GEOLOGIC FACTORS IN THE SIEGE AND FALL OF VICKSBURG, AMERICAN CIVIL WAR, 1863

Vicksburg, “the Gibraltar of the Confederacy”, fell to Federal forces following a 47-day siege at the end of a long campaign to wrest control of the entire Mississippi River from Confederate hands. The geology of the area provided a natural defensive position and led to the siege strategy for victory.

Vicksburg’s main line of defense, although augmented with 31,000 men and some 170 artillery pieces, 60,000 muskets, and a large quantity of ammunition, was primarily one of geomorphology. Sitting astride the axis of the Mississippi Embayment, the land around Vicksburg is dominated by high bluffs, steeply dissected by perennial streams and rivers. The toe of the bluffs rests on Oligocene limestone benches. Overlying the limestone is a sequence of clays, sands, and gravels, capped by a thick interval of wind-blown, quartz silt. This silt, or loess, has unique engineering properties, which made for superb fortifications. In its natural, undisturbed setting, it has high horizontal strength and relatively poor vertical strength, so that it can be graded laterally very easily, yet vertical cuts remain steeply vertical for many years.

The loess was sculpted into steep walled, dirt fortifications and trenches, and topped with cotton bales or wooden parapets. This high strength, naturally absorbent material withstood months of land and river bombardment from the some of the largest guns in the Federal arsenal. When infiltrators moved to tunnel beneath and detonate dynamite charges under Confederate fortifications, much of the intended damage was confined by the natural properties of the loess.