The physical evidence of South Carolina's largest Revolutionary War battle has vanished from view but Charleston archaeologists hope that changes this week as they dig behind a grand home.

Specifically, they hope to unearth evidence of siege lines British soldiers dug in the spring of 1780 in their successful effort to take the South's largest port.

Back then, the city didn't extend beyond Calhoun Street (then called Boundary Street).

The British Navy was embarrassed here in 1776 at the Battle of Sullivan's Island — a patriotic victory commemorated every June 28 on Carolina Day. Almost four years later, however, Sir Henry Clinton and about 8,000 British troops returned to focus on capturing Charles Towne and changing the nature of the war.

As these troops crossed the Ashley River and marched down the peninsula, they dug about 1½ miles of trenches somewhere between modern-day Spring Street and Marion Square.

No one knows exactly where, however, and their elusiveness has confounded Carl Borick, director of the Charleston Museum and author of "A Gallant Defense: The Siege of Charleston, 1780." 

"It's my Holy Grail," he said.

A battleground, then a backyard

The Historic Charleston Foundation has been researching the rear yard behind its Aiken Rhett House property to learn more about how wealthy residents and their slaves shared the large area, which might have contained an ornamental garden as well as work space.

Recent ground-penetrating radar showed evidence of a pit and possibly pathways of brick, shell or compacted earth.

But it also found evidence "consistent with a filled ditch."

That caught the eye of Borick, who has attempted to transpose a 1780 map by British Army mapmaker Charles Blaskowitz onto Charleston's modern street grid.

Borick had suspected either a siege line or an approach trench was dug near present day Elizabeth Street, which runs just outside the Aiken Rhett's rear wall. The house itself didn't exist at the time: It was built around 1820, more than a generation after the war ended.

British troops would have been active near here because they wanted to capture the dam that controlled water flow to a moat along present-day Ann Street. Borick sees the odd bend in Alexander Street, just south of Mary Street, as the approximate location for that dam.

The key to capturing Charleston was the patriots' hornworks at present day Marion Square, where an iron fence today surrounds a small tabby fragment of the once formidable defense.

British soldiers approached this strategic objective by digging a series of trenches about 3 to 4 feet deep, giving them cover as they maneuvered their troops and large guns toward the stronghold.

The siege lines were built east-west between the marshes and creeks just north of the city, beginning around Spring Street on April 1 and getting close to Marion Square by early May.

"The last thing you want to do is storm enemy works. Bunker Hill proved that was a very bad idea," Borick says, referring to the British's 1775 victory outside Boston that came at a horrible cost.

The siege lines worked and the city fell on May 12 resulting in the capture of about 5,200 patriot troops in one of their worst defeats.

As soon as the British seized control of the city — the South's largest port, Borick says they quickly moved to fill in their trenches so the patriots could not use them for their own attack.

The trenches have been largely hiding below the surface ever since.

Final subsection

As the Revolutionary War ended in 1783, Charles Towne incorporated, changed its name to Charleston and experienced one of its greatest periods of prosperity.

What had been a battlefield devoid of trees and homes soon developed into dense, new neighborhoods, including Radcliffeborough and Mazyck Wraggborough, complete with new parks, churches and new shops and homes along King Street. The hornworks were razed, and the area eventually became Marion Square. 

So far, Borick said, the only archaeological evidence of the 1780 siege that's been found includes a 4-pound cannonball unearthed off Vanderhorst Street in the 1970s, a 5½-inch shell recently uncovered during the Gaillard Center work and some grapeshot found in an earlier dig behind the Aiken Rhett House.

"No evidence has ever been found of the siege lines, and to me, that's a big deal," Borick says.

Borick has spent considerable time analyzing Charleston's terrain, such as the bend in Alexander Street and subtle elevation changes, to try to match the current city with defenses depicted on centuries-old maps.

Modern day Ann Street was the approximate location of the moat or canal dug by the patriots to protect their hornworks.

If this dig were to confirm that this was part of the British 3rd parallel or approach trench, from an archaeological standpoint it would give a point of reference for other sections of the British-American lines during the Siege.

This month's dig is a cooperative effort between the College of Charleston's archaeological field school, the Charleston Museum, which employs two of the school's instructor archaeologists and the foundation, which owns and operates the Aiken Rhett House museum at 48 Elizabeth St.

The team of about 10 students and instructors has mapped out seven, 25-square-foot sections they hope will yield evidence of a trench, most likely by noticeable color changes in the soil. If they're lucky, they might find a late 18th century artifact.

"It would be great to find a British soldier's button," Borick says.

Archaeologist Martha Zierden, one of three instructors overseeing the students' dig, says she found previous evidence of a trench-like form in the backyard but wasn't aware of its possible tie to the Revolutionary War.

The current dig began Monday and is expected to last about three weeks. The public can get a glimpse for free by entering from the Mary Street gate between 9 a.m. to 10:30 a.m. Thursdays.

It's unclear how deep the dig will go. "We'll dig until we run out of artifacts and culturally modified soil," Zierden says.

They are expected to get a sense of any possible success soon because Zierden says one only has to dig about a foot down to get to the period of the 1780s. That could happen as soon as Wednesday. As those shovelfuls are carefully laid into wheelbarrows and sifted through screens, more answers will emerge.

It remains to be seen whether one of Charleston's long-lost siege lines is found. 

"It's got to be back here somewhere," Borick says, "but I've been disappointed before."