Abstract
The July 1863 Battle of Gettysburg is the most studied and documented engagement from the Civil War by both historians and geologists. On a strategic level the geology of southern Pennsylvania favored the Confederate Army in almost all circumstances. Lee effectively used the Blue Ridge Mountains to shield his army from observation as he moved north and Cashtown and Carlisle Gaps allowed him to amalgamate his army relatively quickly north of the road junction of Gettysburg. However, with respect to tactics, geology strongly favored the Union defense during the 3 days of combat. Herr and McPherson Ridge were important defensive ridges during the first day’s fighting, and both are formed from resistant sandstone beds from the Heidlersburg Member of the Gettysburg Formation. Lee’s attacks during the second and third day of the battle, including the infamous “Pickett’s Charge”, were both conducted by infantry stationed on diabase dikes, assaulting across flat or gently undulating terrain underlain by softer sedimentary rock, against an enemy positioned on the higher diabase Gettysburg sill. This sill, composed from the York Haven Diabase, outcrops as several of the most well-known battle localities from the Civil War, including Devil’s Den and Little Round Top. Gettysburg was the largest and costliest battle during the war, and there is no other battleground where geology had as large of an influence on the outcome of the fight.
“The blood stood in puddles in some places on the rocks”
—Colonel William Oates of the 15th Alabama, after his unsuccessful attempt to take Little Round Top
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Notes
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Guidebook for the 73rd Field Conference of Pennsylvania Geologists
- 2.
Calculating the ballistic trajectory for Civil War artillery is immensely complicated and the profiles provided in Fig. 3.10 are estimates. Trajectories calculated with the following criteria: 12 Pounder Napoleon; muzzle velocity 439 m/s; solid shot weight 85,750 grains (12.25 pounds), shell 58,310 grains (8.34 pounds); Ballistic Coefficient: solid shot 0.10, shell 0.27; Diameter bore: shot 4.62 in., shell 4.52 in.; elevation 180 m; temperature 90 °F; wind negligible.
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Adkin, M. (2008) The Gettysburg Companion: The Complete Guide to America’s Most Famous Battle (544p). London: Aurum Press.
Catton, B. (1960). The American Heritage Picture History of the Civil War (630p). New York: American Heritage Publishing.
Debelius, M. (1996). Illustrated Atlas of the Civil War (320p). Alexandria: Time Life Books.
McPherson, J. M. (1988). Battle Cry of Freedom: The Civil War Era (905p). New York: Ballantine Books.
Nosworthy, B. (2003). The bloody crucible of courage: Fighting methods and combat experience of the Civil War (753p). New York: Carroll and Graf Publishers.
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Hippensteel, S. (2019). Gettysburg. In: Rocks and Rifles. Advances in Military Geosciences. Springer, Cham. https://doi.org/10.1007/978-3-030-00877-2_3
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