Geology in the Wild - Adirondack Mountain Reserve

Back in January of 2022, we were invited for a winter hike within the Adirondack Mountain Reserve, and while we had a fantastic time, there is something more important underfoot that needs to be discussed. And that is the geology! 

Gates to the Adirondack Mountain Reserve. Photo by Gwendolyn Craig from Adirondack Explorer. 

The Adirondack Mountain Reserve (AMR) is a small park located within the larger Adirondack Park near the small town of Keene, NY:

Location map for Adirondack Mountain Reserve (from the NYS DEC)

Per the AMR website:

The mission of the Adirondack Mountain Reserve is to preserve and protect the forests, lakes, mountain streams, and wildlife on our wilderness tract for the enjoyment of our members and, under the terms of a Conservation Easement in perpetuity to the State of New York, members of the public. We will preserve the forever wild character of this forest land and our property at St. Huberts in a manner appropriate to allow the enjoyment of their wilderness beauty. The mission also includes the operation of a club, with associated facilities, for the pleasure and respectful recreation of its shareholders. 

Adirondack Mountain Reserve map. A digital version can be found the New York Almanac 

The park essentially starts in the northeast with the East Branch Ausable River valley. The entrance gate is located along this northeastern border of the Reserve. The park follows the river valley to the southwest through the Lower and Upper Ausable Lakes with another unnamed lake (or at least a lake I can't figure out the name of) situated at the southwestern point of the reserve boundaries. 

Map of the park with some notable points highlighted.

There are several spots within the park where the geology of the region can easily be seen. However, since we were there in winter, even those locations had less than ideal exposures. These spots that we stopped at are highlighted on the map above.

Beaver Meadow Falls

Hiking southwest from the entrance gate one of the first locations that you can see the geology is at Beaver Meadow Falls. Waterfalls are always a great place to check out the geology.

Rainbow Falls

Continuing towards the southwest along the trail, you then come across Rainbow Falls.

Indian Head Vista

And eventually you can make your way up to the Indian Head Vista, with an absolutely gorgeous view of the Lower Ausable Lake. 

View of the rocks on the Indian Head Vista. 

Pictured above are the rocks at the vista itself. However, if we look at the geological map below, it can be noticed that almost all of the rocks within the Adirondack Mountain Reserve are actually the same rock. They are nearly all the orange color, which is denoted as "a" on the Geological Map of New York. 

Geological Map of of the Adirondack Mountain Reserve. The black box indicates the extents of the map pictured above. 

Reading the Legend from the New York State Geological Map itself, I warn you, there is some heavily geological terminology in the paragraph ahead. Per the Legend, the rocks within the park, denoted as "a", are:

Black to light gray or greenish-gray metanorthosite and anorthositic gneiss (Marcy facies); overprint signifies mafic mineral content in excess of 10 percent (mainly gabbroic or noritic metanorthosite and subordinate anorthositic metagabbro or anorthositic metanorite-Whiteface facies). Subordinate members of this series grade through metagabbro or metanorite, to ilmenite-magnetite-rich metagabbro or metanorite, to pyroxenite rich in ilmenite-magnetite, to ilmenite-magnetite ore. Primary subophitic texture is variably destroyed by granulation and recrystallization resulting in a crystalloclastic fabric. Metamorphic minerals include garnet, clinopyroxene, quartz, hornblende, and a plagioclase less calcic than the original. Mafic mineral percentage is contoured in northwestern portion (St. Regis Quadrangle) of the Marcy Massif, contour value being shown on high side of contour line. 

While that is a lot of technical jargon, I will try and break it down. The rock unit that the AMR sits on it known as the Marcy Anorthosite Massif, named after Mount Marcy, the highest point in NYS, which is located just to the west of the AMR. Anorthosite is an igneous rock made up almost entirely of the mineral plagioclase feldspar. And not only does this rock body cover the park, it is massive in size.

Large view of the Adirondack region of New York, showing the full extent of the Marcy Massif. 

Typically when we have mountain, we expect the geology to be jumbled up with many types of rocks overlaying each other. And that is actually what we see for the most part, outside the orange colored Marcy Anorthosite Massif. The Marcy Massif though sits at the center of the mountains, and it is in essence, the core of a dome structure that makes up most of the Adirondack Mountains.

The core of the Adirondack Mountains, the Marcy Massif, is ~1.1 billion years old (Ga), and covers an area of 3000 square kilometers (~1150 square miles). The rock was initially formed as an igneous intrusive magma body that cooled slowly over time, forming the plagioclase rich anorthosite. After formation this rock underwent periods of metamorphism where parts of the rock were altered, mostly along the outer edges. That is a lot of what is in the description of the rock unit above. These metamorphosed regions have various amounts of minerals that are specific to metamorphic rocks.

Cross section of the Marcy Massif across the Adirondacks. Image courtesy of Regan et al., 2019.

The formation of the Adirondack Mountains is where the unique status of the Adirondack Mountains comes into play. While many people may think that the Adirondack Mountains are just a subsection of the longer Appalachian Mountains, they are, in fact, their own distinct set of mountains. The Appalachian Mountains formed when North America slammed into Africa, forming the supercontinent Pangea, and lifting those mountains upwards as a result of the collision around 300 million years ago. Since the breakup of Pangea, these mountains have generally remained inactive and have been slowly eroding away over time. While the core of the Adirondacks formed during the Grenville Orogeny, over 1 billion years ago, the mountains themselves started to be uplifted ~20 million years ago. The reason for this uplift is currently unknown but the result was a dome structure with the Marcy Massif in the center. The way that domes form, it causes the younger rocks to erode away from the central area of the dome due to fracturing experienced during the uplift. This fracturing and erosion is the reason we have some of the oldest rocks in the Adirondacks exposed in such a huge degree. 

The result is that the AMR sits near the center of a massive dome structure, comprised almost entirely of some of the oldest rocks found within the Adirondack Mountains and there are plenty of places within the park to check them out. (Note: there are some slightly older rocks found elsewhere in the Adirondacks, but not nearly as notable as the Marcy Massif.)

References

https://www.adirondackexplorer.org/communities/economy/adirondack-mountain-reserve-hiking-reservation-system-to-enter-4th-year/

https://www.newyorkalmanack.com/2026/04/adirondack-mountain-reserve-2026/

https://dec.ny.gov/places/adirondack-mountain-reserve-conservation-easement-tract

http://hikeamr.org

https://nysm.nysed.gov/publications/map-chart-series

https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/4/1240/4799269/1240.pdf

https://pubs.geoscienceworld.org/msa/ammin/article-abstract/103/10/1523/548583/In-situ-dating-of-metamorphism-in-Adirondack?redirectedFrom=fulltext

https://www.adirondackexplorer.org/adirondacks-almanack/the-adirondacks-spectacular-historic-and-geographically-unique/

https://wildadirondacks.org/adirondack-geology.html

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.