Lines, Crémaillère

Also, Indented Lines and Serrated Lines. A type of continuous entrenched line that was traced using a pattern of alternating long branches (or faces) and short crochets (or flanks) to form a series of salient and re-entering angles that produced a network of crossing columns of fire over ground immediately in front of the entrenchment. Crémaillère lines could be laid out along right lines with all of the branches and flanks having equal angles and equal lengths or they might be laid out with the branches directed onto a specific point within or exterior to the entrenched line for the purpose of defilading the line or projecting its columns of fire across a specific area. Branches and flanks of a cremaillere line could be laid out using any one or combination of several systems for determining their lengths and angles.

One of the most common methods for laying out a crémaillère line employed a right line as a boundary for marking pre-determined distances between salient angles and using the boundary line to construct crotchets perpendicular to the direction given the branches. The boundary was laid out as a right line that extended the full length of the position to be fortified by a continuous crémaillère line in which all of the branches were parallel to each other and all of the crotchets parallel to each other. Locations of the salients were then marked on the line; these could be anywhere from 60 to 120 yards apart. Lines perpendicular to the boundary line were then laid out toward the rear of the entrenched line; these could be 20 to 40 yards long, depending on the desired length of the crotchets. Diagonal lines were then laid out from the rear extremities of the perpendiculars to the collateral salient markers on the boundary line to mark the line of the interior crests of the branches. Points were then marked on the diagonals to locate the vertices of the re-entering angles of the entrenched line and the extremities of the crotchets. The crotchets were then traced from these points to the salients on the boundary line, making sure that the crotchets were perpendicular to the For a line with salients 90 yards apart and perpendiculars 30 yards long the re-entering vertices would be marked 10 yards from the rear extremities of the perpendiculars on the diagonals to produce crotchets perpendicular to the diagonals.

Length of the branches could also be adjusted to conform to the effective range of small arms employed to defend the entrenched lined and the line laid out so that fire from one salient struck the capital of the next salient in the entrenched line. In this case the effective range was assumed to be equal to the length of a single branch added to the length of a single flank; this allowed the distance between salients to be calculated using the carpenter's theorem (a² + b² = c²). If, for example, the maximum effective range was assumed to be 200 yards and a flank in the entrenched line was assumed to be 40 yards long then a branch would be 160 yards long (200 - 40 = 160). Since the branch and flank constituted the two short legs of a right triangle, then the distance between salients as measured along the boundary line (the longest leg of the right triangle) would be approximately 165 yards. (Calculated as: [40² + 160² =  c²] or [1600 + 25600 = 27200] with the square root of 27200 = 164.92). Once the distance between salients was known the line could be laid out as a regular crémaillère line.

A crémaillère line could be laid out so that all of its branches were directed toward a single point; like the spokes of a wheel the branches were laid out along lines that converged on the same point. The point of convergence could represent inaccessible ground that an enemy could not occupy or be covered by an enclosed field work that protected the line's flank from enfilade fire. Branches could be given almost any suitable length; the flanks were laid out perpendicular to the branches their fire defended and would be given the normal length for flanks (30 to 40 yards) in regular crémaillère lines.

Crémaillère pattern entrenched lines were the most adaptable of all the standard continuous line forms; all of the elements that define the form could be modified to suit the specific fortification requirements of specific extended positions. Linear dimensions of the branches and flanks could be altered to cover specific distances within a position or to provide space along the interior crest of the parapet for a specific number of defenders. Angles joining the flanks and branches could also be modified to make slight changes in the direction of a line or to produce crossing columns of fire over specific areas in front of a line. On irregular extended sites each indentation could be modified independently of all of the others composing the general line to insure that ground in front of the line could be reached by fire from the parapet. Orientation of branches and flanks could be altered within the general line to produce salient and re-entering angles to protect vulnerable points in the line or avenues of approach with crossing columns of fire.

Two excellent examples of the use of crémaillère pattern lines during the Civil War are the Company Canal Line at New Orleans, Louisiana and the bridgehead established on the west bank of the Mississippi River at Bird's Point, Missouri across the river from Cairo, Illinois. The line at Bird's Point was given the general form of an extended redan with two crémaillère pattern lines converging on a central salient point. Linear dimensions of the line were limited to prescribed ranges of smoothbore muskets: branches were 160 to 166 yards long with flanks 33 yards long; angles of the branches and flanks were generally about 90 degrees, with slight modifications made to alter the general direction of the line. Flanks were oriented to produce columns of fire across branches in the direction of the central salient.

A crémaillère pattern line was constructed along the edge of Company (or Company's) Canal that flowed into the Mississippi River from the south across the river from the city of New Orleans, Louisiana. Its object was to prevent hostile forces advancing along the line of the New Orleans, Opelousas, and Great Western Railroad from gaining a position directly opposite the city. Terrain was level and the front narrow; flanks of the line were anchored on impassable swamps to the south and the Mississippi River to the north. The line was sited along the east bank of the canal; from the river to the railroad bridge spanning the canal the line's flanks were oriented to project columns of fire directed north, toward the river, south of the bridge the flanks projected columns of fire toward the south. Branches on either side of the bridge were oriented to cross columns of fire on the bridge and its immediate approaches. Although a parapet traced on a right line would have effectively covered the approaches to the canal, the crémaillère branches insured that an attacking body of troops would be opposed by slanting columns of fire that would have increased the depth of the target and rendered the defenders' fire more destructive.

January, 2003