The geomorphology of Mount Eagle, Virginia, and the Civil War

Alexandria, Virginia — This is my second post about my new home in the Mid Atlantic. I retired from The College of Wooster in August of 2024, and just three weeks ago my wife and I moved from Wooster, Ohio, to Alexandria, Virginia, city just across the Potomac River from Washington, DC, to be closer to family. As a Wooster Geologist, I want to understand my new location in terms of the geology and how that geology has affected human history. Our condominium complex (never thought I’d say those three words together!) is on a dissected ridge called Mount Eagle. In the Civil War map above, Mount Eagle is rendered “Eagle Hill”. We will get to what all those fortifications are about!

My earlier blog post on the geology of Mount Eagle describes how I learned that the gravels on the top of this hill are Pliocene fluvial terrace deposits from the ancient Potomac River as it downcut its wide valley. The geomorphological result is an elevated ridge with a flat top and steep sides overlooking the modern Potomac and the cities of Alexandria and Washington. You may be seeing now why forts are going to be part of this story!

Just this week this sign appeared on the main hiking trail around the condominium campus. Dr. Greg Wiles taught me to be a bit suspicious of tree dates that do not involve tree-ring counting, but it is quite plausible this tree is at least 200 years old.

Here is more of the tree’s trunk. It is the widest tree I can see in our woods. Here is some information on Quercus montana. It likes rocky settings and ridgetops, which fits the setting for this specimen.

Mount Eagle was in the late 18th and 19th centuries a prominent site in the Alexandria area. Bryan Fairfax, a Scottish lord and clergyman, built a mansion and plantation on the top plateau overlooking the Potomac Valley. He was a buddy of George Washington, who visited often from his own nearby plantation at Mount Vernon. Yes, I’m now living in one of those “George Washington Slept Here” places. However, I’m haunted by the fact that there were also at least a dozen enslaved people on this land prior to the Civil War, and that Alexandria had one of the largest slave markets in the country.

This is a photograph of Alexandria looking south sometime between 1861 and 1865 (Library of Congress). In the background you can see the ridges that include Mount Eagle. On May 24, 1861, immediately after Virginia voted to secede from the United States, Union troops crossed the Potomac and occupied the city and surrounding region. Alexandria became the first Confederate city to be occupied (“liberated” would be my term). This was an essential move because the city had a critical port and was the terminus of two railroads. Also, those ridges to the south of the city could be fortified by the Confederates and used to shell Alexandria and Washington. Alexandria quickly became a logistical hub for the Union Army and Navy in the East.

Strong Confederate forces assembled south and east of Washington. Richmond, Virginia — the capitol of the Confederacy — was only about a hundred miles to the south of Washington. The first major battle of the Civil War was fought on July 21, 1861, at Bull Run, about 30 miles west of Alexandria. It was close enough that civilian sightseers from the Washington area could travel to the battle for entertainment. To the shock of the northern establishment, the Confederates forced the Union Army into a catastrophic retreat, winning the battle and immediately threatening the city of Washington. The capital of the United States was quite suddenly vulnerable to an attack from the Rebels. Now that geology we learned lays its critical role.

Soon after Union troops occupied northern Virginia in May 1861, they began to clear the woods and dig artillery emplacements in the ridges south and east of Alexandria and Arlington. The defeat in the First Battle of Bull Run showed that these fortifications must be extended and improved rapidly. General George B. McClellan, the new commander of the Army of the Potomac, ordered an extensive and elaborate ring of forts, redoubts and trenches to encircle the capital region, from the left flank south of Alexandria circling around to southeastern Maryland. The map above shows the dozens of forts in the Washington region, all designed to be within an easy cannon shot of each other and eventually all connected by trenches. From 1862 to the end of the war in 1865, Washington was the most fortified city in the world. The red arrow on the map above points to the Mount Eagle area.

This sketch map by an unknown Union officer is now in the Library of Congress. Southwest of Alexandria is Fort Lyon, one of the largest forts, sitting atop “Eagle Hill”, which is our Mount Eagle. You can see its strategic value as it controlled land access to the capital and Alexandria from the west and south, and could shell targets in the Potomac River.

In this closer view of another map, the flat surfaces of the fluvial terraces are apparent. They made it much easier to construct fortifications on the ridge heights. In the upper right of this map are two buildings referred to as “Johnsons”. The larger one is the Mount Eagle mansion built by Bryan Fairfax. The modern condominium complex where we live occupies the footprint of that mansion and extends west to part of Fort Lyon.

Here is a view inside Fort Lyon in 1863 showing the 26th New York Infantry assembled on the parade grounds. (Library of Congress.)

These gun crews are on station in Fort Lyon. This is an undated image from the Library of Congress. Despite several alarms, Fort Lyon and this southern end of the Washington defenses was never attacked. The forts, then, did their job of protecting the capital. There was, unfortunately, significant loss of life anyway at Fort Lyon. Over twenty soldiers were killed and twenty wounded on June 9, 1863, when the powder magazine exploded by accident.

What is left today of Fort Lyon and its associated defenses? The fort itself is completely covered with modern structures, mainly the Huntington Metro Station, some office buildings, and part of the condominium campus. The above sign is the only visible marker of the fort.

However, a tiny bit of the defensive system can still be seen. In this map Fort Lyon is seen connected to the smaller forts Weed and Farnsworth. Fort Farnsworth is connected by entrenchments to Fort O’Rourke. Between Farnsworth and O’Rourke is a cannon battery in earthworks along the trench.

The site of that battery has been preserved in this patch of woods on the western edge of Mount Eagle Park.

Inside these thick woods are the remnants of gun emplacements. It doesn’t show well with photography because it is so overgrown, but the trench is in the middle, foreground to background. On the right side is the embankment behind which the cannons were placed. See below an example of such a redoubt.

This image is from another Washington fort showing how the Mount Eagle battery guns were positioned.

So now I have at least an early understanding of the surficial geology of our new home on Mount Eagle in Alexandria, Virginia. Our steep hill is topped by a plateau formed from a fluvial terrace of the ancient Potomac River. This explains the topography and the abundant pebbles, cobbles and boulders on the surface. These terraces are higher than Alexandria and Washington to the north, so occupying them in force was necessary to defend the capital from Confederate attack. The flat terrace surfaces and deep, gravelly soil made construction of large fortifications relatively easy. I now feel at least somewhat oriented in my new Virginia home!

Source: https://woostergeologists.scotblogs.wooster.edu/2025/05/10/the-geomorphology-of-mount-eagle-virginia-and-the-civil-war/