MISSISSIPPI LOESS AND THE SIEGE OF VICKSBURG

Vicksburg, MS, is located on Pleistocene loess, an eolian silt about 100 feet thick. The loess has eroded into deep, steep sloped ravines. As a result, the City sits on prominent hills and intervening deep valleys. The properties of the loess greatly influenced the conduct of MG U.S. Grant’s 1863 siege. Union forces invested Vicksburg on 19 May 1863, and saw the dendritic loessal drainage pattern formed a concentric ridge system around the city. Using this topography to advantage, the Confederates had fortified a prominent ridge that circled the City from the Mississippi River north of town to the River on the south. Federal reconnaissance showed a smaller, less continuous ridge outside the Confederate ridge, which they fortified. Thus, the loessal topography brought the armies into proximity, but did not permit close combat since the intervening terrain was impassable. Only six avenues of approach existed between the two ridges, all drainage divides with roads located atop them. The Confederates had erected their strongest fortifications at these loessal avenues where the bloodiest fighting of the siege took place. Undisturbed loess has a weakly cemented structure of calcium carbonate and some particles are clay-coated, giving it cohesion. The material is easy to dig, and vertical cuts stand almost indefinitely. This property facilitated the excavation of thirteen siege approaches by the Union toward the Confederate lines, the several Confederate countermines, miles of infantry trenches, and the construction of artificial caves for the shelter of soldiers and the civilians trapped in the city. Little groundwater exists in the impermeable loess and Vicksburg had long relied on cisterns for water. Union soldiers noted that streams all flowed through the enemy lines and quickly polluted them with dead animals, forcing southern troops to rely on the cisterns, since parties obtaining water at the River were subject to Union fire. The cisterns soon went dry, causing a water shortage among the Confederates. Loessal Geology, exposure, lack of food and water all contributed to the surrender of the City of Vicksburg on 4 July 1863.