When darkness descends on Virginia's Great Falls Park, the animals gather by the river for a drink. Beavers, raccoons and muskrats come on little front feet and large hind feet. Rabbits hop along, their tiny hind together and the larger forefeet slightly apart. Scampering skunks move along four or five inches at a time, while deer take giant 13-inch steps. How do we know all this? The critters leave behind evidence - some of it in the form of paw prints.

Next morning, a Saturday, cars roar into the parking lot, dispensing intrepid junior naturalists. Under the leadership of volunteer ranger Ginny Lo, the kids will find the evidence and cast it in paster of Paris.

"What kind of animals do you think we have in the park?" asked Lo in a briefing session in the visitors center. No, there aren't any bears, she tells them, but there are deer, squirrels, raccoons, skunks, beavers, possums, groundhogs . . .

"Any bluejays?" asks a little boy.

"Yes, all kinds of birds and ducks and eve wild turkeys," Lo tells them. Then she passes out the supplies of cardboard about 2x 7", cellophane tape and paper cups containing about an inch of dry plaster of Paris.

"We'll get water from the river," says Lo.

Using the tape, the kids make the cardboard strips into rings just big enough to fit around a paw print.

When you find a good print, out the cardboard ring aroung it and sprinkle some dry plaster of Paris on it. When you're ready to cast, put some water - about a third as much as you have plaster Paris - into the powder. You should have a doughy solution. Pour it on top of the print into the ring and let it dry about 15 minutes. Then take it off the print, but leave the cardboard on the plaster cast another 15 minutes. And, yes, you get to keep your own casts", says Lo.

Armed with these supplies and with mimeographed sketches of various prints, the group of about 25 kids and assorted parents and den mothers heads toward a trail that follows the river.

A woman walking a beribboned Schnauzer crosses the trail, and the kids yell enthusiastically: "Hey, let's get some dog prints!"

"Well," says Lo diplomatically," let's just hike all the way to the dam and see how many different kinds of prints we can find." A little farther, the trail crosses a stream and the junior naturalists spot some tracks on the mussy bank. They examine the prints, comparing them those on the mimeographed sheets.

"I'm afraid what we have here are some of the most common tracks in the park - dog and human tracks," announces Lo. "There are clues everwhere," she adds, sensing that the junior naturalists are getting a little discouraged. "Don't just look at the ground. Look for nests and beaver lodges - they mean there are animals nearby and there must be tracks. But be careful not to step on the tracks."

Now the path hugs the riverbank and the tracks begin to appear. The first junior naturalist to spot the print gets it, and the others move on.

"It's a little big for a raccoon print," Lo tells one of the finders. "It might be a beaver."

"Or an orangutang?" asks the kid.

Another junior naturalist finds a mystery print he thinks may belong to a muskrat. "But it has a seperate thumb," says Lo, "and a muskrat doesn't".

Could it be a beaver?

"A beaver's fingers are spread farther apart," Lo points out. By process of elimination, they decide it must be a racoon print.

Most of the prints turn out to be raccoon prints, and before long, the river bank is lines with kids pouring out plaster of Paris on the prints.

"I think we used too much water," says one father, watching a chalking white stream of plaster of Paris flow out of the cardboard ring toward the river.

Parents watch the watches, timing the drying process, while the kids skim rocks over the water or swing. Tarzan-like, from hanging branches. Some come to Lo with problems: "There's a bug in my cast," "The ducks keep pecking at the plaster."

One boy signs his work, tracing his initials and the date in the plaster with a twig.

Lo urges the group to hike on. She wants to show them wild flowers and field-mouse nests and trees felled by beavers. "You can pick up the casts on the way back," she tells them.

"Do you think they'll be here?" asks a mother skeptically. "Some little kid whose cast didn't work will probably grab them."

Some hike on, but most stand guard over their casts. Many try to remove the casts prematurely and break them.

"We'll glue it together when we get home," consoles one mother.

"Wait unitl the plaster is dry and doesn't stick to your finger," cautions Lo. "Then turn the ring on its side and let it dry some more before you take off the cardboard. Wait until it's completely dry before taking the mud off the bottom with a brush."

For the patient, the rewards are recognizable raccoon, beaver and muskrat prints, all in one piece.

"I guess the trick now it to get them home before they get smashed to smithereens," says a mother.

You can search for paw prints in Great Falls or along muddy banks in any park or rural area. Bring the supplies listed above and follow Ginny Lo's instructions for making casts. To savor other aspects of nature, you may want to join of the organized hikes.