Antietam

Abstract

Robert E. Lee exploited the geology of central Maryland on the largest scale during the Maryland Campaign and at the Battle of Antietam. In the days preceding the battle, he held the passes through the hard quartzite ridges of South Mountain to delay the Army of the Potomac’s advance towards his isolated Corps. Once McClellan arrived on the carbonate battleground around Sharpsburg, the Federal commander’s offensive tactics proved costly and largely futile. McClellan told each of his six Corps commanders of their specific assignment during the battle, but never the overall battleplan. As a result, the rolling terrain of the limestones, dolostones, and shales on the battlefield prohibited each Corps commander from observing the actions of their colleagues and coordinating and supporting their attacks. As a result, each portion of the battle was a disjointed and unsuccessful attempt to break Lee’s line. The opening morning phase of the battle was fought across the Conococheague Limestone, a rock formation known for consistently weathering and producing undulating terrain. Casualties were especially high during this fighting. The afternoon phase of the battle was concentrated around a Confederate position in the famous sunken lane. Here the Elbrook Formation, composed of alternating beds of limestone, harder dolostone, and softer shale, produced a rolling landscape that the Union officers used for concealment when on the offensive. The final phase of the battle was concentered around the southernmost bridge across Antietam Creek and the contact between the Elbrook and Waynesboro Formations. Burnside’s IX Corps was ordered to attack Lee’s right flank, but delays in capturing and crossing the bridge, and difficulty with the rugged terrain of the Elbrook, cost the Union greatly. The battle ended when A.P. Hill’s Light Division, marching from Harpers Ferry, arrived on the field to deliver the final counter-attack, saving Lee’s army.

“In the open the growing corn was cut from the stalks as with knives, and within the woods limbs of trees were torn away and rocks were splintered by the deadly fire”

—Rev. J. Richards Boyle, Adjutant of the 111th Pennsylvania Regiment, at “Pennsylvania at Antietam Day”, September 17th, 1904