"This is what was here before all the people and the fast-food restaurants and the shopping malls got here," says Pete Schrantz.

We're in the middle of 90-acre Clopper Lake on an electric pontoon boat called the Blue Heron. Pete, a Gaithersburg native and part-time ranger at Seneca Creek State Park, is showing us the Gaithersburg that was.

"This is a man-made lake," he explains to the five adults and seven children aboard. "What they did here was similar to what a beaver would do. They dammed up the creek system with a big piece of earth. They mounded it up to stop the water, and the lake has been filling up since 1976."

The boat's electric motor purrs very quietly, but Schrantz, a zoology major at the University of Maryland, sometimes turns it off, the better to see wildlife.

"We have foxes, raccoons, white-tailed deer, possums . . ."

"Do you have rabbits?" interrupts a little girl.

"Lots," answers the guide. "And groundhogs so big that sometimes kids think they're grizzly bears and come back and tell us we have bears in the park!"

But on an early fall afternoon we see nary a bear, only a fisherman in a folding chair on the bank.

"There are bluegills and bass in the lake," says Schrantz. "You're almost guaranteed to catch a bluegill, even fishing from the dock with a piece of bread for bait. But the adult fishermen are after largemouth bass."

We cruise to a spot where a meadow borders the lake. "This is a beautiful place for deer to graze and for fox dens," says Schrantz. "They need deep grass." As we scan the grass, wood ducks ruffle the meadow and fly up toward the woods.

"Those tiny ducks live in holes in the trees," Schrantz tells us. "But there aren't enough places for them to live, so we supplement the housing supply by putting out wooden boxes for them. They're migratory, but they're not migrating yet. The young have just gotten their wings, and this is a testing period."

Schrantz wants to cruise into a cove where dead black trees are sticking up through the water. The blue heron, the big waterbird that gave its name to our boat, likes to hide here. But so do the bass and, out of consideration for a boat full of fishermen, we retreat.

"There are 150 species of birds in the park. Earlier in the year, you couldn't go anywhere without bumping into somebody with binoculars. This is usually a good place to see the great blue heron," he says, cutting off the motor.

"How big is it?" asks a child.

"About 31/2 feet tall -- bigger than you." answers the guide. "It's slate-blue and has long skinny legs."

What we see, however, is the blue heron in miniature: A green heron, with a fish in its mouth, flies just above the water near our boat.

"See him!" cries Schrantz. "He uses his bill like a sword to spear the fish, and he has a little tuft on his head he raises when he flies."

We pass an old pine plantation, a remnant of the days when part of the park was a farm, and Schrantz points to it, approvingly.

"The pines attract different species of bird, such as woodpeckers. Deer also like a little pine in their diet," he says. "There are watersnakes in the lake, but no water moccasins. And on a hot sunny day, you'd see a lot of turtles lounging on the rocks."

We have almost circumnavigated the lake and, before we return to the boathouse, Schrantz stops for a last look for wildlife.

"Be very quiet and see how many bird songs you hear," he urges.

"You've gotta be quiet so the ducks don't get scared," a child tells her little sister.

Someone hears a cricket.

"This is a big time as far as insects are concerned," says the guide."And did you hear that chirpy-chirp sound? That was a chipmunk."

Suddenly a high-pitched whoop pierces the silence, a cry distinctly human and expressing great joy.

"Someone caught a fish," surmises Schrantz, and as we pull into the dock a boy is reeling in a bluegill. In fact, the boy, his sister and his parents have caught a whole bucket of them, which they plan to have for dinner. But not tonight.

"Tonight we're going to Chunky Cheese," says the boy. CRUISING CLOPPER LAKE Hour-long outings on the Blue Heron cost 75 cents. Cruises are scheduled for 1:30, 3 and 4:30 on weekends through September. Canoes, rowboats, pedalboats and sailboats are also available for rent. Call 924-2127 for details. TO GET THERE -- Take Route 270 to the Montgomery Village exit and follow Quince Orchard Road to Clopper Road. Turn right on Clopper; the Seneca Creek State Park entrance is at 11950. You can pay a $3 entrance fee or park just outside the gate and walk about half a mile to the boathouse. The 60,000-acre park has five hiking trails and an orienteering course. Maps are available at the visitor center..