After hours of scrambling up and down ledges, bushwhacking through underbrush and clambering over stone walls in flurries and sub-20-degree temperatures, our group reached an apparent impasse earlier this week: a tumbling, mostly frozen-over stream.

"Come on," I joked. "It's only waist-deep."

Others had a better idea. They dragged fallen branches and logs across a narrow, less-turbulent stretch and in about 15 minutes constructed a rickety, very temporary bridge, with a handrail no less.

One by one all 10 of us edged safely across — even an oversized Husky built more like a woolly mammoth than a canine — and resumed our hike.

We were traversing a rolling swath of forest straddling the towns of North Stonington, Preston and Griswold that the Avalonia Land Conservancy is close to acquiring, thereby helping extend a greenbelt from northern New London County to Long Island Sound.

With the expected purchase this year of a 409-acre parcel known as the Tri-Town Ridgeline Forest, Avalonia will celebrate its 50th anniversary in grand style.

"This is the missing piece," Sue Sutherland, an Avalonia board member who chairs the nonprofit organization's acquisition committee, said after we thawed out from our 4-mile hike.

Founded in 1968 by seven people who first preserved a 4.6-acre spit of land on Masons Island in Stonington, the Avalonia Land Conservancy — known then as the Mashantucket Land Trust — has grown into one of the region's leading conservation organizations. With 1,800 members and $20 million in assets, the conservancy owns or holds easements on almost 100 properties in eight towns amounting to more than 3,500 acres, including two small islands just over the Connecticut borders in Rhode Island and New York.

Avalonia's efforts received a huge boost last month when the state approved a $555,000 grant to be applied toward the purchase of the Tri-Town Ridgeline Forest, now owned by a Tennessee developer. The organization must raise about $300,000 more through other grants and individual donations to complete the sale.

Chuck Toal, Avalonia's director of development and programs, said the organization is about to launch a fund drive for the Tri-Town Ridgeline Forest acquisition, as well to buy other parcels as part of a plan to protect an additional 2,000 acres.

Sutherland and fellow board member Richard Conant Jr., a retired marine biologist, are confident of success once land-preservation advocates fully appreciate the value of the Tri-Town property.

"It connects a large, unbroken, forested wetland tract," contiguous to 1,400 acres protected by the Nature Conservancy and in Pachaug State Forest, Conant said. Avalonia also owns a separate adjoining tract and has applied for a state grant to acquire a 54-acre adjoining parcel.

Protecting connected acreage helps "preserve an entire ecosystem," Conant said. "That's real conservation."

Safeguarding such parcels from development has benefits that go beyond protecting flora, fauna and watersheds — Avalonia preserves are now open to the public for walking on designated trails and, when appropriate, for launching canoes and kayaks. Visit www.avalonialandconservancy.org to view a map of all the properties and visitation guidelines.

The Tri-Town Ridgeline Forest would be a worthy addition to this network, with two 500-foot hills that provide expansive views of dense woodlands, lakes and streams; impressive chestnut oaks; a "lost pond" formed by a centuries-old stone and earthen dam; and extensive stone walls.

These walls are of particular interest to Avalonia members Carl Tjerandsen and Markham Starr, who for years have traipsed over hill and dale throughout the region to research the origins of these familiar stone structures.

Tjerandsen, who hopes the acquisition of the Tri-Town Ridgeline Forest will help preserve these walls, helped blaze the twisted route our group followed Monday along overgrown logging roads, over slightly more established paths and across the aforementioned swollen, icy stream. He paused every so often to consult his GPS, but mostly to extol the beauty of the forest and the "power of the stones."

During those pauses, Starr, a documentary photographer and author of the 2016 book, "Ceremonial Stonework: The Enduring Native American Presence on the Land," discussed how he differentiates walls built by early Native Americans from those built later by Colonial farmers. For the most part, he said, the farmers built walls to enclose or keep out animals, while the Native Americans built them for more spiritual reasons.

I'll write about their research more fully in a couple of weeks.

Avalonia is one of several land preservation organizations that collectively have protected some of this region's most treasured parks and wildlife areas for future generations. Run by dedicated volunteers and financed in large part by contributions, they deserve our gratitude and support.

Avalonia will celebrate its golden anniversary from 5:30 to 7:30 p.m. Feb. 21 at Mystic Aquarium. Rob Klee, commissioner of the Connecticut Department of Energy and Environmental Protection, will be the guest speaker. More information about the event is available on the organization's website.