One Man’s Opinion

Dick Nicholas

Volume 12, Number 5
May 2004

Before getting started on this month’s primary subject, I’d like to recommend a non-Civil War book that I recently came across: Washington’s Crossing by one of my favorite historians, David Hackett Fischer. Historians these days are often in the business of telling us how badly our myths fall short of reality, so it’s a rare treat when a writer comes along to tell us that reality is sometimes far grander than we imagined. That’s what Fischer has done in Washington’s Crossing, a meticulous and brilliantly colored account of the period surrounding the Virginian George Washington’s famous crossing of the Delaware River in the winter of 1776 to attack the Hessians at Trenton, New Jersey. Fischer eloquently makes the point that the alleged exaggerations about Washington and many of his exploits during our first war for independence were anything but fantasies. Rather his true accomplishments far outweighed the credit that he has been given. Perhaps some of the current crop of Civil War academicians obsessed with the so-called myth of the “Lost Cause” might want to consider the possibility that history may very well be full of many examples where reality really did justify or even exceed the myth!

Each year thousands of sightseers, historians, students, reenactors, military buffs, and others clamber over Civil War battlefields to touch the past. We want to see where the battles were fought. What did the land look like? How high was Cemetery Hill, Malvern Hill, Prospect Hill, Henry Hill, Marye’s Heights, or Clark Mountain? How wide was the Rappahannock, or the Rapidan? Why did Jackson pick Stony Ridge for a defensive position at Second Manassas? These are just a few of the questions that we ask when we visit the scenes of conflict and experience an emotional empathy with the men who fought and died at those places. I, too, walk the battlefields, but I’m usually focused on another factor—the geology, and its influence on the battle ground, and/or the maneuvering that led up to the battle.

There is no question that battlefields enrich our understanding of the Civil War, but only a miniscule few have any concept of the extent to which the movements of the various armies and the battlefields themselves were influenced by the geology of the region and locality. First, a few basics. Rocks guide the formation of topography, that is, the location, size, and shape of mountains, rivers, and valleys. Rocks determine whether a place is flat, or hilly; steep or gentle; rough or smooth. All of which takes us back to the common thread that runs through most of the questions that battlefield visitors most commonly ask—hills, elevations, relief, heights, high land, low land, etc. Why this interest in topography? Because the commander of every battle, that has ever been fought in history, has attempted to gain the advantage of high ground over the enemy. So, if you understand the geology, you will also understand the high ground and hence its military advantage.

Perhaps the most classic example of the influence of geology on the location and outcome of a major Civil War battle is at Gettysburg, Pennsylvania. In fact, one writer goes so far to say that the reason that Gettysburg is “the most studied military action in the world” is because of the terrain considerations in the strategies and tactics adopted by the two opposing armies, the Army of Northern Virginia and the Army of the Potomac. Just as the name “Gettysburg” resonates down through American history, so do the names of the elevated areas on the battle field such as McPherson’s Ridge, seminary Ridge, Cemetery Ridge, Little Round Top, Devil’s Den, and Big Round Top—all of which had significant influence on the battle. And all owing their existence to the presence of an igneous rock that was intruded into that area about 200 million years ago!

The present Gettysburg battlefield landscape owes its configuration to a complex series of geological events that took place during the Triassic and Jurassic geological periods. Of course, in 1863 the science of geology was young, and military geology, as the term is understood today, was years in the future. Yet the principles of military geology, applied, or all too often not applied, influenced decisively not only this battle, but the entire campaign.

The Battle of Gettysburg was essentially an effort by the Confederates to drive the Union army from the outcrop of the north-south trending Gettysburg sill (York Haven Diabase of Jurassic age) south of the town of Gettysburg. The York Haven Diabase is a hard igneous rock that is resistant to weathering relative to the surrounding Triassic sediments. This outcrop of intrusive igneous rock is shaped like a fishhook extending northward about 3 miles from Round Top through Little Round Top and Cemetery Ridge to Cemetery Hill, then east and south to the barb of the fishhook, Culp’s Hill. Cemetery Ridge, underlain by the Gettysburg sill, is high ground and was the main line of defense used by General Meade.

About a mile west of Meade’s defensive line is Seminary Ridge where Robert E. Lee aligned his army on the second and third day of the battle. The latter elevation also trends north-south more-or-less parallel to Cemetery Ridge. Seminary Ridge is also supported by another resistant igneous body called a dike. In between the two igneous diabase-supported ridges is a shallow valley or swale that is underlain by less resistant sands and shales of Triassic age. Although Seminary Ridge and Cemetery Ridge have only about 30 to 40 feet of relief, in 1863 they represented significant and very favorable defensive military positions relative to the intervening lowland. It was across this mile-wide shallow and open lowland underlain by sedimentary rocks that Pickett and others would have to charge on the afternoon of July 3rd to the reach the Federals commanding the high ground along Cemetery Ridge. From a strictly geological/tactical perspective, if Meade had attacked west up the slope of Seminary Ridge, he would probably have sustained the same magnitude of losses that the Confederates took when they charged east up to Cemetery Ridge.

The casual visitor to Gettysburg today is usually somewhat under-whelmed by the gentle slopes of the two low-relief ridges (Seminary and Cemetery) where the climatic battle took place. In contrast, two rocky knobs at the south end of Cemetery Ridge have sharper relief and dominate the topography of the area. With elevations of 100-200 feet above the surrounding countryside, Round Top and Little Round Top provided excellent observation and artillery positions for the Federals. Union artillery positioned on the heights of Little Round Top under the direction of Union General Henry Hunt was able to dominate much of the battlefield during the battle. Again, the importance of high ground was evident.

Another major Civil War battle was fought on rocks of Triassic-Jurassic age was the battle of Second Manassas in August of 1862. And here, as well as Gettysburg, the distribution of the more resistant Jurassic igneous rocks profoundly influenced the positioning of the troops and the outcome of the battle.

The Manassas battlefields are located in what is known geologically as the Culpeper Basin. As you drive north up Route 29, you first cross into the Culpeper Basin about Culpeper. Look carefully in the road cuts as you continue northeast and you will notice that the soil changes to a deep reddish-maroon color—a sure sign that you are now on Triassic red sediments. The isolated knob on your right just south of Culpeper is Mt. Pony, another diabase-supported elevation. The relatively featureless rolling topography of the Triassic continues until you cross Bull Run on I-66 just east of Manassas.

Within the battlefields of both First and Second Manassas, the low relief hills such as Chinn Ridge, Henry Hill, Mathews Hill, Dogan Ridge, etc., all have similar elevations of about 270–290 feet and are supported by sandstones and siltstones. In contrast, the ridges underlain by diabase—Stony Ridge and Stuarts Hill—are about 330–340 feet high. The elevation difference affected the course of Second Manassas. In addition to the differences in heights, land underlain by diabase tends to be rocky and wooded; land underlain by siltstone and sandstone tends to be open farm land.

You know the rest of the story. Jackson entrenched his Corps along the unfinished railroad on the partially wooded southeast slope of Stony Ridge from August 28–30, and fought off a series of onslaughts by Pope’s army over the three-day period before Longstreet attacked from the west and helped win a decisive victory. Located less than a mile north of the crossroads of Groveton on the Route 29 (Warrenton Turnpike), the National Park Service maintains an excellent site to view Stony Ridge and the deep cut of the unfinished railroad. Geologically as you walk a few hundred yards northwest from the Groveton-Sudley Road, you cross a little stream and outcrops of the soft and less resistant red Triassic siltstones and shales before ascending the southeast slope of Stony Ridge and reaching the trace of the unfinished railroad about halfway up the hill. The closer you approach the railroad cut, the more diabase cobbles and boulders appear strewn about the surface. When some of Jackson’s defenders ran out of ammunition on August 30, they resorted to throwing some of these softball-sized diabase cobbles at the charging Yankees.

As at Gettysburg, the defenders of the high ground at Second Manassas possessed a great tactical advantage in the battle. Although the participants had little if any knowledge of the basic geological conditions that controlled the topography and surface, common sense and experience dictated that it was always better to seek the protection of as much of the earth as possible, and to shoot down at an opponent, rather than to have to shoot up at him. Both battles were profoundly influenced by the topography. At Manassas, Stonewall Jackson made masterful use of a resistant ridge formed by igneous rocks to form a strong defensive position. At Gettysburg the commanders of both armies made intelligent use of the terrain, and therefore of geology. In both cases, however, the defenders occupying the higher elevation won the battle.