The Role of Rocks in the Civil War
Scott Hippensteel is an Associate Professor of Earth Sciences at the University of North Carolina at Charlotte
... I was retracing the path of an ancestor who fought at the Battle of Antietam. William Tritt was a member of the 130th Pennsylvania Regiment that assaulted the sunken road (Bloody Lane) at the center of the Confederate defenses. The rocks on this Maryland battlefield are completely different than those found at Gettysburg, only 60 km to the northeast. Instead of the softer sandstones and harder igneous rocks at the Pennsylvania battlefield, Antietam is underlain by carbonate rocks. These calcite-rich limestones and dolostones have weathering characteristics that create terrain that was exploited by skilled commanding officers – in some ways similar to what Union General Meade did with the igneous ridges at Gettysburg.
Apparently this is what happened with my ancestor’s regiment as his entire division approached the Confederate line -- they remained concealed behind the rolling hillcrests in front of the Confederate infantry and artillery. When finally taking up positions for firing and attack, they found themselves on a ridge of especially hard limestone only 75 meters from the sunken road, in a perfect position for enfilading fire from above. At such a distance they could fire and take cover behind the hillcrest to reload, or simply fire and charge with bayonets, knowing the Confederates could fire only a volley or two given the slow reloading rate of rifled-muskets in 1862.
The difference in the terrain across this portion of the Antietam battleground and that of the earlier, and bloodier, morning phase of the battle is the result of geology and climate. The carbonate rocks around the sunken road belong to the Elbrook Formation, a combination of softer limestone, dolostone, and shale. The weathering of this heterogeneous rock produces rolling terrain. Had the 130th been a part of the attacking regiments sent across the cornfield during the earlier morning-portion of the battle to the north, they would have been crossing a completely different type of limestone – a nearly pure carbonate that weathers with little relief. The flat terrain produced significantly higher casualty rates for the attacking soldiers. In short, the differential weathering between limestones created a terrain that favored the attack on the sunken road by my ancestor’s regiment ...
... By 1862 most portions of central Tennessee had been clear-cut of trees for agriculture. Around Murfreesboro, forests only survived if there was limestone close to the surface (preventing tillage). These pockets of woodland provided concealment for the attacking Confederates during their initial massive assault, reducing the effectiveness of both Union artillery and small-arms. The Union line faltered and fell back until the center of the line made a stand in the most ideal defensive position, geologically, to be found on a Civil War battlefield. Karrens (or “cutters”) form a series of natural limestone trenches that are approximately a meter and a half wide and deep. The resulting strong natural defensive breastworks allowed the Union infantry to slow the Confederate advance for many hours, ammunition allowing, until a new defensive position could be reorganized to the rear. It could be argued that these rocks saved the Union army from a complete disaster during the battle of Stones River ...
http://historynewsnetwork.org/article/162968
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Park rangers would love to hear of your find 