The Hessians were out for blood that autumn day in 1777. They marched 10 miles from Haddonfield to Red Bank, hoping to surprise the American defenders of Fort Mercer on the Delaware River.

Instead, they fell into a trap.

Many of Britain's German allies passed over the abandoned earthen walls topped with pointed logs, and then cheered, thinking they'd breached the fort and were close to victory.

On the other side, though, was another wall - and a deadly hail of artillery and musket fire that cut through their ranks like a scythe.

Soon, the ground was blue with the uniforms of 400 dead and dying Hessians - fully a third of the assaulting German force.

Their remains were interred the next day outside the fort, at what is now the Red Bank Battlefield Park in National Park, Gloucester County.

In June, the mass burial sites and surrounding battleground will become an archaeological dig expected to give up numerous artifacts and clues about what happened 238 years ago. Research, including the translation of German and French accounts, is already shedding new light.

The effort - funded with a $46,200 grant from the National Park Service - will be detailed in a free public program at 7 p.m. April 9 in Room 500 of the Health and Sciences Building at Rowan College at Gloucester County in Sewell.

Among the relics likely to be recovered? Belt and shoe buckles, parts of weapons, musket balls, metal badges, shrapnel, cannonballs, and canister - metal balls fired from artillery like a giant shotgun. Their distribution may show where the fight was hottest.

What happened at Red Bank is a great story, said Jennifer Janofsky, curator of the Red Bank Battlefield Park and Whitall House and also the Giordano Fellow in Public History at Rowan University. "I think it's just as compelling a story as Valley Forge.

"The tide of the Revolution began to turn in the fall of 1777 with the tenacity of the soldiers fighting at Fort Mercer," she said. "This is what gave Washington the momentum he needed."

The archaeology work at the site will "help us sift through the myths, distill them, and remove the stories that are irrelevant so we can better understand and interpret the battlefield," said Wade Catts, regional director of cultural resources for JMA, the West Chester firm that will conduct the archaeological work.

"This project will be challenging but manageable compared to Gettysburg," he said. "Red Bank is very, very exciting because nothing quite like this has ever been done there."

Fort Mercer on the New Jersey side of the Delaware River and Fort Mifflin on the Pennsylvania side were major obstacles to the British forces in 1777. The forts' artillery fire along with chevaux-de-frise - iron-tipped logs set in the river to sink or disable enemy ships - prevented a dozen British ships from supplying the 18,000-strong redcoat army occupying Philadelphia.

Without the supplies, the British couldn't hold the city, much less pursue the Continental Army under Gen. George Washington, who told Fort Mifflin's commander to hold the fort to "the last extremity."

The stakes were high on Oct. 22, 1777, as the Hessians left Haddonfield. Fort Mercer's defenders knew the Germans were likely to attack, and they reconfigured the fort - with a French engineer's help - to make it more defensible, Janofsky said.

A local 18-year-old, Jonas Cattell, who heard of the enemy's plans in Haddonfield and ran ahead to the fort, provided the extra warning needed to make final preparations.

Up to 600 soldiers, including many African Americans, manned Mercer, which was commanded by Col. Christopher Greene, a Rhode Islander. They faced an attack by about 1,200 enemy soldiers, part of a larger force of 2,300, according to the latest research.

One Hessian column, American Capt. Stephen Olney wrote in a journal, "had made its way into that part of the fort which we had evacuated, and supposing they were masters of the fort, huzzaed! and came, perhaps, to cut up their prisoners.".

That's when the defenders opened up from the reengineered fort, mowing down the Hessians under Count Emil Kurt von Donop.

The enemy's numbers were further thinned by bar-shot - two cannon balls connected by an iron bar - fired from American vessels in the river. At least one Hessian soldier was decapitated.

Donop was mortally wounded, and much of the command staff was killed as other attackers were "repelled with great loss," Hessian Capt. Johann Ewald wrote in his journal.

The British faced more bad news the next day. The 64-gun frigate Augusta came under heavy fire, struck the shoals on the New Jersey side, caught fire, and blew up.

At the Red Bank Battlefield, ground-penetrating radar, metal detectors, and excavation will help locate and recover artifacts. If human remains are encountered along "with grave goods, my preference is to leave them in place," Catts said. "We'll make that decision when we come to it.

"This will be done carefully. On some level, this is sacred ground."

Identifying the original footprint of the fort will be another goal of the work, said Janofsky, who plans to give tours to about 200 local schoolchildren on June 8 and 10.

"I think a lot will be found - everything from buttons to cannon shot," she said. "This is the first systematic archaeological study of the battlefield and it should give us a more comprehensive story."

ecolimore@phillynews.com

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