AFTER 50 YEARS of building public hiking trails all over the Washington area's outback, the Potomac Appalachian Trail Club has come home to Rock Creek Park.

"The idea was so obvious I was embarrassed when it finally was brought up," said Tom Floyd, PATC trails construction supervisor. "There's not a single properly laid out, marked and maintained foot trail in the whole park, yet there is more need for trails here than anywhere elese we've ever worked."

Trails there will be, some 15 to 25 miles of them, to be built by the hand labor of volunteers from PATC and FORCE (Friends of Rock Creek's Environment), whose Jeff Norman was the one who started the ball rolling.

Floyd, who has negotiated rights of way, laid out routes and led work parties on hundreds of miles of Blue Ridge and Allegheny Mountain trails, wasn't sure there would be much enthusiasm for building them in the middle of town. He couldn't have been more wrong; the turnout for the first day's effort was 31 people, largest in PATC's history.

"They were cutting brush so fast they were running up my back," Floyd said as he took a visitor over a section of what will be known as the Theodore Roosevelt Trail near Pierce Mill. "We had to mark a couple of side trails to keep people from bunching up."

He paused to inspect a white oak and a beech that had fought over the same patch of earth and finally fused together. "I drive through this park all the time, and I should have seen the need. At every parking area and picnic ground, all year round, there are people just standing around waiting for trails. Maybe they don't recognize it themselves, but I do. You see somebody get out of his car and walk up and down the pavement for a few minutes and then drive away. What that man was looking for was a trail: a path, and a sign that says, 'Come this way, there is something to be seen.'"

PATC volunteers have put man-years (and woman-years and child-years) of hand labor into answering that need, not only on the famed Appalachian Trail but on hundreds of miles of other hiking and backpacking paths. Round trips of five or six hours are routine, and often the work parties camp overnight along the trail in the worst of weathers to save commuting time.

"A lot of our members can walk to work with us in the park," Floyd said. "And they also can sleep an hour later, which probably is the main reason for the big turnout."

The project is expected to take about two years, with work parties scheduled every six weeks or so (the next one will be April 30). "We're going to try to give Rock Creek as much priority as possible because of the need," Floyd said, "but we have so many trails to build and maintain that there are very few open weekends."

A couple of questions spring to mind about such a trial system. When the pathways are open, will it be safe to walk them? And is there enough "park" in road-ravaged Rock Creek Park to make hiking there worthwhile?

"The park is by no means a high-crime area, and the rate has been dropping for the past several years," said Park Manager James J. Redmond. "The problems are mainly on the fringes, which is really a street problem rather than a park proble, and in the picnic areas, which is largely a crowding and traffic problem. We now have an agreement with the Metropolitan Police that puts more of their scooter men in the park, and of course the new trails will be patrolled, just as the bike and horse trails are. I think the result of this project will be to greatly improve our surveillance of the park."

"And just as important," Floyd put in, "the opening of these trails will bring more of the good people of this town into the park, people who care about this city and who don't just walk away when they see something that's wrong. We're going to bring eyes in here, and that will automatically tend to discourage criminals, who are generally pretty furtive."

Floyd's reaponse to the question of what Rock Creek Park has to offer the hiker was a brisk five-minute walk from busy Beach Drive to a ridgetop that seemed as deserted as any in the Appalachians. The sound of traffic was drowned out by the calls of Carolina wrens.

"Imagine how much more quiet and remote this will seem after the trees leaf out," he said. "And this is about as narrow as the park gets. There are long stretches of the park that are a mile or more wide, where our trails will be hundreds of yards from the nearest road. I was surprised to find out, going over the maps, that only something like 10 per cent of the park is visible from the roads."

The 2,000-acre park stretches some six miles through the heart of the city, and continues unbroken for many more miles into Maryland.

Many of the trails will follow old paths used by Indians, who built fish weirs in the creek and quarried quartzite for axes and arrow-heads from mile-long stairstep rock formations that mark the meeting of the Piedmont and Coastal Plain geologic provinces. Others will pass pioneer homesteads that were vanishing ruins when the park was established at the turn of this century.

Trails currently in use will be 'improved and in many cases relocated, because, Floyd said, "some of them are badly eroded and others bypass the most interesting areas. We're not trying to get hikers from point to point, we want to give them the best exercise and experience possible with the least damage to the environment."

The footways will introduce many Washingtonians to what local birders long have known: Rock Creek is rich in flora, including virgin trees, ferns and wildflowers, and fauna, ranging from the whitefooted mouse to the white-tailed deer.

There are even three unique species, a tiny snail and two minute isopod crustaceans whose only known habitation on earth are certain isolated springs upon whose pure and constant flow they are totally dependent. "The trails will avoid the springs," Ranger Bob Ford said, "because a couple of thirsty hikers could conceivably swallow and entire species, and anyway, most of the groundwater in the park is so polluted the only safe thing to do is pack your own water or get it frokm a hydrant."

Some 500,000 people live within Rock Creek's 77 square-mile watershed, a recent park management study noted, and the park "is located at the bowels . . . of this drainage system. The creek still is badly polluted but is not the stinking sewer it was before Montgomery County got serious about wastewater treatment. Above Piney Branch, where Washington's ancient and notorious combined sanitary/storm sewer system flushes into the stream, Rock Creek is decent enough for wading and occasional kayaking.

An hour's stroll over perhaps a mile of Teddy Roosevelt Trail and two side trails one frosty afternoon turned up, besides the Carolina wren, red bellied and downy woodpeckers, white-breasted nuthatches and white-throated sparrows, cardinals, mourning doves and dark-eyed (formerly slate-colored) juncos, a red-tailed hawk foraging for pigeons and being bedeviled, as usual, by crows, and a great flock of turkey vultures returning to their roost at the Kennedy-Warren apartments.

There were so many American beeches in the park, many of them scarred by graffiti dating back as far as 1921, that Ranger Ford said they seem to be establishing dominance in certain areas even though they are tender and slow-growing.

There were tulip trees of majestic proportions, greak oaks of several species and many hybrid strains, massive but feathery eastern hemlocks, pines of several persuasions including at least one exotic pitchpine, hornbeam (ironwood, musclewood) and others whose identifications remained unsettled after detailed examination by Ford and other Rock Creek trail boosters who came along for the walk, including Becky Field of the National Zoo and Phil Barringer, a Defense Department negotiator.

They also picked off mountain laurel, huckleberry, honeysuckle, strawberry bush, greenbrier, Christmas fern, wintergreen, trailing arbutus and our old friend poison ivy, which is protected - sometimes and in some places - by the NPS because, Ford said, "It's one of the few native 'weeds,' noxious or not, that survive that thrive among the weeds imported from Europe and Asia by the early settlers."

Although deer venture into the park only rarely, probably because of dogs, some scat of the season was found at the edge of an old meadow above Blagden Avenue. More common mammals include the gray squirel, chipmunk, raccoon, opossum, short-tailed shrew, and red and gray foxes; also seen, by the observant, are the flying squirrel, skunk, rabbit and red, little brown and big brown bats.

One exotic species discovered during the afternoon was a corpse wrapped in Kleenex and sealed in a Tropicana orange juice bottle. With it was a note, written in a clear girlis hand on lined paper:

Marceau Dyson b. Aug. 1975 d. Dec. 1976 A good hamster. I loved him.