Civil War Field Fortifications

Cremaillere and Indented Lines

Cremaillere, or indented, lines were composed of a series of long faces (called branches) and short flanks (called crotchets) arranged to produce a network of crossing and parallel columns of fire immediately in front of the line. This type of line could be adapted to almost all types of terrain and could be modified in a wide variety of ways to project very dissimilar columns of fire. A standard cremaillere line had all of its branches and crotchets arranged in a regular repeating pattern that slanted the branches' columns of fire slightly to one or the other flank. The regular pattern could be reversed at any of the re-entering angles (where a branch joined a crotchet) to increase the number of columns of fire that were directed toward the center of the line.  The branches could also be arranged to form a central salient angle in the line that projected the columns of fire from each face of the salient toward the flanks of the line. Whenever necessary the flanks of cremaillere lines could be anchored on and protected by enclosed works with a strong relief.

The standard measure for the length of branches was about 100 yards and that of the crotchets about 30 yards, but the standard could be altered to suit individual fortifications or even sections of the same cremaillere line. The Federal fortification at Bird's Point, Missouri which covered the confluence of the Ohio and Mississippi rivers had branches that ranged from 204 feet (68 yards) to 556 feet (185 yards) long with crotchets a standard 100 feet (33 yards) deep. Exact measurements depended on the terrain covered by the fortification and the necessity for lengthening or contracting branches and crotchets to effectively direct columns of fire onto ground immediately in front of the line of works.

In general cremaillere lines produced flawed columns of fire that did not effectively cover all of the ground immediately in front of the line with both direct and flanking columns of fire. As illustrated in the accompanying graphic the columns of fire projecting from the branches were complementary and parallel, but not coincident. A gap equal to the thickness of the parapet, the exterior slope, and berme existed between each column of fire projecting from both the branches and the crotchets. The effect of the series of gaps between columns of fire was exaggerated by the excessive number of salient angles incident to repeating cremaillere pattern. Each salient resulted in a wide sector without fire that had to be covered by slanting fire from the branches and flanking fire from the crotchets. The columns of fire projecting from the crotchets were extremely narrow and only covered an area about 20 to 24 yards wide to the front of collateral branches. This was partially mitigated by the successive effect of fire from the crotchets; as an attacking force approached the front of a branch or salient angle it would pass through successive, but not overlapping columns of fire from the crotchets within range with still being hit by a direct or slanting fire from the branches immediately in front or slanting from its line of advance. The ditch itself was not well covered by flanking fire. The shortness of the crotchets combined with the large number of re-entering angles provided an enemy body of troops protection against fire delivered from the parapet once it entered the ditch. Extra labor and defensive precautions were necessary to provide the means necessary to prevent the enemy from using the re-entering angles as rallying points where they could easily mounted the berme and exterior slope of the parapet.

Since the branches were pushed forward from the general line of the work they were vulnerable to enfilade fire delivered from a distance.  When one branch could be enfiladed, the collateral branches could also be enfiladed. The same was true of the crotchets. The most common method for mitigating this defect was to increase the height of the crotchets and an adjoining section of the branches to give the parapet the protective effect of a traverse.

Despite the various defects inherent their form, cremaillere lines were still considered an attractive alternative to long straight stretches of parapet which could not project crossing columns of fire. As a simple matter of economy of force, labor, time, and materials cremaillere lines were superior to both redan and tenaille lines. While redan lines assumed the construction of field works with strong reliefs at intervals and tenaille lines required a very great depth of ground, cremaillere lines did not require particularly thick parapets and required much less depth than tenaille lines.

Perhaps the single most common type of line used during the Civil was the irregular indented line.  An irregular indented line was composed of a series of straight faces joined to form obtuse salient and re-entering angles without regard to specific patterns. The lengths and directions of the faces and the sharpness of the angles were determined by the terrain rather than designed adherence to the replication of any of the prescribed linear forms, though they often did include sections designed according to one or more of the specific prescribed forms. Irregular lines imparted a degree of design flexibility that was not available to other more exacting forms.

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