Fortification Friday: “a redan or a lunette, is thrown up on the exterior to cover the outlet”

Before we close the discussion of openings for forts (see what I did there?), let me circle back to compare Mahan and Wheeler in regard to one of the fine points considered.  That being the use of a detached redan or lunette in advance of the outlet.  Recall that in pre-war writing, Mahan suggested:

In very frequented passages, a redan or a lunette, is thrown up on the exterior to cover the outlet, and thus ensure its safety in case of surprise.

And Wheeler, in the post-war, mentioned a similar arrangement, but perhaps narrowed the application to those larger outlets, for sorties, where simple interior traverses would not be practical.

Mahan offered two figures that illustrated the redan to the front of an outlet:


Figure 48 offers a wide redan in front of an outlet, which is further covered and flanked by by the “horns” of the larger work.  A very well protected outlet, we might say.  Mahan considered this a Redan Line.

On Figure 49, we see much more complexity.  Particularly with the defensive lines of fire.  The outlet is nested within a redan of a larger line.  On both sides are faces within redans of differing angles. This is considered a Tenaille Line – a proper definition we will discuss later.  But the point being the covering redan, to the front of the outlet, was absolutely necessary here in order to protect that weak spot.  The covering redan is somewhat off center of the outlet, perhaps to limit exposure at the expense of accessibility.

Wheeler, as you may recall, gave us only a simple rendition of the covering redan:


The question I have in regard to these advanced, detached “parts” covering openings is… just how often were these employed during the Civil War?

When examining surviving earthworks, we often find the area around the outlets obliterated.  Sometimes, due to necessity, that is done to facilitate visitor access.  But more often, just a case where the structures around the outlets were the most susceptible to erosion.

And when examining wartime plans, we see some use of these redans… but more often not.  Consider Fortress Rosecrans outside Murfreesboro:


This was, some have said, the largest fort built during the war.  And in this plan we see examples of many features suggested by Mahan.  Specific to the outlets, we see up near the top that Battery Cruft was a detached lunette (maybe a “half lunette”) covering an outlet.  Elsewhere, such as next to Lunette McCook at the bottom right, we see an outlet (an existing road) without a covering redan or traverse.  Though we do see obstacles erected to the right of Lunette McCook.  And certainly that named work was positioned to dominate the approaches to the outlet.  Furthermore, what you don’t see in my “snip” are works in advance of the fortress that covered the railroad and road.  Though those were oriented south and not regarded as covering the outlet in question.

Another plan to consider is from Virginia, at Deep Bottom:


Here we see five road crossings at the main line of the works.  One of those is blocked entirely by a redan.  The other four (including one that appears to be a path cut just to clear a redan) have no traverses or covering works.  Just obstacles placed in front.

If we are assessing the protection of outlets, with Mahan’s suggestions in mind, we find a mixed application of those covering redans.  Seems to me the use of that sort of feature was based on the engineering assessment of need.

Now considering such use under Wheeler’s suggested implementation, let’s look to the location of a few large scale sorties.  First, how about the works were the Crater assault was mounted:


And further around the lines, and further forward in the historical timeline, to the sector around Fort Mahone:


And to the left of that sector near where the Federal Sixth Corps mounted their sortie:


Now the scale of these maps mean these are not so much “plans” as operational maps.  So we know there are structures that escaped the pen here.  But what stands out, with double underlines, is the use of something far more elaborate than Mahan and Wheeler discussed.  We see entire sections of works advanced in a manner to provide staging grounds for those formations preparing for the assaults. Major assaults, mind you, involving whole divisions.  These were, you see, works built for the offensive.  Grand offensives!  In that light, might we say the entire Federal line was one large “covering work” in front of an array of staging areas and supply depots?


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