Earlier this week I mentioned examinations aimed at place a battery in the marshes behind Morris Island in late July 1863. Let me follow that up with details about how the engineers surveyed the marsh and what they found. Colonel Edward Serrell, 1st New York Engineers, recorded those details in his journal of operations on Morris Island, incorporated into his official report. The survey technique was simple and straight forward. The team used 30 foot long, ¾-inch diameter iron rod:
The extreme edges of the swamps on the small creeks are hard, and frequently filled with oysters and oyster shells, but at a few feet from the water they become very soft, and within 10 or 12 feet the mud will not afford foothold for a man.
In these marshes, back from the harder edges of the creeks, the mud is from 18 to 23 feet deep, generally about 20 feet deep. It is so soft that the weight of the sounding iron will carry it down 8 or 10 feet, and a man can with one hand push it the remainder of the distance.
Down below the mud was hard packed sand. Serrell went on to describe the surface features of the marsh:
On the mud there is a growth of very coarse grass, Spartina glabra and Uniola spicata, which is 4 or 5 feet high. It does not, however, form a sod, and the roots are not deep, but fine; they afford but little resistance to anything sinking through them. Extreme high water covers the surface of the mud.
Geologically the marsh is held to be sedimentary deposits of the very finest particles, brought down by the fresh-water streams, and are mostly vegetable. The blowing sands from the outer beaches, which are less recent in their formation, are sometimes mixed with the mud.
The resistance is increased by quantities of small shells, Auri-cola bidentata and Littorina irroratus, and occasionally muscles.
In other words, a lot of this stuff:
Crawling with these:
Hey, I like to touch all the bases… not just the cannon blasts and gleaming bayonets. But you get the picture.
Serrell described the marsh mud in terms of its load bearing capacity:
A man’s foot, having a surface of from 30 to 35 inches, and sustaining a weight equal to 150 pounds, would sink into the mud 18 to 25 inches every step, and, if these were not made with some rapidity, much deeper. Two elements are involved here not in the other case; first, that of the motion of the foot, and, second, the suction of the mud against the leg, one tending to favor the penetration, the other retarding it; neither of these conditions applied where the load was static and rested on the surface.
Yes, things will sink into the mud. Which is not a good thing. Furthermore, the mud’s consistency was that of jelly (Serrell’s word, mind you.) Any movement sent vibrations out. Men working on top of a plank would often send out vibrations over hundreds of feet across the surface.
But Serrell countered those issues with some practical observations:
In the case of a man attempting to walk, it was shown that, under the conditions he presented, something like a force of 500 or 550 pounds to the square foot could not be sustained by the marsh, but here there was the heavy weight of the body brought on the small point of the toe, or the side of the foot, or upon some other part of the sole of the shoe, in motion. If a battery was to be built, so long as the guns were not fired the forces would essentially be static, and the condition of rest become an important element in the calculation.
Reasoning a platform could be built on the marsh, Serrell then focused on the other problems – how to place such a platform on the marsh and how to deal with vibrations created when the gun fired. We’ll look at that in the next installment on the Swamp Angel.
(Citation from Serrell’s report, OR, Series I, Volume 28, Part I, Serial 46, pages 230-2.)