Gettysburg

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.