Fortification Friday: “The dimensions of the exterior side must vary with the relief.”

Over the last couple of installments, we’ve looked at the geometry of a bastion fort.  A point to drive home, which hopefully readers have taken home, is the way the exterior side is somewhat the foundation for all planning.  The orientation and placement of that line governs almost all other particulars of the fort.  And a refresher… this is the exterior side of a bastion fort:


Recall this is the first line drawn when planning out layout of the bastion fort.  Likewise, when resolving the defects of that type of fort, the exterior side serves as a governing factor.  The engineer could not exceed the space defined by the exterior sides, and he had to defend every corner of that defined space.

In his instruction, Mahan distilled the somewhat abstract principles surrounding bastion forts to derive some realistic planning guides that students might apply to the field.  In the case of the exterior side, Mahan offered a “rule of thumb” about the length of such a line… along with justification and a basis for refinement:

The exterior sides of the bastion fort should not exceed 250 yards, nor be less than 125 yards, otherwise the flanking arrangements will be imperfect. With a relief of 24 feet, which is the greatest that, in most cases, can be given to field works, and an exterior side of 250 yards, the ditch of the curtain will be perfectly swept by the fire of the flanks, the lines of defense will be nearly 180 yards, a length which admits of good defense, and the flanks will be nearly 30 yards.  With a relief of 14 feet, the least that will present a tolerable obstacle to an assault, and an exterior side of 125 yards, the ditch of the curtain will be well flanked, the flanks will be nearly 20 yards in length, and the faces between 30 and 40 yards. Between these limits, the dimensions of the exterior side must vary with the relief.

We sometimes go to the conclusion the range of musketry governed the dimensions of the fort.  The true nature of that relationship was a “sort of.”  Certainly the dimensions of the line of defense, offered here by Mahan as 180 yards, should be within the effective range of musketry.  But, at the same time, as Mahan describes, it is the geometrical rational which imposed the 250 yard exterior side, from which the 180 yard line of defense is produced.  I submit the answer here is that both musketry range and geometrical rules were equally in play.

The main thrust of this paragraph, however, is how topographic variations … that is relief… was an important factor when setting the exterior side.  We see that Mahan recommended, for the bastion fort, a variation of 24 to 14 feet in elevation.  And he gives ample justification.  Too much relief, and the guns on the parapet cannot cover the ground in front of the ditch properly.  Too little and the works offer no obstacle to the attacker.

So if you wanted to place a fort, you’d want to have terrain with some undulation up to around 14 feet.  Right?  Well what if you are building a fort out at … say…. Morris Island in 1863?  Not a whole lot of elevation variation in that beach front property. And what elevation there is tends to be temporary and much dependent upon the wind-blown sands.  The solution there, as witnessed with detailed plans from Battery Wagner, was to dig deeper and pile higher.  Though there was a limit to how deep one could dig there on the beach, meaning more was “piled higher.”  this gave Battery Wagner’s parapet about a 12 to 15 foot height above the ditch.  It also meant that work’s plan differed significantly from a classic bastion fort – both to provide the required coverage and address any faults to the layout.

How about the other extreme?  Say building a fort on top of Maryland Heights?  The side of that mountain drops off far more than 24 feet within 200 yards.  However, it was clear the attacker was not going to vault up that steep slope.  The exterior side that directed the layout of Stone Fort was that which lay across the plateau on top of the Heights.  Accordingly, we should assess the layout of that work – a redoubt with two bastions – based on that assessment of the threat.

Military science often devolves back to some basic common sense.  In this case, one lays out a fort in reference to the most likely enemy line of attack.  From that, one places the most cumbersome obstacles and clearest fields of fire so as to deter the attack in the first place. However, like so much that is common sense, practical application of something so simple often introduces more complexities than a simple set of rules can address.  That, of course, is why one would have to “plan” these fortifications.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 15-6.)

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

One thought on “Fortification Friday: “The dimensions of the exterior side must vary with the relief.”

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