WIND GAPS, WEVERTON LEDGES, AND STONE WALLS: MILITARY GEOGRAPHY AND GEOLOGY OF THE BATTLE OF SOUTH MOUNTAIN, FREDERICK AND WASHINGTON COUNTIES, MARYLAND—14 SEPTEMBER 1862

On 4-7 September 1862, Gen. Robert E. Lee and his Army of Northern Virginia crossed the Potomac River at fords near Leesburg, VA, to initiate the first Confederate invasion of the North, a complex troop movement that culminated in the battle of Antietam. The Confederates marched N to Frederick, MD, and then W across Catoctin and South Mountains toward the Great Valley, drawing Gen. George McClellan’s Union Army of the Potomac out of positions around Washington, D.C. On 13 September McClellan was handed the “Lost Order,” a copy of Lee’s dispatch to his generals dividing his forces into four scattered parts W of South Mountain. The stage was set for one of the great missed opportunities of the Civil War.

South Mountain is a narrow N-S ridge on the W side of the 7-mi-wide Middletown Valley, the E side being formed by parallel Catoctin Mountain. These three topographic elements, along with Pleasant Valley and Elk Ridge to the W, constitute the Blue Ridge physiographic province in Maryland. The ridges mark the outcrop belts of resistant Lower Cambrian Chilhowee quartzites and phyllites; Neoproterozoic Catoctin metavolcanics and Mesoproterozoic gneisses underlie the valleys. Along most of its length, South Mountain is 1200 ft high, its narrow summit being formed by the Weverton quartzite. Turner’s, Fox’s, and Crampton’s Gaps are prominent wind gaps where roads still pass over the mountain. The physiography and road network combined to confine the most severe fighting to these gaps.

On the morning of 14 September, McClellan belatedly initiated engagement of the undermanned Confederate defenses on South Mountain, his right wing marching against Turner’s and Fox’s Gaps W of Middletown and his left wing against Crampton’s Gap, 5 mi to the S. The Confederates stoutly resisted Union attacks up the steep, boulder-covered eastern slope of the mountain from behind stone walls and quartzite ledges. At nightfall, the Confederates had effectively lost control of the summit and gradually withdrew to a defensive line along Antietam Creek in the Great Valley to the W. Meanwhile, they had effectively invested Harpers Ferry, VA (now WV) from three sides and forced surrender of the Union garrison there. By the morning of 17 September, Lee had reassembled most of his army on the Antietam—and McClellan had missed his chance to defeat his opponent “in detail.”