Fortification Friday: The simple, effective Abattis

Abattis?  Abatis?  Abbattis?

And how do you pronounce it?  Ah-ba-tee?  Ah-bae-tus?  Or, for those in the deep south… Awe-bat-us.

How about Russian? Zaseka.  And the Russians knew a thing or two about abattis.  Their Zasechnaya Cherta was a thousand kilometer line against the Tatars, dating to the 12th and 13th centuries, built initially from felled trees.  Thus, on a grand scale rivaling the Great Wall of China, the Russians used a basic obstacle to form a defense against cavalry raids.  We find reference to felled tree or well placed limb obstacles from ancient times right up through the twentieth century.

For this post, I will stick with Mahan’s spelling of the word … Abattis, which he described as such:

Abattis. The large limbs of trees are selected for an abattis.  The smaller branches are chopped off, and the ends, pointed and interlaced with some care, are presented towards the enemy.  The large end of the limb is secured to the ground by a crochet-picket, and may be partly imbedded to prevent its being readily torn up.

One of the best methods for forming an abattis, and which is peculiarly adapted to strengthening the skirts of a wood occupied by light troops, is to fell the trees so that their branches will interlace, cutting the trunk in such a way that the tree will hang to the stump by a portion uncut.  The stumps may be left high enough to cover a man in the act of firing.

If we are particular, there are two variety of abattis described here.  The first is that of limbs arranged, and preferably pinned, in front of the works.  Mahan offered this illustration for that form of abattis:

PlateIVFig29

On the left we see the pickets designed to retain the limbs. Mahan offered specific instructions for laying these sort of obstacles:

Abattis are placed in front of the ditch; in this position they must be covered from the enemy’s fire by a small glacis. They are sometimes placed in the ditch against the counterscarp.

Note on the right side of Figure 29 above the glacis.  And think about how this would work in the defense.  An attacker would advance up that glacis, every step bringing them into greater profile within the defender’s view.  At the height of the glacis, the attacker is faced with the need to descend into a mess of twisted branches…all while the defender has a clear shot.  And if the attacker does chose to deal with the abattis, all the work is done in plain view… and within range of… the defender.  At least that is how it was supposed to work against infantry and cavalry.  Junius Wheeler, in his post-war update to Mahan’s lesson plans, offered this illustration:

WheelerFig66

As for artillery, the main reason Mahan suggested the glacis is to make difficult any attempt to break up the abattis by shot or shell.  If well constructed, the glacis would serve to ricochet the projectile over the abattis.  And if the rest of the fort were properly constructed, the projectile would continue to sail over the parapet and all vital areas… expending at some point well to the rear of the defense.  We see that illustrated, in reference to the other components, in Figure 26:

PlateIIIFig26

The obstacle at the bottom of the ditch in this case is a small picket.  But we might refer to some wartime photos to see an abattis used in the ditch, laid against the counterscarp, as Mahan suggested:

32419u

We see an abattis laid against the counterscarp, which we are looking over, in the foreground.  In the background we see palisades and other obstacles… which we will discuss in due time.  Wheeler, writing post-war, offered one other alternative along this theme.  He suggested planting the abattis upright in the ditch as so:

WheelerFig65

The other variation mentioned by Mahan, felled trees still attached to the stump, was perhaps more so a field expedient.  As he said, perhaps where light troops were defending a wood line.  Beyond just forming a supplement to the main fortification, the abattis might be called to serve as the main line of defense in some situations:

This is an excellent obstacle in a wooded country, and admits of good defense, if a slight parapet is thrown up behind it.  The parapet may be made of the trunks of trees laid on each other with a shallow ditch, or trench, behind them; the earth from which is thrown against the trunks. In an open position it may be relied on as a security against surprise, particularly of cavalry.

Abattis were relatively simple, as far as obstacles go.  Very little effort needed to create them.  And the materials were usually easy to come by.  Likewise, once in place the abattis were easy to maintain. So we see a lot of them in Civil War photographs.  One of my favorite studies in that regard is Fort McAllister:

03371a_Crop

This is the view of Hazen’s Division attacking the fort on December 13, 1864.  Not hard to put yourself in those shoes.

We don’t see much in the way of a glacis protecting these abattis.  And these are very far out from the main fortification’s ditch.  Another point of view shows a section of abattis closer to the fort’s ditch:

03138a_Crop

I think we are have several factors involved with the placement of abattis at Fort McAllister.  To begin with, the Ogeechee River’s alluvial plain did not offer much in the way of relief for the defender’s advantage.  So to build a glacis, one needed to displace a lot of sand.   And that is sand, not earth.  And when dry, sand does not stand up well, bringing the need for some form of revetment.  Bottom line, a lot more work.

An operational factor at play is the nature of the defense.  Fort McAllister was built to stop Federal gunboats and ironclads from venturing upriver.  So its facings were strengthened accordingly.  The marshes and other natural obstacles would deter any flanking operation from a sea-based attacker. So the Federals flanked the works by way of a march from Atlanta.

Lastly, the distance of the abattis from the ditch is, I think, significant.  It being a tactical factor. By 1864 both armies were keenly aware of longer engagement ranges.  The defenders of Fort McAllister could push out those abattis and feel comfortable their artillery and musketry would range.  But keep in mind, when viewing the photos, the abattis appears to be “out far” in some sections but close in at other points.  All were, I would submit, placed with respect to tactical needs.

One more photo from Fort McAllister that reinforces Mahan’s discussion of abattis:

03139r

Look at this mass of twisted limbs and branches.  It is not the heavy limbs that Mahan may have preferred.  But it is an obstacle none-the-less.  Again, imagine having to step through that mess in order to get at the next layer of obstacle… all while under fire from the defender.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 45.)

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