THE GETTYSBURG BATTLEFIELD: GEOLOGY'S IMPACT UPON MILITARY HISTORY

The Gettysburg battlefield is a mile-wide lowland with low ridges on W and E. The lowland is on soft red shale to fine sandstone (lower Gettysburg Fmn; U. Triassic; floodplain or lake/playa deposits). The ridges are on hard basal-Jurassic diabase: Seminary Ridge on the W on a thin vertical dike of Rossville lower-Ti diabase, and Cemetery Ridge on the E on a thick west-dipping sill (Gettysburg Sill) of slightly older York Haven higher-Ti diabase. Both ends of Cemetery Ridge rise into higher hills eroded from the Gettysburg Sill: Cemetery + Culp's Hills to the NE, Little + Big Round Tops to the S. Further W of Seminary Ridge are low hills (McPherson + Herr Ridges), but developed on hard gray sandstone, redbeds, argillite, and black shale (Heidlersburg Member, mid-Gettysburg). By analogy with the Newark Basin, these strata appear cyclic, orbitally forced, arid and monsoonal tropical climates.

During the mid-Civil War (1863), 75 000 Confederates under Lee slowly moved Nward behind the Blue Ridge – South Mountain barrier, while 90 000 Union troops under Hooker (later Meade) remained E on the Piedmont to shield Washington. By late June, both armies converged on Gettysburg, due to the landscape's grain, the many roads radiating out from that town, and reports of possible supplies available there.

Early July 1, Confederates moving SE toward Gettysburg ran into Union troops coming N just W of town. Both sides began to fight vigorously, especially for McPherson Ridge, and ever more units fed into that conflict all day. By late afternoon, the Confederates prevailed and spread S along Seminary Ridge, while the Federals retreated through town and prepared positions on its S edge on Cemetery Ridge + Hill. Intense fighting continued July 2 as the Confederates attacked (unsuccessfully) the ends of the Union position, the S (Little Round Top) in the afternoon, the N (Cemetery + Culp's Hills) in the evening + night. July 3, Lee struck Meade's center on Cemetery Ridge. Like 1859's battle at Solferino, Lee heavily cannonaded the Union line, and then sent a massive infantry assault (under Pickett) across the lowland. As Pickett's troops neared Cemetery Ridge, the Union artillery opened a devastating fire, so that few Confederates were left when they got up to the Union line; hence, this frontal charge also failed. July 4, under heavy rain, both armies sat exhausted. The following night, the Confederates began withdrawing back to Virginia.