Civil War Field Fortifications
Parallels
Parallels were siege works used to provide defensive positions that allowed the besieging army to hold the ground gained in its approaches directed toward the point of attack. As the approaches advanced the parallels were also used as forward supply depots where siege materials necessary to complete the trenches in front were collected. Their name suggests their general orientation to the point of attack: parallels were laid out either parallel to the point of attack or on a concentric line that enveloped the point of attack. The First Parallel could be opened anywhere from 1,500 to 600 yards from the point of attack and was usually constructed by means of the simple sap. Each subsequent parallel was positioned within canister range of the preceding parallel so that the full extent of ground covered by the approaches would be well swept by defensive fire. The Second and any subsequent parallel within enemy artillery range, but beyond musket range, were usually constructed by means of the flying sap; parallels within musket range of the salient of the point of attack were constructed by means of the full sap. Unlike boyaux, parallels were provided with banquettes that allowed infantry to fire over the parallels' parapets. Batteries and magazines could be constructed within parallels by widen the trench, or secondary boyaux could be extended from the parallels to battery positions between parallels. Parallels could be anywhere from eight to 12 feet wide to insure unencumbered circulation of troops and material through the approaches.
When the approaches were threaten by enemy sorties the fronts of the parallels could be covered by obstacles such as abatis, palisadings, and entanglements, though the extra labor involved in constructing extensive obstacles was usually too expensive in time and material for most attacking armies. In the event of an enemy sortie against the head of the approaches the saps forward of the last completed parallel were quickly abandoned and troops were concentrated in the parallel to repel the sortie. On Morris Island, South Carolina the attacking Federal force covered the front and flanks of its parallels with very elaborate obstacles, which caused an important delay in the advance of their approaches, due the fear that a successful Confederate sortie would drive the besieging force into the Atlantic Ocean. Their fear of sorties was almost completely eliminated when the head of their sap ran into a line of mechanically detonated torpedoes about 200 yards from the ditch of Fort Wagner. The torpedoes strongly suggested that the Confederates would limit themselves to a passive defense of the ground in front of Fort Wagner. From that point the Federal approaches were able to proceed more rapidly and without the need to construct defensive works to protect the head of the sap. At Vicksburg the Confederates generally conducted a rather passive defense which allowed Federal approaches to move quickly through ground where they would otherwise have been forced to construct defensive works. In one of the few cases in which the Confederates did launch a sortie, they were able to capture about 50 yards of a Federal sap and fill it in before withdrawing to their main defensive line.
Saps Lines of Circumvallation and Countervallation Mortar Batteries Trench Cavalier
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