Civil War Field Fortifications

Mortar Batteries

Stone mortars worked on the same principle as the fougass: high velocity rocks are dangerous to the health and well-being of human bodies. Their sole purpose was to rain a hail of small stones down on a defending body of troops fighting from behind cover, making it impossible for the troops to maintain their position at the decisive point, in this case, the covered way. By the beginning of the Civil War stone mortars had fallen out of service and had been replaced by small Coehorn Mortars that could be handled by two or four men. Coehorns were light enough that they did not require a platform and could be placed at any convenient location in a trench. Most Coehorns fired 12 or 24 pound shells at ranges from 50 to 1,200 yards. Like stone mortars, they were much too light to be able to do significant damage to earthworks, rather, the high trajectory of their shells negated the enemy's cover and could inflict heavy casualties on troops  not protected by bomb or splinter proof shelters.

Coehorns were used quite extensively by both sides during the Siege of Petersburg, Virginia, though the Confederates never seemed to have enough to go around. During the final bombardment of Fort Wagner on Morris Island, South Carolina that compelled the Confederates to evacuate the place the Federal force had a battery of three Coehorns in their fifth parallel. In the western theater neither side seems to have been able to find a decent Coehorn when it was really needed. At Vicksburg an ingenious Federal engineer found a way to shrink iron bands around tree trunks to make a reasonably effective replacement for Coehorn mortars. When a Federal mine was exploded under the Third Louisiana Redan on July 1, 1863 the wooden mortars fired relentlessly into the Confederate reserves who had been called up to repel the expected attack on the crater (which did not materialize) causing very heavy casualties. Again at the Siege of Spanish Fort, Alabama the pioneers of the First Division of the Sixteenth Army Corps fabricated their own wooden mortars and used them to answer the fire of the Confederates' nine real Coehorns.

Larger mortars were another matter entirely. If hauling around and setting up a Coehorn was easy, the extremely heavy and immobile siege and sea-coast mortars required extensive preparation, a little heavy moving equipment, and no small amount of rough cajolery to place in battery and get them ready to fire. Mortar batteries were usually established in the front or rear of the second parallel and, because mortars didn't generally require anything else, were almost always sunken and did not have embrasures. The length of the terre-plein depended on the number of mortars that were to be placed in the battery; 15 feet was allowed between each mortar, another six feet for a central traverse, and 10 feet between the mortars on each flank and the epaulments. The width of the terre-plein ranged from 13 1/2 feet to a maximum of 20 feet. It was sunken to a depth of  3 1/2 feet from the natural level of the ground in front and 4 feet in the rear to provide a good slope for drainage. The front slope from ground level to the terre-plein  had a base of two feet and in the rear the slope fell one foot for every two feet of depth. A one foot wide berme was left between the crest of the slopes into the terre-plein and the foot of the parapet and epaulments. The parapet and epaulments ranged from 4 to 4 1/2 feet high and their interior slopes were not revetted unless the consistency of the soil required it. Parapets could be between 12 and 18 feet thick, while the thickness of the epaulments depended on the type of fire they had to resist. As with gun and howitzer batteries, the soil to construct the parapet and epaulments was taken from a shallow ditch excavated around the exterior of the battery.

As soon as the terre-plein had been leveled heavy platforms were laid and positioned so that the mortars' shells would pass at least three feet above the interior crest of the parapet. A wide variety of mortar platforms were used during the Civil War, more often than not they had to be tailored to suit the requirements of specific battery sites. Since mortars were mounted on beds rather than carriages, solid, level platforms were absolutely necessary for mortars to be pointed, elevated, and fired accurately. Smaller heavy mortars, oddly, tended to wear out their platforms rather quickly while the heaviest, the 13 inch mortars, had beds that were large enough to spread the shock of the pieces' discharge more evenly over the surface of their platforms which imparted a longer useful life. Wear could be reduced by moving mortars around and firing them from different points on their platforms.

Much of the labor involved in constructing mortar batteries could be saved by taking advantage of natural depressions or the reverse side of ridges for battery positions. Since mortars used vertical fire, they were not aimed in the same way as other types of direct fire pieces and did not necessarily have to be able to see their targets to fire accurately. Any convenient ravine with soil capable of supporting a mortar's weight could be leveled, equppied with platforms, and turned into a battery position with a natural ready-made parapet.

Heavy mortars could be quite effective at reducing a garrison's ability to withstand a prolonged siege. At the Siege of Fort Morgan, Alabama the besieging Federal force used 14 mortars to search the interior spaces of the fort with their fire. The citadel of the fort, which had not been demolished as it should have been prior to the siege, was ignited by mortar fire and the resultant conflagration consumed much of the garrison's food supply. Mortar fire also induced the commander of the fort to haul 80,000 pounds of powder out of the magazine and have it destroyed by flooding. Since the garrison was pretty much confined to the shelter of the casemates inside the fort by the mortar fire, when the Federals' heavy guns started breaking into the casemates the commander felt compelled to surrender before the casemates were breached to the point of collapse. In other cases mortar fire was not very effective. At the reduction of Fort Pulaski, near Savannah, Georgia less than half of the Federal mortar shells landed inside the fort, and the fire of the heaviest mortars, conducted at ranges beyond 2,000 yards, was erratic and unrelaible. Brigadier General Qunicy A. Gillmore concluded that mortars were incapable of breaching casemated sea coast fortifications even when they could strike the earthwork of the terre-plein on the barbette level of the walls above the casemates. During the Siege of Vicksburg Federal mortar fire from the mortar boats of the Mississppi Squadron consistently failed to hit any significant targets, though they did cause at least one major fire that consumed several city blocks. Otherwise, their only effect was to put a bad scare into the civilians unfortunate enough to be trapped in the besieged city.

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