The sun had long since sunk over the salt marsh and the Maryland youngsters were engrossed in their favorite pastimes.

They were scattered across the floor of the old duck hunting lodge, 12 and 13-year-olds hovering under the flickering light of oil lamps. Some were reading Mad and Cracked Magazines, others were matching wits in the endless card game "spite and malice."

John Page Williams, field trip director for the Chesapeake Bay Foundation, strode in dressed in his muddy jeans, carrying a bucket and a sieve full of goo under his arms.

He propped the glop up on a crude coffee table, shined a flashlight into the bucket and filled the room with his basso profundo.

"All right, people, let's talk about the food web."

The 14 kids dropped their cards and comics and gathered around in the dim light. Outside a full moon shone and beneath the floor wavelets lapped at the lodge's underpinning of pilings.

"In the bucket we have a bottom sample we took today from the channel," said Williams. "How would you describe it?"

"Yukky," said a blond girl.

"Jello," said another.

The bucket was passed from hand to hand and each student sniffed and jiggled it. Night breezes picked up the pungent odor of sulfur fumes.

"That's right, it's like jello," said Williams, "and as you can see there's nothing much in it but mud. If we ran it through a sieve there wouldn't be anything left over."

"Now here we have two other samples taken from 30 and 40 feet of water. One the left is from 40 feet, and it's sandy and muddy. On the right, from 30 feet, we have oyster shells.

"Now what do you see in them?"

The youngsters poked fingers into the gruel and stirred it around, uncovering shells and fragments of tiny crabs and clams.

"Okay," said Williams, "we had croakers and sea trout for supper tonight. When we cleaned them on the dock, what did we find in their tummies?"

The youngsters remembered the tiny crabs and clams that came from the croaker and sea trout bellies. "Right," said Williams. "And now you know why we look for a hard bottom when we go fishing."

It was one of a number of small discoveries that the selected crew of "talented and gifted" public school students would make on a two-week jaunt through the waters of the Chesapeake under the auspices of the state and the CBF.

They had seen fresh water thundering to salt on a canoe cruise down the Patuxent, and recorded the changes in flora and fauna as they cruised and camped downstream. They had explored the half-salt environment of Meredith Creek near Annapolis and now they were locked into life in the salt marsh at Great Fox.

Locked solid. That morning third year biology student Ingrid Burke from Middlebury College had led three canoes full of playful younsters down one of the channels in the marsh to a sand bar. They waded into the thigh-deep water, plunged hands into the sand and mud and dug to their armpits chasing razor clams.

"Look for a little hole the size of your finger," Burke told them. "When you find one cram your hand in as deep as it will go and dig as fast as you can.

"The clams will try to get away. They dig into the mud with suction, so you've got to hurry."

The kids did just that, hurling great globs of bottom sediment into the air and plunging their heads under water for the last lunge at the fleeing clams.

The clams would get eaten, along with eels and crabs from the (CBF crab pots, hog chokers (little flat fish), shiner minnows and grass shrimp fried crisp brown and buckets full of croaker, sea trout, flounder and bluefish.

The following day Don Jackson, caretaker at Great Fox and a biology student at Maryland, would take the younsters to neighboring Watts Island, a heron rookery.

Watts once was inhabited, but erosion has eaten it away to uselessness. When man left, the birds moved in in force, and today it's alive with heron and osprey nests, skimmers, oyster-catcher birds, black ducks, egrets, willets, glossy ibises and cormorants.

The birds are so at home that the youngsters' canoe voyage through narrow inland channels went practically unnoticed.

Birds sprang from the marsh a stone's throw away from the boat.

Great Fox and Watts are teaching paradises. CBF got its foothold on Fox, just off Crisfield, Md., when a group of Baltimore hunters decided to give up the lodge after duck populations declined.

The hunters presented the 16-room lodge and 110-acre marsh to CBF three years ago and field trips have been on the increase ever since.

Hunters and conservationists make strange bedfellows, but that's exactly what they are here. The beds that tomorrow's nature lovers sleep in were hunters' beds five years ago.

It's a turn of events that leaves Williams and the others at CBF chuckling.

"You just couldn't build a place like this today," said Williams. "For one thing, we'd oppose it ourselves. The marsh is too fragile, and the laws on tidal wetlands are much stricker now."

CBF, the nonprofit organization that coined the phrase "Save the Bay," runs educational trips to Fox and elsewhere on the Bay and its tributaries from early spring to late fall.

For information, write Chesapeake Bay Foundation. Box 1709. Annapolis. Md. 21404 or call Clara Petrini at 261-2350 (D.C. area number).