LEE VS. MEADE AT GETTYSBURG (JULY 1-3, 1863): THE INFLUENCE OF TOPOGRAPHY AND GEOLOGY ON COMMAND DECISIONS AND BATTLEFIELD TACTICS

The battle of Gettysburg was fought in a 15-sq.-mi. area in the Mesozoic-age Gettysburg rift basin in south-central Pennsylvania. Battlefield terrain consists of northeast-trending lowlands underlain by red shales and sandstones of Late Triassic-age Gettysburg Formation and rocky ridges developed on Early Jurassic-age diabase. The militarily most significant diabase hills are formed on a York Haven Diabase sill running from Culp’s Hill (625 feet) in the north through Cemetery Hill, Cemetery Ridge, and Little Round Top to Big Round Top (785 feet) in the south—the Union “fishhook.” A lower, north-south ridge to the west, Seminary Ridge, is ribbed by a Rossville Diabase dike.

The middle of the Civil War saw the Confederate Army of Northern Virginia (ANV) marching north through the Great Valley into Pennsylvania using the Neoproterozoic Catoctin volcanics and overlying Cambrian clastics of South Mountain to shield themselves. Meanwhile, the Union Army of the Potomac (AOP) maneuvered between the ANV and Washington, D.C., until R.E. Lee ordered his corps commanders to proceed to Gettysburg, the hub for ten roads. In response, G.G. Meade marched north and ordered Federal cavalry to Gettysburg to screen the AOP.

Initial clashes occurred to the northwest of Gettysburg early on July 1; three deep cuts on an unfinished railroad figured in the action there. By late afternoon, the Union had fallen back to defensive positions on the diabase-sill “fishhook.” Fighting continued on July 2 as the Confederates on Seminary Ridge attempted to cross the lowland to dislodge the Union left. An unauthorized advance by AOP D.E. Sickles to move westward from the “fishhook” resulted in the ANV gaining some ground, but by evening the AOP occupied its original defensive position. On the evening of July 2, Confederates attempted to dislodge the Federal right, but breastworks of timber and diabase boulders on Culp's Hill were formidable. Following an artillery bombardment of the Union center on the afternoon of July 3, the Confederates launched a massive, but futile assault (“Pickett’s Charge”) from Seminary Ridge against Cemetery Ridge. Under cover of heavy rains on July 4/5, the mauled ANV retreated into Virginia. Despite the difficulty of entrenching, crude fences of diabase boulders had provided the Federals with adequate cover in many sectors.