SEDIMENTARY GEOLOGY AND THE EFFECTIVENESS OF CONFEDERATE TEMPORARY COASTAL FORTIFICATIONS

Two general categories of defensive fortifications were delineated by military engineers during the Civil War. Permanent works, such as Forts Pulaski and Sumter, were built during peacetime and were intended to last for decades. Temporary works, in contrast, were built during wartime to face an immediate threat and lost much of their strategic value after fighting ceased. Temporary fortifications, including Battery Wagner and Fort Fisher, proved especially difficult to capture for the Union, in large part because of sedimentary geology.

Increases in artillery technology during the 1850’s rendered permanent brick fortifications obsolete. For example, in April 1862 Union Brig. Gen. Quincy Gillmore ordered the bombardment of Fort Pulaski, GA. This brick fort lasted less than a day under Gillmore’s rifled artillery before surrendering. Convinced of the efficacy of his guns, Gillmore moved north and conducted an even larger bombardment of Battery Wagner on Morris Island, SC. On July 18, 1863, the Union Army and Navy sent a cross-fire of nearly 10,000 heavy artillery rounds into the sand fort. Confident of the nearly total reduction of the battery and its 1,800-man garrison, Gillmore ordered an assault from 5,000 of his infantry. To the great dismay of the infantry and their commanding officer, however, what appeared to be a battered fort with a wounded garrison was instead a fully functional battery with a nearly complete defensive capability. The ten-hour bombardment had killed only eight men. Gillmore’s grand assault turned disastrous -- his men suffered 1,515 casualties (>30% of the attacking force) and the outnumbered Confederate defenders lost only 222 men.

The defensive capability of Battery Wagner is related to sedimentary geology. The geomorphology of the barrier island dictated attack from one predictable angle: directly down the beach. Assault across the muddy back-barrier marsh or dune field was impossible. The aeolian sand also fouled rifles and pistols alike, and soldiers had no means of cleaning their weapons in the field. Finally and most importantly, the sand used for parapet and bombproof construction rendered incoming artillery rounds ineffective. The fine/medium sand allowed penetration of shells to a depth where the explosion would be suppressed and inter-grain friction slowed expansion of shrapnel.