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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Crazy Horse Memorial

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

https://www.deadwood.com/business/attractions/crazy-horse-memorial/

https://www.nps.gov/moru/learn/nature/geologicactivity.htm

https://www.govinfo.gov/content/pkg/GOVPUB-I29-PURL-gpo79412/pdf/GOVPUB-I29-PURL-gpo79412.pdf

https://etc.usf.edu/clipart/63000/63044/63044_black_hills.htm

Geology in the Wild - The Thistle Landslide

Landslides are unfortunately a common problem around the globe, especially in areas of growth where humans alter the landscape. But landslides can happen even when there isn't human activity to artificially steepen slopes or reduce the cohesiveness of the land surface. One such landslide was the Thistle Landslide, that ended up destroying the town of Thistle, Utah.

The picture above is overlooking the landslide itself, which came down the valley in the center of the picture. The landslide filled the Spanish Fork River valley, which is located down below the train tracks as seen here. Fall of 1982 and winter/spring of 1983 had been extra wet, and a late snowfall in April along with a quick thaw caused 15 million cubic meters of earth to start sliding down the slope. The valley was populated with eroded debris from the North Horn and Ankareh Formations (a mixture of shales, silstones, and sandstones) that were nestled in a "trough-shaped depression", AKA a paleovalley.   

The slide started on April 13th, causing the Denver and Rio Grande Western Railroad tracks to start shifting over a few inches (the railroad was previously located further downslope than it is located today). By the 15th, the railroad had been shut down and the surface of the road, Highway 6, (also located along the valley floor) started to buckle. By the 17th, despite attempts at dredging the river, it was deemed impossible and the town of Thistle, located around the mountain towards the left of the photo, was abandoned. The landslide drowned the houses of Thistle and the entire town was left as a ghost town. At top speed the landslide reached speeds of 3.5 feet per hour and was 1000 feet wide, 200 feet thick, and over a mile long. The part of the landslide that dammed the Spanish Fork River edned up being about 220 feet high. 

View of Thistle Lake after the landslide. Image from the USGS. 

After the landslide, the railroad and river had to be rerouted through the mountain as seen in the image above and the highway was rerouted to just behind where I took the picture. In total, direct damage cost over $200 million (in 1983 dollars) and remains one of the costliest landslide in US history. 

References

https://www.wunderground.com/blog/weatherhistorian/worst-landslides-in-us-history.html

https://www.utahhumanities.org/stories/items/show/94#:~:text=A%20giant%20landslide%20in%20April,Salt%20Lake%20City%20with%20Denver.&text=By%20Sunday%2C%20Thistle's%20fate%20was%20sealed

https://geology.utah.gov/map-pub/survey-notes/geosights/thistle-landslide/

http://luirig.altervista.org/pics/index4.php?search=Thistle,+Utah,+landslide+April+1983.+Lake+Thistle+was+created...&page=1

Geology in the Wild - Roadside Faults

While traveling home a couple of years ago, I happened to notice some nice faults along the side of the road. So, like any good geologist, I got out to take a few pictures.


The location is in Utah, within the Eocene Green River Formation east of the Tie Fork Rest Area between Price and Provo along Route 6. It's the fancy newish rest area that has a bunch of train displays.


The image below was taken along Route 6 facing north, with the west to the left and east to the right. You can get a pretty good view of it within Google Street View Here.

West                                                                                                                                      East

Taking a closer look at the western fault above.


And an even closer look with the fault highlighter. The beds show significant offset by matching up that double bed located just above the arrows in the image below.

 This type of fault is what is known as a Normal Fault, as illustrated in the image below. It results from extensional forces where each side of the fault are moving away from each other. A Reverse Fault would be where the two blocks are being pushed towards each other.

Looking at the eastern portion of the top image.

We can see there are two, nearly vertical, faults lying very close to each other.

The bend in the fault line above is just due to the degree of erosion revealing the fault at a weird angle and also the perspective from which I am viewing the fault. In actuality, the fault is not bent like that.

Both of these faults are also Normal Faults because of the same extensional activity as to the west. This makes sense because if the fault to the west resulted from extensional activity, it would be highly likely the entire area, if not the region, had undergone extensional pressures. This means that these couple of faults here would probably not be the only normal faults in the area and there are  more normal faults spread throughout the area and/or region.

This is what is known as a scientific hypothesis. We looked the available data on the region and came up with a testable hypothesis based on it.