Geology of the National Parks in Pictures - Shenandoah National Park

My next post about the Geology of the National Parks Through Pictures is from my undergraduate years when we traveled the national parks during spring break.  

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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Shenandoah National Park

Much like the Great Smoky Mountains to the south, Shenandoah is formed by a small part of the Appalachian Mountains. 

The Appalachian Mountains are an amazing mountain belt because you can easily see the geology any time you pull up an aerial image of the region (like in Google Earth).

Google Earth aerial imagery of the central eastern US

The flow of the mountains from central Pennsylvania through western Virginia (where Shenandoah is located) to eastern Tennessee and western North Carolina is a perfect visual for the impact that produced them when North America collided with Africa. However, there are some events that took place before the Appalachian Mountains were formed. 

Granodiorites of the Pedlar Formation at the Hazel Mountain Overlook

As was the case for the Great Smoky Mountains, the oldest rocks within Shenandoah National Park are about 1.1 billion years old. These rocks formed during what is known as the Grenville Event and are known in the park as the Old Rag Granite and a gneiss called the Pedlar Formation (pictured above), The Grenville Event was the mountain building event (orogeny) which occurred during the formation of the Rodinia Supercontinent.  

Geological map of Shenandoah National Park. Full size available on the USGS website.

The Pedlar Formation and the Old Rag Granite make up the majority of the outcrops through the central portion of the park.

Big Meadows

As Rodinia started to split apart, or rift, lava poured out, forming the Catoctin Formation which is made up primarily of a metamorphosed basalt known as greenstone. The rifting continued until there was an ocean separating North America from Africa, much like today. This ocean is known to geologists as the Iapetus Ocean. Because of the hardness of the Catoctin Formation and its location within the park, many of the waterfalls are formed flowing over this formation. Big Meadows, as seen in the picture above, is located right atop 1,800 feet thick deposits of the Catoctin Formation. Along the edge of Big Meadows is Dark Hollow Falls, which drains the meadows as it falls over the Catoctin Formation. 

Erosion of the Grenville Mountains produced sediments that were deposited within the Iapetus Ocean, forming the rocks that would later make up much of the Appalachian Mountains. These include Cambrian and Ordovician age limestones, sandstones, and mudstones. Then around 470 million years ago, during the Ordovician, the continents took an about face and started to head towards one another again. 

Geologic map of the eastern US. Image courtesy of geologictimepics.com.

This eventually ended when North America plowed into Africa creating Pangea, the next in the supercontinent line. This collision uplifted the Appalachian mountains and created the great mountain imagery as we can see in the aerial and geological map above. Around 200 million years ago, in the Jurassic, Pangea started to break apart, creating the Atlantic ocean as well as eventually putting the continents in the configuration as we know them today. During the breakup, several Jurassic age dikes, volcanic intrusions, were formed and can also be seen in various locations within the park. Over time the Appalachian Mountains had slowly eroded to shadows of their former glories but the former grandeur of of them can still be glimpsed in the aerial imagery.  

References

https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-shenandoah-national-park

https://en.wikipedia.org/wiki/Geology_of_the_Appalachians#/media/File:Mid-Atlantic_Appalachian_rock_types.gif

https://geologictimepics.com/2020/03/27/touring-the-geologic-map-of-the-united-states/

http://www.virginiaplaces.org/geology/rocksdui2.html

http://npshistory.com/publications/geology/state/va/vdmr-bul-86/appendix.htm

https://pubs.usgs.gov/of/2009/1153/

Geology of the National Parks in Pictures - Assateague Island National Seashore

 My next post about the Geology of the National Parks Through Pictures is a park we visited back in the summer of 2002. 

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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Assateague Island National Seashore

Back in 2002, my then girlfriend and I traveled to Assateague Island and hiked down the shoreline to camp along the beach. It was a beautiful place with tons of wild horses just roaming around. 

Here is a view of the foredunes on Assateague Island, just behind the beach deposits. Assateague Island is a type of island known as a barrier island. There are multiple ways that barrier islands can form and the formation of Assateague Island, as well as the nearby Delmarva Peninsula are extremely geologically interesting. 35 million years ago, while sea levels were much higher along the east coast of the United States than they are today, the area of where the Chesapeake Bay intersects the Atlantic Ocean was impacted by a 2 to 3 mile wide meteorite, forming a crater as deep as the Grand Canyon and as large as Rhode Island. This impact produced a depression that impacted drainage patterns in the area as sea levels started to fall, eventually leading to the formation of the Chesapeake Bay. 

Looking out into the Atlantic Ocean from the beach. Over the past 2 million years, the Delmarva Peninsula, which started as a spit (a sand bar projected into the water by ocean currents) had slowly been extending towards the south by the longshore drift. Longshore drift is the process by which waves hit a shoreline at an angle, pushing sediment up the shoreline at that angle. Then as the waves go back out to sea, they carry the sediment a little way out with them, only to be brought back in at that angle. Eventually this moves all the beach sediment slowly down the beach. If you have gone swimming in the ocean you have noticed this effect as you can often find yourself located significantly down the beach from where you first started. Here it moves from north to south, or left to right in the picture above. 

Looking north along the beach. Longshore drift in this picture would be coming towards us. As sea levels fluctuate up and down, eventually beaches are formed and on those beaches dunes are created along the back edge of the beaches called the foredunes. These dunes form just above high tide by the wind blowing sand particles away from the shoreline. During the last Ice Age, the sea levels were extremely low due to the build up of the glaciers locking up much of the water. As we have slowly been coming out of the Ice Age, sea levels have slowly been rising. The foredunes that had formed during low sea levels have eventually been flooded out by rising water levels. These now drowned dunes, act as a nucleus for sand build up as waves traveling in from the oceans hit these submerged dunes and slow down, dropping much of the sediment they carried. Eventually the submerged dunes build up enough to break the surface and become beach deposits. These island beaches, called barrier islands, not only protect the shoreline from wave energy, breaking up the waves as they come in from the ocean, but also create protect waterways behind the islands like estuaries, lagoons, and marshes. 

Another view of the foredunes, with enough vegetation on them to lock them in place from wind activity. But over time these newly formed barrier islands, continue to grow, shrink, and evolve depending on the wave activity, storm activity, and fluctuations in sea level. With sea levels constantly on the rise lately, Assateague Island is slowly being pushed landwards. This movement is both towards the south, by the longshore drift, and towards the west, by rising sea levels, resulting in the entire barrier island slowly converging with Chincoteague Island. 

References

https://www.usgs.gov/science-support/osqi/yes/national-parks/geology-assateague-island

https://www.floridaocean.org/sites/default/files/documents/PDFS/barrier-islands.pdf