Revising the Swale

The software I used to map the parabolic arc of the Minie-ball is Ballistics Explorer V6.4.6. It can be found at The developer was extremely helpful in my quest. I was able to input the muzzle velocity, bullet weight, percent of slope (we are firing downhill), and the zero range. I also provided the ground elevation along the cross section which I read from the GIS mapping provided by the city of Fredericksburg. I used vertical elevation, measured in feet, and horizontal distance, measured in yards. The output was in the form of three traces or bullet paths. This enabled me to read the amount of clearance a Union soldier had while taking cover in the Swale in defilade. I displayed this information on my maps. Any fault in portraying the data or interpreting it is entirely mine. Typical output looked like this.

Screenprint-Ballistics-Explorer-webScreen shot of Ballistics Explorer V 6.4.6

In applying the parabolic flight of the Minie-ball to my previous cross section data, I discovered that I needed to add a number of cross sections to improve the picture of the impact on the terrain. I ended up with 16 cross sections in all. I added three north of Mercer Street and three at the southern end. Next, I inserted one between each of the existing cross sections. I assumed that the Confederate soldier would attempt to shoot at anyone who poked his head up out of the Swale. In practical terms, this means that the bullet would just kiss the closest edge of the Swale, continuing on until it hit the dirt somewhere beyond.
A map of these cross sections is as follows.

Expanded-cross-sections-webMap of expanded cross sections

A typical diagram of one of the cross sections displays the ground (green), the Line of Sight (blue) and the parabolic flight of the bullet (red). The vertical elevation is provided in feet, at five foot increments. The horizontal distance is provided in yards, at five yard increments. Also noted are the modern day locations of various roads: Sunken Road with its Stone Wall, and Willis Street, Littlepage Street. I also note the originally identified “Zone of Protection” where a Union soldier might possibly be in defilade. This is seen below.
[Insert 100’ N of Lafayette cross section]

A closer view of the point at the top of the Swale is as follows. On the cross section, I note the area of true defilade. This accounts for sufficient space to allow the Union soldier to stay below the Line of Sight of the Confederate soldier. This necessarily reduces the original zone of protection or potential defilade.


Integrating all of the data into one view provides a fuller understanding of what the civil war soldiers felt and saw. I show two lines; the Top of Slope (brown) and where the Line of Sight (blue) strikes the ground. This is the original Zone of Protection.


In the next view, I display three new lines. The Line of Safety at the side nearest to the Confederate soldier (orange), where the Minie-ball bullet would strike the ground, and the Line of Safety farthest from the Confederate soldier(orange). Now, a few points concerning the Line of Safety. It begins where the Union soldier would have two feet of vertical clearance between the ground and the line of sight. This is not adjacent to the Top of Slope, but back down the slope to account for the depth needed to be in defilade. The Line of Safety far side also has limits. I used one foot of vertical clearance in this instance. It is important to note that approximately half of the area between the two Lines of Safety would be bullet free. No matter where a Union soldier lay in the region closest to the Sunken Road, no bullet fired from the Sunken Road would hit. In the second region between where a bullet could hit and the Line of Safety far side, a bullet might hit, but only by pure chance. As long as the Union soldier poked his head or body up above the Line of Sight. If he didn’t, he was invisible. If he did, and was seen, then a Confederate soldier could attempt to guess his location and shoot in that vicinity, again, only by pure chance.


Putting both line maps together, we get this composite view.


At the north end between Hanover Street and Mercer Street, I have a number of concerns. This area did not map as well. In fact, I stopped my delineation of the lines at the alley located between Hanover Street and Mercer Street. This alley parallels Hanover angling northeast. The ground in this area has been significantly graded in recent times as new houses replaced older dwellings. Regardless, the Swale does exist, even in this area north of Mercer Street.
You will also notice dashed lines in the vicinity of Mercer Street. I used this to indicate that the reliability of the data along the street is somewhat suspect, especially on Mercer Street. If you look at the version of the map that displays the satellite view, most of this region is taken up by the full width of the road and sidewalk improvements. These are important post-battle changes to the terrain. Further south, the map at Lafayette Boulevard, also reflects the road improvements.


Uses of the Swale:

1. During the battle of Fredericksburg, Union troops:

  • a. Advanced only as far as the Swale, or
  • b. Retreated to the Swale from locations closer to the Sunken Road
  • c. Took refuge in the Swale
  • d. Returned fire from the Swale

2. The Swale:

  • a. As a terrain feature, did not impede movement
  • b. Late in the battle, the large number of Union troops taking shelter in the Swale was a major impediment to movement of attacking troops.

3. After the battle, the Union troops increased the Swale’s depth during the nights of 13 and 14 December, especially in the north end between Hanover Street and Mercer Street.
4. Following the battle, the northern section of the Swale, between Hanover Street and Mercer Street, was used as a burial trench for over 600 Union dead.
1. The Swale at Mercer Square was a significant terrain feature in the Battle of Fredericksburg. It reduced the number of Union casualties by providing cover and concealment from the fire by Confederates located in the Sunken Road.
2. The width of the area in defilade, and therefore the number of troops that could take shelter in the Swale, has been under appreciated by historians and therefore undervalued.
In the next blog, we will take a look at the impact of Union troops firing back at the Confederate soldiers in the Sunken Road and on top of Marye’s and Willis Hills.



About Peter Glyer

I am retired with a lifelong interest in history, primarily the Civil War and WWII - Europe. I was an Army engineer, hence my interest in terrain. I graduated with a degree in City and Regional Planning and a Masters in International Relations.
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2 Responses to Revising the Swale

  1. Pingback: National Cemetery (Part 3) – Swale Trench and Wallace’s Ice House | Mercer Square

  2. sabers8th says:

    Great post and blog! I am curious were you able to break out what the regiments were armed with in order to document all weapons that might have been used e.g. Enfield rifle, Smoothbores. I think it would fascinating to see the other weapons incorporated. Are you looking to expand it into artillery as well?



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