The Patterson viaduct, what's left of it, is a forgotten remnant of Howard County's history.

One stone arch stands by the banks of the Patapsco River just south of Ellicott City, barely visible from the road and overgrown with vegetation. It is a depository for debris that the river leaves behind after a storm. A broken telephone pole hangs over its side, and fragments from teen-age parties and hobos' bonfires are scattered under it.

Nonetheless, railroad history buffs consider the remnants of the bridge, one of the oldest railroad spans in the world, historically significant and are worried that plans to construct a new bridge to replace the 40-year-old Ilchester Road crossing nearby could destroy the viaduct.

In 1830 the Patterson viaduct, amid great fanfare, carried the Baltimore & Ohio Railroad's first train, actually a horse-drawn carriage, over the river to Ellicott's Mills.

This inaugural journey, between Baltimore and Ellicott's Mills (now known as Ellicott City) is significant, the preservationists say, because it proved that the design and construction concepts of this country's first railroad engineers could be successfully implemented and that the railroad could compete with the canal as a method of transportation.

But few people outside a handful of railroad enthusiasts know about the viaduct's heritage. So the enthusiasts are seeking to draw attention to the bridge and its predicament, and are asking the county to consider its fragile condition before going ahead with plans for the new bridge. They fear that the one remaining arch might be threatened by the Ilchester Road construction.

"The county should be made aware of the significance of the arch," said Gary Shlerf, president of the B&O Historical Society. "We'd like to see the site protected and a study done to make sure that there is no further visual impact on the structure."

County officials, who acknowledge they were unaware of the Patterson viaduct when planning the new bridge, said they plan to move forward with construction and do not believe the historic site will be damaged.

The old Ilchester Road bridge, which was built 40 years ago, has deteriorated and must be replaced, said William Riley, the county's chief engineer. He doesn't think the Patterson viaduct will be harmed.

"It's been there all those years I don't see any reason why it would be affected,'' Riley said.

A contract was awarded in October for design of the $355,000 project, and it doesn't include funds for an impact study, Riley said. "We will make sure we look at it when we do the hydroelectric study."

John Hankey, a railroad historian and former curator at the Baltimore & Ohio Railroad museum, said that the arch might be endangered by erosion caused by the stripping away of surrounding vegetation, by careless use of heavy construction equipment, or by being buried by fill dirt.

Council member Angela Beltram, who has an interest in historic preservation, said she would like to see the county take steps to ensure that the viaduct is preserved.

"We have so much development and so many historic sites we need to do something" to make sure they are preserved, she said.

The county is studying preservation laws in neighboring counties and expects to draft its own after hearing a consultant's report, Beltram said.

The viaduct is on the National Register of Historic Places and, as part of the original main line of the B&O, is on the list of landmarks designated by the American Society of Civil Engineers.

A number of railroad-related historic sites already are recognized by the county, including the B&O Railroad Station in Ellicott City, and two bridges, the Thomas Viaduct in Elkridge and the Bullman truss bridge in Savage. Hankey said Howard County has "the finest concentration of surviving railroad structures in {the} country."

The viaduct, named for William Patterson, one of the B&O's first directors, was almost destroyed in the flood of 1868. Three arches were swept away during that disaster and the remaining arch became the abutment for an iron bridge, which was dismantled in 1903 after the railroad was rerouted.

John Morgan, a University of Maryland history student who has done extensive research on the viaduct, said he would "hate to see the arch go." He envisions it "semirestored" one day and used as part of foot path to Patapsco State Park across the river.

Robert Vogel, curator of civil engineering at the Smithsonian Institution, said that even a lone arch is important to the historical picture of an area as a whole.

"It is one of the few remaining structures on the old main line of the B&O. Even fragments become monuments when so little is left," he said.