In the light rain and the tumbling leaves, the city kids rounded a bend on the forest trail and looked out across the river. David Parker stopped in his tracks and pulled back his hood for a better view.

"What is that island?" asked the 8-year-old.

"That's not an island," said naturalist Bob Boxwell, who was leading the group toward the Potomac. "That's Virginia."

It was a day of discoveries large and small for the second- and third-grade students from John Hanson Montessori School in downtown Oxon Hill. On the walk down to the river, they had learned to spot the hairy red thatch of poison ivy snaking up the tree trunks in Chapman's Forest in Charles County. They kicked through the oak and poplar leaves to find the acorns the squirrels had missed. And they craned their necks to look at the holes where the woodpeckers lived.

"I bet it would take a hundred days to climb that tree," said 8-year-old Aja McLeod of Temple Hills.

For many of the youngsters, the week-long series of field trips that ended Friday offered a rare opportunity to leave the Beltway bustle and explore under a quiet canopy of trees. The excursions are part of a pilot program put together by the Chapman Forest Foundation, funded by $3,500 in grants from the Sierra Club and the Chesapeake Bay Foundation, to teach public school students about nature preservation and the Potomac environment. Only John Hanson is involved, but organizers said they hoped to expand the program in the spring to include other schools in Prince George's and Charles counties.

"They get to see the trees, leaves, bugs, mosses," said Angela Cooper, 31, whose daughter, Davina, participated in the field trip. "There's so much variety that they don't get exposed to in the city."

Chapman's Forest, a swath of more than 2,000 acres along the Potomac just north of Indian Head, was the site of a long battle by environmentalists who opposed a plan to build a 4,600-house development there. In 1998, Gov. Parris N. Glendening (D) negotiated the purchase of the land to preserve the forest.

The morning in the forest began with Boxwell, chairman of the Sierra Club Southern Maryland Group, standing on the porch of Mount Aventine, a manor house built in 1840 that overlooks the Potomac, showing the 18 children examples of turtle shells and blue jays that live in Chapman's Forest. Then they broke up into groups and trudged off down the trail.

Boxwell's group stopped frequently, observing a clump of feathers of the yellow-shafted flicker that decorated a rotting log, picking wild grapes or learning that a floppy-leaved plant was called a papaw.

"Look at all the papaw, guys, it's all over the place," said Marissa Austin, 8, of Upper Marlboro.

Others skipped down the path chanting "Papaw, papaw."

One focus of the instruction was water quality. The students were shown ditches in the dirt trail caused by erosion and told how cutting down trees can lead to more erosion and increased sediment in rivers and streams. At the beach along the river, students performed a "Secchi dip in" experiment, in which they filled a plastic tube with Potomac water and measured how much they could see through before the water became too murky. Nutrients such as phosphorous and nitrogen as well as sediment runoff in the water can block light from reaching aquatic vegetation.

"Potomac River water quality improved as a result of the Clean Water Act passed in the 1970s," said Jim Long of the Chapman Forest Foundation. "However, like the Chesapeake Bay, today it still suffers from excess nutrients and poor clarity."

After the field trip, the students returned to their classrooms with seeds and acorns to try to plant trees at the school, said Juliana Collier, their teacher. They also will write poetry and draw pictures, taking inspiration from the trip, she said.

"I could stand there and talk to them about how important forest preservation is or how water quality is important, but they'd miss the beauty, and that's what they see here," she said. "It gives them a frame of reference that they otherwise wouldn't have."

But their urban upbringing wouldn't be replaced by two hours in the woods. On the walk back to Mount Aventine, Boxwell stopped to examine a stump that was scattered with crumbled acorns where squirrels had been eating. Aja put her hands in the pockets of her silver coat and cocked her head.

"Hey," she said. "This is like T.G.I. Fridays."

Piri Jenkins, left, talks with Amina Dunn, Natassja S.C. Pupuma and Gregory Bacon. A series of trips took the students to the woods north of Indian Head.Gregory holds an oyster found on the banks of the Potomac River. The Oxon Hill second- and third-graders learned about the river's water quality.Amina Dunn checks the leaves and other items she picked up. Eighteen students went on the field trip.