as a counterweight, forming a giant see-saw. This remodeled T. rex also gives us a much more accurate depiction of its size, where an adult can grow up to ~12 feet tall at the hip and ~40 feet long from tip of the tail to snout. But one of the most notable features of a T. rex were its teeth, that ranged in size up to 12 inches long, which included the root, still leaving approximately a 6 inch tooth exposed. And despite this animatronic being installed in the Wall Drug around 2006, it still does seem to maintain that more upright, incorrect, pose, than the one more closely associated with the Jurassic Park franchise, where the T. rex is balanced horizontally across its legs. The teeth however in this model do appear to be appropriately sized, although they be a little small in comparison to the smaller size of this T. rex compared to a full grown one. 
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Friday, March 13, 2026

Geology of the National Parks in Pictures - Wind Cave National Park

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 8th 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

Geological map of the Black Hills, including Wind Cave National Park (noted just below the southernmost point of the purple rocks). Image courtesy of the NPS.

Like the Mount Rushmore National Memorial and the Crazy Horse Memorial, Wind Cave National Park is located within the Black Hills dome. But unlike those parks, Wind Cave National Park is not located within the Harney Peak Granite. The rocks in this region started forming about 1.6 billion years ago, during the Proterozoic. This is when magma began to work its way up through the surrounding rocks in this area at the time. While still well below the surface of the earth, that magma cooled slowly to form the Harney Peak Granite. Due to the hardness of the granite, from its formation at 1.6 billion years to about 500 million years ago, several rocks formations were likely deposited on top of it and then eroded away leaving no trace.  However, starting around 500 million years ago, new rocks that were deposited on the Harney Peak Granite have been preserved. These rocks include the green "Limestone Plateau" rocks as pictured in the map above. 

Geological map of Wind Cave National Park. Map courtesy of the NPS. 

Legend for the Wind Cave NP Geological Map. Image courtesy of the NPS. 

The "Limestone Plateau" can be subdivided into numerous different rock formations, once we zoom in on the part of the dome within Wind Cave National Park. Deposited directly on top of the Harney Peak Granite is the Ordovician Age Deadwood Formation (~480 million years old). This is a mixture of sandstones and limestones, as well as some other rocks, that represent a transgression, where the sea level started to rise and the oceans started to cover this region. On top of that is the Mississippian Age Englewood Limestone (~ 363 –358 million years old). The Englewood represents a shallow marine environment. 


And on top of the Englewood Formation is the primary rock of interest for Wind Cave National Park, the Pahasapa Limestone (AKA Madison Limestone). This is a Mississippian Age limestone that dates to about 358 to 341 million years old. This is the limestone from which Wind Cave is formed within. The Pahasapa Limestone is up to 420 feet thick and formed from the deposition of seashell fragments within a warm shallow sea across the region. Following deposition of the Pahasapa Limestone, the seas started to recede away ~320 million years ago. This is when the first parts of Wind Cave started to form. 

When the seas started to come back, (~300 million years ago) rocks were deposited on top of the Pahasapa Limestone. This time they were made up of limestones, sandstones, and red clay shales. This formation, called the Minnelusa Formation, created a semi-permeable barrier on top of the Pahasapa, limiting water flow down through the rock from the surface. An interesting thing to note is that the Minnelusa Formation can actually also be seen within Wind Cave as well, with some of the red clay visible in higher parts of the cave near the Garden of Eden and Fairgrounds rooms. The inclusion of the Minnelusa Formation within Wind Cave is how scientists know that the cave started forming over 300 million years ago, making it one of the oldest known cave systems in the world. Following deposition of the Minnelusa Formation, sea levels continued to rise and fall, slowing dissolving out the cavern system. This transgression-regression cycle of the seas continued until around 70 million years ago when the entire area started to be uplifted into the Black Hills dome. As the dome was being uplifted, fracturing of the rocks allowing more water to flow through the rocks, quickening the pace of cave formation.

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. 


Most limestones are comprised primarily of the mineral calcite, which is a variety of calcium carbonate (CaCO3). Calcite is the mineral that most sea shells are made out of. Calcite has a physical property that it will dissolve in slightly acid waters, which is where you get the formations of caves. One of the unique properties of the Pahasapa Limestone though, is that when it formed, it had significant quantities of gypsum, another type of mineral, within the rock. Gypsum (CaSO4) has the ability to absorb large quantities of water, and when it does so it, expands and then contracts when that water is expelled. Over time, as water entered and left the limestone, the gypsum expanded and contracted, fracturing the surrounding rocks. As time progressed, the water that flowed through these fractures within the limestone slowly started to replace the gypsum with calcite. 

Boxwork Formations

Geologically speaking, Wind Cave is most well known for its boxwork speleothems (cave formations) (as seen in the image above). The boxwork is a result of these gypsum fractures that were refilled with calcite. The calcite crystals that filled in the fractures were more resistant to dissolution than the surrounding limestone was, so as the limestone rock dissolved away, revealing the cave, the calcite crystals that filled the gypsum fractures remained behind. These calcite crystals are what primarily form the boxwork speleothems. The fracture pattern, and therefore the speleothem pattern, forms a very angular, boxy configuration, giving the speleothems the term boxwork. 

Map of Wind Cave

Even looking at a map of the cave itself, it also forms an angular and boxy configuration, producing a fractal pattern (where a pattern is repeated over different scales). Overall, the cave descends 643 feet and extends over 167 miles (as of 2024). As the dome was uplifted, the water started to drain out of the cave about 40 to 50 million years ago. The current day water levels are located about 500 feet below the surface in an area of the cave known as "the Lakes". 


Although the most well know, boxwork isn't the only cave formation within Wind Cave. There are other formations such as popcorn ceiling and frostwork. However, formations such as flowstones and dripstones, like stalactites and stalagmites, are more rare here due to the drier climate and the semi-permeable Minnelusa Formation overlying the cave that limits the amount of water flowing through.  

References
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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. 

References
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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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