“The whole battle is about trying to take an igneous intrusion away from another army,” says Robert Whisonant, a geologist at Radford University in Virginia. Whisonant is quoted in Erin Wayman’s fascinating article about geology and the Civil War, which appears in the November issue of Earth magazine (formerly Geotimes.)
Wayman focuses on two of the most important battles in the war: Antietam and Gettysburg. Whisonant and his fellow researcher, Judy Ehlen, observe that one of the deadliest battlefields at Antietam was the Cornfield, where three times more solders died than at the adjacent Burnside Bridge. They attribute the totals in part to the Cornfield’s underlying limestone, which has weathered flat, whereas the more resistant dolomite and shale of Burnside had eroded into a safer terrain of hills and ridges, allowing soldiers to hide and maneuver unseen.
Whisonant’s quote refers to Gettysburg where a diabase (dark igneous rock) had intruded into softer sediments and formed a boulder-rich, hilly terrain. The Union army held the 25-meter-high Cemetery Ridge and the Confederates were on the lower Seminary Ridge. Between the two was a plain, formed by the erosion of the soft sediments. More than 12,000 Confederate soldiers died when they tried to cross the unprotected plain during the infamous Pickett’s Charge. With that loss the South retreated.
For me, the other interesting story of Gettysburg is the stone itself. The battle-influencing diabase formed around 200 million years ago, as North America and Africa separated during the breakup of Pangaea. The split produced a series of rift valleys that stretch up the Atlantic Coast, including one now found in Connecticut and Massachusetts where dinosaurs left behind thousands of tracks. Collected in the middle 1800s by Edward Hitchcock, the tracks can be seen at the Amherst College Museum of Natural History. The area is also famous for producing a brown sandstone used in the building trade throughout the east. The rock is better known as the material covering hundreds of brownstone rowhouses.
If you are interested in more information, the Pennsylvania Geological Survey has two publications on line. The first is a reprint of a 1962 report on Gettysburg and the second is 2008 field guide to the battlefield. The National Humanities Council also has a great reference spot with more articles by Whisonant.
General Robert E. Lee may have been a brilliant tactician but he appears to not have been a good geologist. But in his defense, geology was still a young science and most of what we know now could not have even been imagined in the 1860s.