"Hence the saying: If you know the enemy and know yourself, your victory will not stand in doubt; if you know Heaven and know Earth, you may make your victory complete."

The Art of War, by Sun Tzù

In 1863, after more than two years of Civil War, the Confederate Army of Northern Virginia launches a decisive attack towards the north, in direction of the town of Harrisburg in Pennsylvania. Spotted by the Union Army a skirmish near the small farm town of Gettysburg starts, soon escalating into one of the bloodiest battles in U.S. history. In three days (July 1 to July 3) nearly 50.000 Americans were killed or wounded. The outcome of this military campaign and battle was strongly influenced by geological events almost one billion years in the making.

Fig.1. Simplified geological map of the Gettysburg campaign (after EHRLICH 2009; BROWN 2006; CUFFEY 2008, modified, with edits suggested by BENTLEY 2014), for more and a better geological & tectonic description of the Appalachian Mountains and surrounding areas see also Callan Bentley´s Mountain Beltway Blog.

The Gettysburg campaign started almost a month earlier in the town of Fredericksburg, where both armies had passed the winter along the shores of the Rappahannock River. An easily defendable terrain, the attacks of the Union Army towards south were successfully stopped by the Confederates. June 3 part of the Confederate troops marches first towards west and follows then the south-north trending Great Valley.

Both the Piedmont area, where Fredericksburg is located, as the nearby Blue Ridge Mountains, are formed by hard igneous and metamorphic rocks, more than a billion years old. Such rocks, when eroded, form a rocky and rough terrain, not a very good ground for the soldiers or horses to pass there. However the Blue Ridge Mountains provided shelter from Union spies and attacks by the enemy's cavalry.

In the flat Great Valley, filled by soft, erodible ancient sediments, the roads were easily trafficable and the Confederate soldiers quickly advanced. The well developed, and therefore fertile soils, provided also plenty of food to support the advancing army.

The roughly parallel bands of mountains and large plains, of great tactical use for the Confederate Army, follows the outline of the Appalachian Mountains, formed in the late Paleozoic, some 400 million years ago, during the collision of ancestral North America with ancestral Africa.

The gently rolling landscape with small hills in the Gettysburg area and in similar sedimentary basin regions, crossed by the Union Armies during their pursuit of the Confederates, is formed by sediments deposited in a second tectonic phase. During the Triassic, some 220 to 200 million years ago, the supercontinent of Pangaea broke apart and large rift-basins opened. This chain of tectonic basins (Culpeper and the Gettysburg-Newark basin) were subsequently filled with sediments coming from the slowly eroding Appalachians, mostly layers of hard sandstone, interbedded in soft siltstone and shale-formations.

Not only large-scale orogenic processes, also local fault systems played an important role in the Gettysburg campaign, as trafficable gaps in the Blue Ridge and South Mountain formed where tectonic movements have crushed the rocks. The weakened rocks are easily removed by weathering and running water and so form broad valleys in the mountain barriers.

In the night of June 28 general Robert Edward Lee (1807-1870) learned from a spy that the Union Army was concentrated around the town of Frederick. Lee therefore positioned his soldiers in the towns of Chambersburg, Cashtown and Carlisle. The Cashtown gap was the only trafficable road for heavy artillery at the time and shaped by tectonic movements along the Cashtown strike-slip fault. Lee could here quickly cross the South Mountain ridge into the Gettysburg basin and eventually face the enemy.

On the same day general George G. Meade (1815-1872) replaced Joseph Hooker (1814-1879) as commander of the Union Army. Meade, anticipating an attack from the west, immediately send troops from Frederick to the nearby Parrs Ridge, to secure this strategic advantageous topographic height. Similar to the Blue Ridge, also Parrs Ridge is formed by weathering-resistant schist and quartzite.

Meade send also the cavalry and three infantry corps to Gettysburg, however the flat terrain of the sedimentary basin couldn't provide cover and was therefore not considered good battleground.

Unfortunately and unprepared the Union soldiers encountered the scouting Confederate soldiers. What at first seemed to be a lesser skirmish soon developed to a fierce battle when more and more Confederate reinforcements pulled through the tectonic gaps and the rest of the Union Army rapidly advanced towards Gettysburg on the sedimentary flats.

To be continued…

Bibliography:

BROWN, A. (2006): Geology and the Gettysburg Campaign. Pennsylvania Geological Survey Educational Series 5, published by the Commonwealth of Pennsylvania/Department of Conservation and Natural Resources/ Bureau of Topographic and Geological Survey: 14

CUFFEY, R.J. et al. (2008): Geology of the Gettysburg Battlefield: How Mesozoic Events and Processes Impacted American History. Geological Society of America, Field Guide 8 Excursions in Geology and History: Field Trips in the Middle Atlantic States: 1-16

EHRLICH, T.T. (2009): Gettysburg National Military Park & Eisenhower National Historic Site - Geologic Resources Inventory Report. National Park Service of the U.S. Department of the Interior: 49

ROSE. E.P.F. & NATHANAIL, C.P. (2000): Geology and Warfare: Examples of the Influence of Terrain and Geologists on Military Operations. Geological Society of London: 498

UNDERWOOD, J.R. & GUTH, P.L. (eds.) (1998): Military Geology in War and Peace. Reviews in Engineering Geology, Vol. 13, The Geological Society of America: 237