No Place to Run, No Place to Hide: Geology, Terrain, and Casualties at Gettysburg, Chancellorsville, and Antietam

Geologic and geomorphic analyses of the 25 bloodiest battles of the American Civil War show connections between bedrock geology, terrain, and casualties. We discuss three of the largest battles to further illustrate these connections. Gettysburg is located in a Mesozoic Basin composed of sedimentary rocks intruded by igneous dikes and sills. Cemetery Ridge, a fishhook-shaped diabase ridge occupied by the Union army, constituted key terrain; Confederate forces had to attack across open ground developed on soft siltstones and shales. Chancellorsville was fought on deeply weathered crystalline rocks in the Appalachian Piedmont province. The battlefield surface is highly dissected by small streams, ravines, and gullies. Key terrain includes a quartzite ridge occupied by Robert E. Lee's troops and high ground at Hazel Grove, a superb artillery position first held by Union soldiers and then by Confederates. Antietam battlefield, situated in the Appalachian Valley and Ridge province, has two distinctly different types of topography – open ground developed on relatively pure limestone and a dissected area formed on a shalier, more dolomitic carbonate formation. Carbonate lithologies such as this tend to produce a more open type of topography upon which higher casualty rates occurred. Key terrain includes the Cornfield, a sunken lane in the central part of the battlefield, and steep slopes near Burnside Bridge.