Low in Montgomery County's midsection is a square patch of woods, its scrubby trees rooted in soil pocked with stone. Directly north is a huge terraced pit, where workers at the Rockville Crushed Stone Quarry cut and grind a thick seam of rock.

Immediately south of the woods is a collection of opulent new homes, with basements that jut half out of the ground because the same unyielding rock is just underneath. The woods between these million-dollar houses and the massive quarry grow on one of the last unbuilt areas in Montgomery that lies atop an unusual geological remnant of earth's early life.

The woods are known as the Travilah Serpentine Area, taking the name from Travilah Road to the north and the property's abundant serpentine, a mineral that gives the tough bedrock beneath the surface a green, scaly look like snakeskin. The 258 acres on the west side of Piney Meetinghouse Road in Potomac make up Montgomery County's most unusual forest, a rare conglomeration of trees, plants and grasses that has mostly disappeared from the mid-Atlantic region.

A few years ago, the county passed on an opportunity to buy the land, despite pleas from environmentalists and the county's own naturalists that this was a remarkable botanical haven found in few other places in the world. Now a McLean developer owns it and wants to build on it.

This fall, officials begin debating whether they should take steps to help the developer build in the Travilah Serpentine Area, as the county has done with much of the surrounding land. Some argue the county should take a different path, one that leads to the woods.

It was an uphill trudge, and the day's clinging heat suffused the forest gloom. Kraig Walsleben, environmental consultant to developer Miller and Smith of McLean, led the group of visitors to a spot on the hilltop where the ground's blanket of oak leaves grew spongy, and a skirt of dark water encircled a clump of red maple.

The maples' roots had hopped out of the ground, Walsleben explained, in an effort to absorb oxygen they couldn't draw from the pond's saturated soils. The hopping had been done on nature's timetable, allowing moss and brush to fasten and ride along on the raised roots.

"This is a wonderful woods in a very developed area of the county," said Carole Bergmann, a forest expert for the county park and planning agency and a tour participant. "It's worth saving."

That's what the developer's proposal would do, said Charles F. Stuart Jr., project manager for Miller and Smith. Slightly more than half the property, 130 acres, would become a conservation park under a plan called The Reserve. The forest's best features, its unusual canopy and the rare and endangered plants found there, would be showcased in the park. The property's other half would contain as many as 80 single-family homes on 20 acres, another 200 homes for senior citizens on 37 acres and a 70-acre campus for a private school.

"We've taken the ground that's less [biologically] diverse and made it an environmentally friendly cluster development," Walsleben said.

Bergmann politely, insistently, disagreed. "There's a real diversity of species all over the property," she said. Bergmann, like other members of the county's park staff, wants to see the whole 258 acres preserved, so that sizable populations of rare plant species are protected.

The Montgomery County Planning Board has ranked the property high on its wish list for park purchase since 1994. Last spring, members of a citizens board debating revisions to the Potomac area's master plan endorsed the land's public purchase.

"When you're talking about the last best place [of its kind in Montgomery], it's not either, or, or maybe," said Ginny Barnes, a Potomac resident and member of the citizens board.

The county had a chance in 1996-97 to buy before Miller and Smith, but didn't because its money for park purchases was committed, said Bill Gries, who negotiates those purchases for the county. At that time, the county spent most of its parkland money on stream buffers in the headwaters of the Paint Branch stream in eastern Montgomery.

"You can't take advantage of every opportunity," said Gries, adding that the county still would like to acquire the property.

That the Travilah Serpentine Area even exists is testament to a geological mishap 400-500 million years ago. At that time, primeval Montgomery County was a barren rocky expanse, the only possible signs of life a few primitive lichens and seaweeds and bug-like invertebrates that dared to creep ashore from the water's edge.

Underneath, the earth was heaving. A heavy piece of ocean floor from an ancient sea to the east that geologists call the Iapetus Ocean ground against the empty continent, and the land began to ripple against the pressure. As the first bald humps of the Appalachian mountains began to rise, a sliver of the Iapetus Ocean floor sheared off, wedging itself high into the continent's stony folds.

This sliver, known to geologists as the Baltimore Complex, is now a broken belt of serpentine rock running parallel to the Appalachians. It reaches from the Travilah area at its southern end through Maryland and into Pennsylvania, said Bruce Lipin, with the U.S. Geological Survey in Reston.

The Baltimore Complex once hosted 50,000 acres of serpentine habitat in Maryland, but most of it has been lost to mining or development. Of the 2,500 acres that remain, the largest site is 1,900 acres of state-owned land called Soldier's Delight near Owings Mills in Baltimore County. Travilah is Maryland's second-largest undeveloped serpentine area, situated at the bottom of a teardrop-shaped rock mass about a mile wide and four miles long.

The thin soils that gradually eroded from the tough serpentine rock are rich in the elements magnesium and iron, with little of the calcium that plants derive from most local soils. There also are concentrations of metals such as chromium, nickel and cobalt, making the soil toxic to many kinds of plants and giving rise to the traditional description of serpentine lands as barrens.

Plants that inhabit this harsh terrain make a tenacious accommodation with their surroundings. Grasses such as bluestem, broomsedge and Indian grass, which can regenerate from fire or drought, are found in a cleared 250-foot right-of-way for a high-voltage Potomac Electric Power Co. power line that bisects the Travilah property. Among the grasses grow wildflowers, such as the increasingly uncommon whorled coreopsis and cornel-leaf aster.

Travilah also has yellow pine, usually found in the dry, sandy soils of Virginia's coastal plain. There are stunted post and blackjack oaks that hint of old age not by their size, but by the way their rough trunks flare at the bottom to secure their foothold. The land harbors seven state-listed endangered and threatened plant species, such as the potato dandelion, a rare woodland cousin of the more common backyard variety. Thirteen more plant species with dwindling populations, such as the spring forget-me-not and small-flowered bittercress also are found there.

Building on this land means "destroying critical habitat for threatened and endangered species," said Silver Spring botanist John Parrish, who has catalogued Travilah's array of plant life. "It's not the right of developers to take Maryland's natural heritage away."

But landowners do have wide latitude when it comes to plants. State botanist Chris Frye explained that plants, unlike animals or birds, are considered part of the property they inhabit, and the landowner can exercise traditional property rights.

The exception is if the plants are found in wetlands, where state and local regulations apply. "If it's dry land and it's private property, then there's no regulation of threatened and endangered [plant] species," Frye said.

Still, the county controls a key to the future of Travilah. The property now has no sewer service, though sewers serve neighboring subdivisions. County government must authorize the extension of sewer lines by the Washington Suburban Sanitary Commission.

"We think the [sewer] utilities should be available to the property, given the fact that they're available everywhere else," Stuart said. The developer's engineers say they can reduce environmental damage by running sewer lines in The Reserve's street right-of-ways, rather than along stream valleys crossing the property.

County planners said they aren't sure that's good enough. They look at Palatine, the 287-acre development of luxury homes just south of Travilah. In the early 1990s, Palatine builders buried the sewer lines to their two-acre lots in street right-of-ways, but they had to blast the serpentine rock to get them in and heavily grade the thin soils. The forest that was to envelop the mini-estates is showing signs of stress, with many dead and dying trees.

"This type of forest community is very sensitive to disturbance," said county environmental planner Katherine Nelson.

The Planning Board will tackle the sewer dilemma in the coming months as its staff readies the first draft of a new master plan for Potomac's 66 square-mile area. During this time, Travilah's developers have their building plans on hold, as they wait to see whether the county will extend sewers.

The staff draft, which goes to the planning board early next year, probably will urge that the county buy the Travilah Serpentine Area because "we're extremely concerned about the potential loss of habitat" if it receives sewer service, said Callum Murray, lead planner for the region.

The county has hired a consultant to describe the environmental impact of extending sewer service to the area and another 160 acres of land farther south of Travilah. The consultant's report is due this month.

"Because sewers spur growth, they drive everything in the [master] plan," Murray said. "The sewer issue is our most difficult issue."

If denied sewer service, Travilah's owners could try building with septic tanks, because the land already carries low-density residential zoning. The developer's engineers admit getting septic tanks to drain properly would be a challenge in an area with abundant rock and poor soil.

Tom Gallagher, a member of the Potomac citizens board helping with the master plan, wonders what the Travilah land is worth if it has no sewers and can't accommodate septic tanks.

"When a landowner goes into an investment, it's incumbent upon them to understand what they're buying," said Gallagher, a lawyer who works for a development firm with two shopping centers in Potomac.

But Miller and Smith knows Travilah still looks valuable to the county as parkland.

"We're not unwilling to sell it to the county," project manager Stuart said. The fair market value of the land, he said, should reflect the fact that improved, unbuilt lots in the nearby Palatine subdivision are selling for $475,000.

"We think we got a good deal," Stuart said, referring to the $4.1 million price Miller and Smith paid when it bought the land from Potomac Electric Power Co.'s subsidiary, Potomac Capital Investment Corp., in late 1997. "Why would we ever sell it for what we paid for it?"

William Hussmann, chairman of the planning board, said he believes that if the new Potomac master plan calls for Travilah's public purchase, it can happen. It may be another year before Potomac's master plan works its way through public hearings to its final stop, the County Council, which must adopt the plan.

"If it's on the [county land-buying] list, we buy it sooner or later," Hussmann said.

Although Travilah's ownership by Miller and Smith "puts us in a more expensive position," he added, "I think we have the power and resources to protect the land and acquire it."