The Rocks at Gettysburg

Geology and the Battle

    Tourists used to   the vocabulary of battle expect to hear cannons, cavalry, and rifles are not ready to hear a discussion of diabase, shale, and sandstone. However, understanding the geology of Gettysburg is a key to understanding troop movement and activity as well as the famous fishhook line of the Union. The first term to know is diabase, seen here at Devil's Den. Diabase is a term that is confusing because the same composition of rock is called dolerite or black granite in other areas. An igneous rock, diabase is very hard; it is also speckled and is sometimes used for monuments, as at Gettysburg. Diabase is prone to spheroidal weathering, one of the terms that I always remember from my geology classes because it is so easy to spot.
Spheroidal means rounded, or like a sphere, so there aren't sharp corners. Weathering refers to the process of wearing away, by the action of water, cold, or pressure or by chemical actions. The odd rounded shapes that the diabase outcroppings have is a result of these actions. Pictured above is Devil's Den, located on Sickles Avenue, west of Little Round Top. Like other diabase features in Gettysburg National Military Park, Devil's Den was formed 200 million years ago when the molten igneous rock flowed through the sandstone and shale of the plain. The varied shapes at this location are the result of differential weathering. Some of the cracks are actually ancient "cooling fractures" which began millions of years ago; others occurred more recently as the result of weathering.

 In Pictures After the Battle

     This famous (or infamous) staged picture shows a Confederate soldier who was killed in the fighting near Devil's Den. After the battle, his body was moved to this site for a picture titled "Confederate sharpshooter at Devil's Den."  This position probably was used by a sharpshooter, but not this man. Iconic photographs of Devil's Den over many years have made this rock outcropping among the most famous of the rock formations in the United States

How Did Diabase Affect the Union Positions?

     Any military strategist or history student knows the importance of the high ground in a battle. Through some planned decisions, and some mere coincidences, the Union came to hold a series of higher points that formed a letter J or a fishhook. These high points are composed of diabase. From Culp's Hill to the east and north to the end of the hook at Little Round Top, the Union positions were on the diabase outcrops -- places where the harder, resistant diabase had not worn down over time. Cemetery Ridge, the center of the Union line which was attacked on July 3, 1863, rises about 570 feet above sea level. At the hook of the Union line, Big Round Top is 785 above sea level and Little Round Top is 650 feet above sea level. These higher places were the positions the Union successfully defended. You might notice differences in color or texture in the rock outcroppings as you walk across parts of the battlefield. These are a result of slight differences in mineral composition in the various rocks and boulders. You will also see the deep tan color of the sandstone and the red of the shale at lower elevations, such as the railroad cut along Reynolds Avenue.

     The diabase bedrock and boulders had several other effects on the soldiers during the three days of battles. First, boulders provided cover from which sharpshooters and others could fire at the enemy. The diabase rocks that lie along the hillside of Little Round Top were used in that way, as were the famous positions at Devil's Den. A problem was created by the diabase, however. Soldiers were in general unable to dig trenches into the hard, resistant diabase that was close to the surface. At many places, you can see the diabase appear almost like a floor of stone.

     Reading the Rocks

    Few witness trees remain at Gettysburg. Many of the field hospitals are private residences. Criss-crossed by avenues which did not exist in 1863, the terrain offers little that remains as the men of the Army of the Potomac and the Army of Northern Virginia saw it. But the rocks remain. Time -- even a hundred fifty years--has little effect on the hard diabase born in the molten depths of the earth. The rock outcroppings and weird juxtapositions still are as they saw them. When we touch the rough surface of a boulder, our experience is just as theirs was. Perhaps that is why so many frequent visitors have an affinity for the rocks at Gettysburg. We remember standing in a certain place, beneath a rock or beside a boulder, and thinking about those who fought here. For many of the soldiers who survived and were present at the placement of the monuments, the diabase rocks at Gettysburg served as a guide. They remembered the rocks and used those locations to decide on where the right and left flanks of their line stood, and where the center of the line was in 1863.

     Back along an old trail on Big Round Top is this, the second monument to the 118th Pennsylvania Infantry Regiment. This monument, placed atop a boulder on the rock strewn hillside of Big Round Top, commemorates the service of the regiment known as the Corn Exchange regiment. The Maltese cross visible on the monument side indicates that it was part of the Fifth Division. In an earlier post, you will find a position marker for an Ohio regiment on Culp's Hill perched atop a huge boulder in the forest. The trees were ripped to shreds on Culp's Hill by days of fierce fighting, but the rocks remained.

For further information:

http://www.dcnr.state.pa.us/topogeo/education/es5/es5.pdf