IT'S THE time of year when families should be on the lookout for an outbreak of a seasonal illness. Uncertain as to how it spreads, parents nevertheless will recognize the major symptoms: general malaise and simple boredom (characterized by the common chant "there's nothing to do"), to more violent fits of crying, fighting and bouncing off the walls.

Yes, it's cabin fever time again, and its manifestation in parents is often an uncontrollable urge to scream, "Take a hike!" Fortunately, taking a hike may be the best cure for cabin fever, and one of the best hiking trails in the area is the Billy Goat Trail along the Potomac River near Great Falls, Md.

Hiking this trail is a great family activity any time of the year, but it's especially enjoyable on sunny days during the winter months when the leaves are off the trees and the spectacular views along the river are not hidden. And, while the steeper portions of this hike quickly get the blood moving and build up body heat that's uncomfortable in the summer, you'll find the cool winter air refreshing and the hike truly invigorating.

Come prepared for this hike. The trail is aptly named and the holes and ridges that dot it mean that the three-mile round trip will take at least three hours. They also mean that while the Billy Goat Trail is not a dangerous or overly strenuous hike, children under the age of 6 will probably find it too difficult. Plan to leave toddlers and preschoolers at home with a baby sitter when the rest of the family needs the tonic of this trail.

Be sure to wear proper shoes and bring lots of water and a daypack or two filled with lunch. Don't make the same mistake I've seen others make by struggling with a cooler. There are places where you'll need both hands for climbing. And wear several layers of clothing, along with a hat and gloves, that can be removed as your body heat builds up. An empty daypack to carry discarded items of clothing is a good idea, too.

The hike begins at the Great Falls Tavern in Potomac. The two-story tavern has been restored to what it looked like about 1830, when it fed and housed travelers who tended the mule-powered barges that plied the C&O Canal. The tavern now serves as a museum and information center.

Cross over the canal on the bridge in front of the tavern and head south on the towpath. You'll soon pass three lift locks in close succession on your left and leave behind the somersaulting churn of a branch of the Potomac River on your right. Reminders of the past continue to appear, and if you're observant, you'll see the ruins of a lock keeper's house and the slumping remains of a gold mine near Locks 16 and 17.

The far-off, hoof-pounding sound of churning water will hook your curiosity. That noise, and the faint, but unmistakable smell of the water, will pull you ahead. Just ahead is a guardrail fence and 100 feet below that is the swirling channel of the Potomac. Across from this channel are the Rocky Islands, and beyond that, a wider and even wilder section of the river.

Continue along the canal towpath. Before crossing under an elevated footbridge, you'll see two blue blazes on a tree to your right. These blue blazes mark the start of the trail and are your road map through the wilderness. A double blaze always denotes a change in direction, so turn right into the wooded area and follow the trail.

A rocky path, stitched together with soft mosses and canopied by fir trees, leads you back and forth between woods and rocks. Layers of pine needles in the gaps between the tree roots and lichen-covered boulders muffle your footsteps, but the path is not taken in silence. Even in the winter months, wrens, blue jays, cardinals and woodpeckers provide musical accompaniment. The canopy of trees and carpet of mosses seem to protect the area's moistness, so that a good, earthy smell seeps up from the ground.

The path eventually leads again to the boulder-edged Potomac. The river is wide at this point, and is frequently dotted with cigar-shaped kayaks whose helmeted pilots navigate through the white foam. Across the river, rappellers dangle like colorful spiders on the steeper Virginia side.

The drop from rock to river here is about 50 feet, but the swirling, swift-moving current seems much farther down, especially without the security of a guardrail.

This awesome power of nature is even felt by the few fir trees surviving along the edge. They are stunted, almost like natural bonsai trees, and most have branches only on one side. But they grace the cliffs with an elegant simplicity and starkness that is missing in the thicker wooded areas, which the trail returns to before once again coming out along the cliff's edge.

Parallel again with the river, you'll cross a long stretch of corrugated granite where the rough holes and ridges are most apparent. Watch your footing along this stretch, but be sure to take notice of the hidden niches of life. There are countless holes in these rocks, carved eons ago by the Potomac. The children will probably be ahead, motioning eagerly for you to look at the mini-ponds. In the spring and summer, these water holes are full with the wonder of tadpoles, dragonflies laying eggs, the hidden curve of a frog beneath a leaf or the green froth of a fern beneath a boulder. But now is the time to admire the ice patterns and just imagine these spring-time wonders.

After following this stretch of boulders, the trail descends to the level of the Potomac, where you'll reach a tiny beach. Like any good beach, it has sand and mollusk shells. From this beach, the trail ascends straight up a crease in a cliff. It's not a bad climb, really, maybe 30 degrees, with lots of nooks and crannies for footholds and handholds, but a climb where you definitely want the use of both feet and hands.

The next leg of the trail is interspersed with bands of black, pocked volcanic rocks between layers of sedimentary rock heaved up and turned sideways. Somewhere along this stretch we usually stop for lunch in a warm, wind-sheltered nook and enjoy food and refreshment, the warmth of the sun and the sea-like aroma of the Potomac, before locating the next blue blaze and returning to the forest cover.

The landscape here has been altered by beavers. Conical tree stubs, about two feet high, surround a small pond which is dammed by felled logs and debris. A stream trickles from the dam and makes its way to the river.

Still steep in places, the trail continues through the wooded area and soon intersects the C&O Canal and towpath at "Wide Water." The canal, normally wide enough to allow only one barge to pass through, needed a switching place, and this is it. At this point, you have two choices: You may turn left (north) on the towpath and return to your starting point, or you may turn right and follow the towpath for about a half mile to MacArthur Boulevard and the Old Angler's Inn, where you might enjoy some refreshments, often served next to a blazing fire in the fireplace, before heading back to your car.

GET YOUR GOAT -- To get to the Billy Goat Trail from the Beltway, take Exit 41 toward Carderock at the American Legion Bridge. The parkway dead-ends at MacArthur Boulevard; turn left on MacArthur and follow to the park entrance at Great Falls. The park is open daily during daylight hours. Parking is $3 per vehicle, $10 for an annual pass good for both the Maryland and Virginia sides of the Potomac. There is free parking near the Old Angler's Inn but it fills quickly. Be aware that while the Billy Goat Trail is a delightful hike all year long, flooding of the Potomac may close it down. If you're planning a hike after heavy rains, check that the trail's open by calling the park office at 301/299-3613.

Karen Kinser last wrote for Weekend about wildflowers along the C&O Canal.