For about as long as roads have led travelers through the Whitemarsh Valley, vast, rolling pastures and crop fields have distinguished the hillsides just outside Philadelphia's northwest border.

The 426 acres of Erdenheim Farm have endured the centuries as open, agricultural space, an artifact of William Penn's era still alive and green amid a sea of suburbs. In their time, colonial grantees, plantation owners, and Gilded Age millionaires each had a hand in this legacy, though it became threatened when longtime owner F. Eugene "Fitz" Dixon died in 2006. Developers looked at the prime tracts and saw a once-in-a-generation opportunity to move in and build.

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That, however, likely has passed.

Tomorrow, if a 10-way real estate deal for an unspecified sum goes through as painstakingly negotiated over the last decade, Erdenheim Farm's status will be locked in: farmland in perpetuity, its future the same as its past.

"This is the largest land-conservation deal that has ever taken place in Montgomery County, if not Southeast Pennsylvania," said Hugh Moulton, head of the Whitemarsh Foundation, a nonprofit founded in 2001 with the singular focus of preserving Erdenheim.

How the foundation reached the cusp of its goal in eight years required complicated deal-making, massive sums from the public and private sectors, the cooperation of Dixon's heirs and their favored nonprofit, a Whitemarsh tax referendum, and, ultimately, a Radnor executive with an itch to farm.

"This will be remembered hundreds of years from now," said Paul W. Meyer, director of the Morris Arboretum, which adjoins Erdenheim. "It's a tremendous achievement."

Erdenheim Farm is so treasured, Meyer and others explained, largely because of how metropolitan Philadelphia has spread around it. It is the centerpiece of a 2,000-acre stretch of Wissahickon Valley open space that connects Fairmount Park to Fort Washington State Park.

Erdenheim's endurance in private hands connected the arboretum, smaller parks, and country clubs to form the patchwork green link in this chain, which preservationists have tried to protect from encroachment since 1899.

"It's the Central Park of Southeastern Pennsylvania," Meyer said. "I used to think that we were on the edge of the city. Now we're in the middle of the city."

What the Fairmount Park Commission started trying to do with a plan for a park-to-park carriageway in the 19th century likely will become official tomorrow. The inked deal will formally preserve the open space and set out public trails to connect the Fairmount and Fort Washington parks.

It also reassures locals that where they grew up counting Black Angus cattle and Border Cheviot sheep will remain as they have always known it.

"It is truly an iconic piece of ground, and I couldn't imagine not having it," Montgomery County Commissioner Bruce L. Castor Jr. said.

Records show that Erdenheim Farm passed from Delaware Indian territory into William Penn's Manor of Springfield, with Penn himself inspecting it in 1684. It has been private agricultural land ever since, via a series of deep-pocketed landholders that led to Dixon.

The onetime 76ers owner inherited the farm in 1971 from his uncle George D. Widener, but the will stipulated that a choice 117-acre tract at Stenton Avenue and Flourtown Road go to the Natural Lands Trust, a conservation nonprofit, upon Dixon's death.

Dixon resisted proposals for decades to sell off the rest of the land, but he sold about 50 acres on its western edge about a decade ago for a retirement-housing community, the Hill at Whitemarsh, where his widow lives. Edith Dixon could not be reached for comment.

In that deal, the Whitemarsh Foundation was set up to handle preserving the rest of the farm.

A 2003 tax referendum in Whitemarsh funneled some revenue into a fund to buy the farm when it became available. Dixon's death triggered the transactions that likely will culminate tomorrow.

The Whitemarsh Foundation acquired the farm's 98-acre Angus Tract in 2008 with $13.5 million in tax revenue, county, and state grants and a $5 million settlement from a Merck chemical spill into Wissahickon Creek, which runs through the farm.

Tomorrow, in a 10-party closing, the foundation is to buy 91 more acres of the Sheep Tract with $12.5 million in county and local funding.

The crowded meeting will be held in Radnor, where a local executive has his own part to play. Peter McCausland, chief executive officer of Airgas, is expected to buy 243 acres from the Dixon family and the Natural Lands Trust to use as farmland. None of the parties would disclose that price.

The land will come with a permanent conservation easement, which is fine by McCausland. As a first-time farmer, he will hire the Dixon family's staff to help manage the cattle and sheep.

"We had an interest in owning a farm that goes way back," McCausland said Friday. "Another important motivation was to preserve the farm as a farm, because it's been that way since the founders."

He is, however, contemplating some changes: adding an orchard and perhaps a second breed of cattle. Word around the farm is that a pumpkin patch is in discussion. But that's about it.

McCausland's family will move into Dixon's 60-room mansion, which overlooks the Wissahickon Creek and the rolling farmland just about as far as the eye can see.

"We keep it because we like to look out at the countryside," Dixon said in a 1990 Inquirer interview.

Contact staff writer Derrick Nunnally at 610-313-8212 or dnunnally@phillynews.com.