Two roads diverged in a wood, and I -- I took the one less traveled by . . . -- Robert Frost.

The black gum leaves are deep red this week along the Big Blue Trail, and the dogwood berries are bright. Cinnamon fern is fading pale brown, the occasional maple is fire yellow and poplars are just starting to turn.

But the oaks have yet to respond to cool nights and dry, bright days, which means there's a week or so left before mountain foliage reaches a peak.

Many Washingtonians regard this as the season to drag out their "Circuit Hikes of the Shenandoah" handbooks and schedule a trek to the Skyline Drive, to walk a piece of the Appalachian Trail or one of its side trails.

But as much as Elizabeth Johnston loves the AT -- she lives three miles from it in Waynesboro, Pa. -- she can do without it in autumn.

"On weekends it's like the Beltway, for goodness sake. You see hordes of Boy Scouts tramping along, and then the Wanderbirds (Hiking Club) come roaring through," she said.

Johnston and others from the Potomac Appalachian Trail Club would love for a few people to take the road less traveled by -- the Big Blue, which lies a few miles west of and about a half-century behind the AT.

"It's a much more backwoods environment," said Johnston. "You'll pass deserted homesteads with barns tumbling down and old one-room schoolhouses filled with hay. It's really quite beautiful.

"You get a feeling of 19th century pastoral America here," said Johnston, a schoolteacher who last year completed two Big Blue Trail handbooks commissioned by PATC -- one for the north section and one for the south section of the trail. Each begins with the Robert Frost quotation mentioned above, and it's appropriate.

"You may see a few people on the Big Blue when there are roads nearby," said Johnston, who did her research by hiking and measuring the 144-mile trail forward and backward over three years, "but in the backwoods, you probably won't see anyone."

The Big Blue is the product of concern back in the 1960s that private landowners might revoke access to some sections of the AT. PATC began work on an alternative trail to the west, and on Oct. 11, 1981, the Big Blue was completed from Matthews Arm campground in Shenandoah National Park to Hancock, Md., where it joins the Tuscarora Trail and continues through Pennsylvania.

About halfway through this exercise, the federal government stepped in to guarantee no interruption of the treasured AT, which runs from Maine to Georgia, and began buying rights of way where needed.

By then the Big Blue was far enough along, Johnston said, that no one wished to turn back. It was completed largely through the efforts of PATC veteran Tom Floyd of Arlington, who scratched out agreements with landowners or found ways around ground he couldn't win the right to cross.

Now, said Johnston, the problem is getting people to hike the Big Blue. "If a trail isn't used," she said, "it disappears."

She took me on a brief tour of the Devil's Backbone section on Great North Mountain near Gore last week, and although the blue blazes marking the trail still were bright and easy to spot, it was clear that the trail is underused.

Here in the height of hiking season, the trail was littered with fallen acorns and leaves still were piled up on some footpaths from last winter.

There were more deer tracks than human tracks. Johnston said she often bumps into wild turkeys, grouse and other normally skittish wildlife.

The Big Blue is still developing, which means that some stretches run along country roads, rather than out in the woods. We took some of these paths, and they were wonderful. Old, unpainted mountain shacks stood balefully by the road, dry ponds waited for the next rain, and the way was so clearly evident you could concentrate on the scenery, rather than keeping track of the trail blazes.

"It would be a perfect hike for families with young children," said Johnston.

She recommended three starting points for Big Blue hikes:

*SLEEPY CREEK hunting and fishing area, between Berkeley Springs and Martinsburg, W. Va., a starting spot for hiking through hardwoods near a 200-acre mountain lake that draws a variety of wildlife.

*MASSANUTTEN WEST, from Elizabeth Furnace picnic grounds, four miles south of Waterlick, Va. (which is five miles west of Front Royal), for a trek through a low bog and then up Massanutten Mountain.

*DEVIL'S BACKBONE at Gore, which is 12 miles west of Winchester, for a hike up Great North Mountain to a spectacular campsite at the pinnacles, where you can listen to the west wind blowing all night while you lie protected by west-facing cliffs. Great views west to the Allegheny Ridge.

The guidebooks, which contain maps, directions and information on camping, restaurants and a variety of other data, are $6.25 each from PATC, 1718 N St. NW, Washington D.C. 20036.