Dinojim.com - Geology Across America

Back in 2002, my girlfriend (at the time) and I were working our way through various parks along the east coast. We hit up a small park within Maryland and had some fun there. I don't have much in the way of geological related photos but lets see what we can ring out.

Antietam National Battlefield

To simulate Civil War war medicine, here I am having my leg amputated ("bite down on this"). This is because Civil War battlefields are notorious for quick and dirty surgeries, but can that be tied to the geology of the region? Battlefields, are often difficult to ascertain the geology of because, typically they are open, flat lying areas where the rocks are located below layers of soil. But that, in and of itself, is because of the geology.

Antietam is identified as the bloodiest battle in American history with over 12,000 American casualties over 3 hours of fighting. This is because the landscape of the area is made up of primary two types of bedrock, limestone and dolomite. These are represented by the Chambersburg Limestone (Ordovician age), New Market Limestone (Ordovician age), Pinesburg Station Dolomite (early Ordovician age), Rockdale Run Formation (early Ordovician age), Stonehenge Limestone (Ordovician age), and Tomstown Formation (Early Cambrian age).

Due to the erodibility of the limestone versus the dolomite, the limestone areas are often much flatter than the dolomite areas. This has a tendency to divert the fighting towards the limestone terrain. It has actually been shown that Civil War battles have a tendency to occur on limestone bedrock because of the flatter, open areas that they represent. And although limestone is known for its ability to dissolve in contact with water over time, without a caprock to prevent continuous dissolution, the area has a tendency to erode more evenly.

References

https://www.smithsonianmag.com/history/civil-war-geology-123489220/

https://irma.nps.gov/DataStore/DownloadFile/426451

Assateague Island National Seashore

Visited in 2002

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.

Assateague Island National Seashore

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.

Assateague Island National Seashore

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.

Assateague Island National Seashore

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.

Assateague Island National Seashore

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