Carbonate rocks and American Civil War infantry tactics

Abstract

The most studied battleground from the American Civil War, from a geological perspective, is the rolling terrain surrounding Gettysburg, Pennsylvania. Here, the mixture of harder diabase and softer sedimentary rocks produced famous geomorphic features such as Cemetery Hill and Little Round Top that provided strong defensive positions for the Union Army. Another even more common type of rock—carbonates—provided similarly formidable defensive positions at numerous other battlefields in both the eastern and western theaters of conflict. Limestones and dolostones shaped the terrain of multiple important battle sites, including Antietam, Stones River, Chickamauga, Franklin, Nashville, and Monocacy, and these rock types proved consequential with respect to the tactics employed by both Union and Confederate commanders. On many battlefields, outcropping limestone proved beneficial for attacking troops; differential weathering within carbonate formations produced rolling terrain that limited the range and effectiveness of both artillery and small arms (e.g., “Sunken Road” at Antietam). Thin regoliths above limestone also prevented tillage, and the resulting forests provided concealment and cover for advancing troops (e.g., Stones River). From a defensive perspective, on a larger geographic scale, carbonates provided natural high ground from chert-enriched limestones and dolostones (e.g., Missionary Ridge, Chickamauga, Franklin, and Nashville). On a smaller scale, erosion of these same rocks produced karrens that provided natural rock-lined trenches for defending troops (e.g., “Slaughter Pen” at Stones River). Analysis of casualty figures (killed and wounded only) indicates similar losses (∼15% of troops engaged) whether soldiers were attacking across carbonate rock, non-carbonate rock, or unconsolidated sediments. Soldiers defending ground underlain by limestones and dolostones had a slightly higher casualty rate (14%) than those defending terrain above non-carbonate rocks or unconsolidated sediments (12%). This suggests, in a limited manner, that the local smaller-scale defensive advantages provided by limestone, such as karrens, were not as important as the regional-scale advantages for attacking troops, including rolling terrain or forest cover.