Library of CongressUnion soldiers dig a trench on Morris Island, South Carolina, during the Civil War.
What do the battlegrounds at Gettysburg, Kennesaw Mountain, and Murfreesboro have in common? From a tactical perspective, large portions of these fighting fields resisted digging, prohibiting entrenching as a defensive force multiplier. The thin soils and shallow bedrock that rendered the pick and spade worthless are elements of local geology, and the presence of a particular rock type anywhere can influence everything from the lay of the land to the effectiveness of artillery to the sight lines across a battlefield.
Gettysburg is the most studied battle in terms of both military and natural history. Starting around the centennial of the July 1863 engagement, historians and geologists began to explore how the rocks there influenced the fighting. One overarching theme emerged from this collaborative research: older, softer sedimentary rocks produced flatter fields at Gettysburg, while slightly younger, and much harder, igneous rocks formed hills and ridges and outcroppings as fascinating terrain features like Devil’s Den.
Another oft-repeated story about geology and combat at Gettysburg explains the lack of trenches across the fields south of town: The same hard rock that produced the coveted high ground also barred any type of digging. Instead of digging trenches, soldiers piled the diabase boulders in the tens of thousands to form bulletproof walls.
This claim about the geological impossibility of entrenching at Gettysburg is ubiquitous. In my own recent work on geology and the Civil War, I have come across dozens of such explanations of the futility of “digging in” at Cemetery Ridge. Certainly there are some portions of the battleground at Gettysburg that were impossible to excavate. However, the majority of the battlefield, including much of the elevated terrain, is underlain by thick soils fully capable of being excavated for trenches and earthworks. Consider, for a moment, the name of one of the battlefield’s iconic terrain features: Cemetery Ridge, where about 10 years earlier Evergreen Cemetery had been established as a public burial ground.
Scott HippensteelDiabase, a durable igneous rock, still shows on the Brian Farm at Gettysburg. James Longstreet’s attack on July 3, 1863, proceeded across this field toward the stone wall in the foreground. The reason the men of the 12th New Jersey Infantry didn’t dig entrenchments here is plain to see.
In short, the fluidity of the battle lines, the abundance of preexisting breastworks in the form of stone walls, and the desire to avoid digging up the dead, explain the lack of entrenching by the Union army at Gettysburg more than does the geology they met underfoot.
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Rock type, more than any other factor, determines the nature of battle terrain. The most interesting geology and the most compelling terrain result from a mixture of different types of rocks. Igneous rocks, which were once molten material, and metamorphic rocks, which have been changed by pressure and heat, tend to be harder and more resistant to erosion. As a result, they form higher, more defendable ground like at Kennesaw Mountain, Little Round Top, and South Mountain. Different types of sedimentary rock can have the same effect of differential weathering, where softer rock like shale forms valleys and harder rock like dolostone or sandstone forms ridges. This is why the battlefields at Monocacy, Perryville, Antietam, Chickamauga, and Stones River are so similar: They are all underlain by the same combination of different carbonate rocks resulting in interesting combinations of rolling and undulating terrain.
When Braxton Bragg’s Confederate Army of Tennessee arrived along Stones River at the end of 1862, the local geology prohibited extensive entrenching on much of what would become battleground, and the rocks also influenced the defensive tactics in more subtle, underappreciated ways. The rocks on this Tennessee battlefield are twice as old as those at Gettysburg, and formed in a completely different environment. (The geology of the Gettysburg Battlefield is identical to that at Second Manassas—two battlegrounds unlike any others from the war.) The limestone at Stones River is close enough to the surface to prohibit entrenching on much of the landscape, although one portion of the fighting ground—not coincidentally at the heart of the Union defensive line—had very unusual outcrops called “karrens.” Karrens (or cutters) are an uncommon geological feature that resemble a series of natural limestone trenches. If situated perpendicular to the approaching enemy (as they were in 1862), these rock-lined gullies are ideal for providing protection and cover to soldiers firing muzzle-loading rifles.
Scott HippensteelKarrens provided an excellent defensive position for Philip Sheridan’s men at Stones River. The rocks also impeded the movement of pursuing Confederate artillery after Sheridan’s men ran low on ammunition and abandoned the position.
The carbonate at this battlefield had other influences on the fighting as well. By the 1860s, most of Rutherford County had been converted from dense forest to farm fields. Rocky areas were left as woodlands and, as a result, geology indirectly determined where forest cover was available to troops for concealment or protection. In essence, limestone played a role, if incidental, in determining the sight lines for the soldiers and commanders and the timber available for the construction of breastworks or fill-material for earthworks.
At the start of the Civil War, geology was in its infancy as a science. Officers took coursework in topography and field fortifications, both topics related to geology, but the underlying reason behind the changes in relief—the rock under the surface—was not part of the curriculum. Examples of how geology influenced the fighting could not have been hard to see for an officer or a common soldier. Take, for example, one critical portion of the Battle of Second Manassas: Members of Stonewall Jackson’s corps who are desperately defending Stony Ridge run out of ammunition and begin throwing stones. Interestingly, the rocks being thrown with concussive effect at Second Manassas have the same chemistry, age, and origin as those being piled at Cemetery Ridge and Little Round Top in anticipation of a Confederate assault.
While the influence of geology on the fighting in the Civil War was not understood in the 1860s and only first studied in the 1960s, the debate over the degree of influence continues to this day. Historians have, for the most part, recognized and welcomed cross-disciplinary contributions by scientists. Some have been more critical. Earl J. Hess, professor emeritus of history at Lincoln Memorial University, recently mentioned two of my books in a praiseworthy review in the Register of the Kentucky Historical Society. But in one passage, Hess writes, “The only limitation is that both books are much stronger on earth sciences than on military history…. Hippensteel tends to assume too much about how geomorphology limits military options.” He explains, “one example from the book will illustrate this point. He argues … that the explosive mine dug by the 48th Pennsylvania at Petersburg in 1864 “would never have been possible” if the campaign had taken place west of the Fall Zone. Its location at that zone allowed the Federals to dig the gallery through sands and clays. But Hippensteel does not know that military miners throughout history have dug shafts and galleries through a wide variety of soil types and even through solid rock (by hand).”
I debated whether I wanted to dig into a discussion about the history of military mining—I didn’t. The foundation of his argument is that Union engineers could have tunneled through more than 500 feet of the Petersburg Granite, one of the hardest and most durable rocks underlying any battlefield, clandestinely and without explosives, before the Petersburg battle lines changed position. In the summer of 1864 Lieutenant Colonel Henry Pleasants’ men took almost a month to dig through soft sand and mud to construct their (semi-)secret tunnel in the soft strata of the Coastal Plain. Had a similar tunnel been attempted in the crystalline granite west of the Fall Zone, I would argue that it could not be completed by hand—in a stealthy manner—before the end of the war.
Nature, in the form of the James River, has been carving into the Petersburg Granite for millions of years, with limited effect (thus the Falls of the James). No company, regiment, or brigade of Civil War soldiers could cut through the same rock in a period of time that would have had any type of strategic influence on the fighting around Petersburg, regardless of the history of military mining.
The natural sciences and the military sciences can be seen to have evolved together. The most influential professor of military science before the Civil War, Dennis Hart Mahan, the West Point engineer, published his most important work during the same decade that Charles Lyell, the Scottish geologist, published the first two textbooks on geology. Thus, from the 1830s until the present day, there has been overlap between the two sciences, and two centuries make clear that interdisciplinary investigation is always beneficial between sciences.
Scott Hippensteel is professor of earth sciences at the University of North Carolina at Charlotte, where he focuses on coastal geology, geoarchaeology, and environmental micropaleontology. He has written three books about using science to illuminate military history: Sand, Science, and the Civil War: Sedimentary Geology and Combat (University of Georgia Press, 2023), Myths of the Civil War: The Fact, Fiction, and Science behind the Civil War’s Most-Told Stories (Rowman & Littlefield, 2021), and Rocks and Rifles: The Influence of Geology on Combat and Tactics during the American Civil War (Spring Nature, 2018).
