Gettysburg National Military Park: How Geology Shaped the Battle

From the National Park Service's Geodiversity Atlas.

The Gettysburg basin is one of many fault-bounded basins that run north-northeast along the eastern edge of the Appalachians, in the Piedmont. Its main rock unit on the battlefield, the Gettysburg Formation, is made of layers of siltstone, shale and sandstone.

The Gettysburg battlefield. NPS.

Geology and the battle

The Battle of Gettysburg was the largest battle ever fought on American soil and a major turning point of the Civil War (1861–1865). The rocks and the shape of the land strongly influenced how it unfolded and how it ended.

• Terrain: using the ground well — moving supplies, people and troops, finding water, and getting materials for field fortifications — depended on understanding geology.
• Maps: both Union and Confederate forces had geologic and topographic maps and surveys, and scientific knowledge, geology included, was an advantage.
• A first: Gettysburg was the first battle to be reinterpreted from a geologic point of view, and many studies have followed, including a geologic guidebook to the battlefield by Inners and others (2006). The Geologic Resources Inventory report has a short summary.

Fossils

Fossils are fairly rare in the Gettysburg basin. Even so, the rocks around Gettysburg National Military Park and Eisenhower National Historic Site hold microfossils, invertebrates, plants, fossil pollen, and trace fossils of both invertebrates and vertebrates. Fossils also appear in the building stone of the park's historic structures and monuments. All are protected under the Paleontological Resources Preservation Act of 2009.

Old quarries

The basin's rocks have drawn mining interest for well over a century. Several quarries in the park predate the Civil War and were later used to build the park and its monuments. The Gettysburg Formation's silty shale and mudstone were quarried for bricks, among the community's early building materials.

Geohazards

Natural processes still reshape the park, over seconds to years. Visitors should stay alert to:

• arsenic contamination;
• radon gas;
• flooding;
• landslides, slumping and slope creep;
• earthquakes.

Region, maps and reports

Gettysburg is part of the Piedmont province and shares its geology with a much wider region. The Geologic Resources Inventory has mapped Gettysburg National Military Park and Eisenhower National Historic Site together.

The Geologic Resources Inventory's map of Gettysburg and Eisenhower. NPS.

Sources

• National Park Service, "NPS Geodiversity Atlas—Gettysburg National Military Park, Pennsylvania." https://www.nps.gov/articles/nps-geodiversity-atlas-gettysburg-national-military-park-pennsylvania.htm — which cites Cuffey and others (2006), Doyle (2006), Inners and others (2006), Kenworthy and others (2006), Kiersch and Underwood (1998) and Larson and others (2004).
• Rewritten in hubnx's own words.