In a bucolic corner of Bucks County, a rust-hued dirt road winds beside a shallow creek trickling quietly through green fields.

Where the evocatively named Sheep Hole Road intersects with Headquarters Road, though, the pastoral charm is shattered by an orange and white "Road Closed" sign, barring access to a crumbling concrete bridge.

The78-foot-long structure over Tinicum Creek, Headquarters Road Bridge, is dangerous and has to go, PennDot officials say. It hasn’t been safe for cars to cross since 2011.

"The final nail in the coffin was the big four-foot hole that opened up in the bridge deck," said Ryan Whittington, a consultant project manager contracted by PennDot.

Addressing the crumbling structure has been on the state's to-do list since 2004, officials said, and PennDot is seeking to start construction on a new bridge by 2018.

But what appears to be a decaying roadway is actually a historic treasure, said members of the Delaware Riverkeeper Network, a group trying to preserve the structure.

"We know of no other pier-to-pier bridges of this age in the country," said Rob Reynolds, an assistant history professor at Kutztown University. "It's an amazing survivor."

While the bridge's deck is concrete, the foundation and the two mid-stream piers that support it --  masonry pillars built of lime and sand encased in local stone -- date to 1812. The region's 19th century residents put planks over the piers and replaced them as they weathered to cross between what are now Ottsville and Erwinna. The planks weren't replaced by concrete until the early 20th century, but the piers remain, and that substructure makes the bridge the ninth oldest in the state, the Delaware Riverkeeper Network reported.

The area around the bridge is the 575-acre Ridge Valley Rural Historic District, a site on the National Register of Historic Places. The Riverkeeper Network is also concerned about the environmental and cultural damage that could be done by getting rid of the bridge.

The group wants PennDot to rehabilitate the structure, and notes that a 2013 study found a rehab project would cost about $2.65 million while a replacement would cost $4.14 million.

"PennDot has a history of going in and ripping and replacing," said Ed Rogers of the Delaware Riverkeeper Network.

PennDot noted it has won awards for its bridge preservation program, and said it is not ignorant of the bridge's history. Indeed, officials don’t dispute rehab would be cheaper than replacement, but only in the short term. But the 16-foot-wide, one-lane span is too far gone to restore, the agency said.  Years of erosion from the creek have taken a toll on the foundation. The original stone masonry is crumbling and the concrete deck is falling apart. 

The cost of maintaining the original piers would eventually exceed the cost of a replacement, and repairs wouldn't change the fact that the existing bridge's configuration can't support modern traffic.

PennDot officials' preferred alternative is a new 24-foot-wide, two-lane structure that would accommodate larger vehicles such as fire trucks. PennDot says its early estimate for a new bridge is $2.4 million, far less than the network's.

The State Historic Preservation Office has said PennDot's proposed replacement plan takes into account the need for historic preservation, a decision the Riverkeeper Network disputes. The group plans to keep fighting, and expected to voice concerns at a public meeting Wednesday that will address mitigating the environmental impact and ensuring the new construction will blend with the surrounding historic district.

"If you can't save one of the last bridges in this country of this age and size," Reynolds said, "what can you save?"