FOR CAMPING and nature enthusiasts, Charles County has two large state forests -- Cedarville and Doncaster, both with good, natural picnic and camping sites.

But if you dream of thick woods gouged by deep ravines, look to Westmoreland State Park in Montross, Virginia. Here, the forest extends right to the brink of cliffs that tower 150 feet over the Potomac. A tiny beach links a wet marsh to the cliffs.

The best way to explore Westmoreland Park is via the oval trail, about two hours of easy walking on rolling terrain.

A start at Horsehead Cliffs lets you peer down at the river and take in the sweeping vista. A short hike on the Big Meadow Interpretive Trail, just past the information center, will take you into the woods, a must-see in the spring when the mixed hardwood forest teems with flowering dogwood, spicebush, rhododendron and mountain laurel. The trees and bushes vie for attention with rainbow carpets of spring wildflowers, including three species of orchids.

A small beach lies to the left of the trail junction, where you may take in ospreys soaring overhead, waterfowl dotting the river, and songbirds in the woods at your back. The very lucky hiker may even spot one of the bald eagles from any of the 10 or so active aeries in Westmoreland County.

From this point at the beach, you can see the multicolored clay, sand and silt layers of Horsehead Cliffs, which date back 5 to 15 million years, when a shallow sea covered this ground, and the climate was subtropical. Sediment that settled to the sea floor built up the cliffs. There are billions of fossils embedded in their steep walls, including the occasional porpoise, mastodon or rhinoceros. Avoid walking under the cliff ledges, as they are very unstable.

The Horsehead Cliffs are retreating at the rate of about a foot per year, undercut by wind and waves. The once-narrow Potomac Valley itself was widened by the erosion of rushing water when the glaciers melted 20,000 years ago.

Back at the trail junction, a left ontoa boardwalk leads to the marsh observation tower. In early spring, the lush cover of catttail shoots reminds the visitor of a grassy field -- an image jarred only by the splashing and croaking of frogs.

This marsh, loaded with arrowhead and jack-in-the-pulpit, was created not by mighty glaciers but by lowly beavers, who dammed a stream about 60 years ago.

More than 100 species of birds inhabit Westmoreland State Park, among them diving ducks, loons and migrating warblers.

WESTMORELAND STATE PARK -- To get there, take Route 301 to Va. Rte. 3, turn east (left) and go about 19 miles to Va. Rte. 347. Turn left into the park and go about 1 mile to the contact station. Sandy beach, pool, bathhouse, fishing, boating, (ramp, rentals). Picnic areas and playgrounds; tent and trailer sites available April-November, 30 cabins May-September. Visitor center and pool complex are handicapped-accessible. Open year round: hiking in the winter, daylight to dusk; summer hours are 6 a.m. to 10 p.m. For more information about facilities, write to the park at Route 1, Box 600, Montross, VA 22520. 804/493-8821.

OTHER PLACES TO PARK IT

DONCASTER STATE FOREST -- 16 miles west of La Plata on Md. Rte. 6 near Doncaster. Heavily forested with sweet gum, red and white oaks, pine and yellow poplar. There is hunting, fishing and picnicking in the park's 1,485 acres. Open year round daylight to sunset. Route 1, Box 425, Indian Head, MD 20640. 301/934-2282.

SMALLWOOD STATE PARK -- Near Rison, Md., 16 miles west of La Plata via Md. 225, right on Md. 224, 7 miles to entrance on right. Retreat home of Revolutionary General William Smallwood. (Bigwigs of the era had no Ocean City condos in which to escape the steady stream of meetings and live-in guests in their homes.) Open year round for picnicking and strolling. Guided tours in the summer, historical programs, picnicking. Open daily 8 a.m. to 6 p.m. Information: Smallwood State Park, Marbury, MD 20658. 301/743-7613

CEDARVILLE STATE FOREST -- From Route 301, 4 miles east of Waldorf on Cedarville Road. The Piscataway Indians once made this their winter home because of the mild climate and plentiful game. There are 3,500 acres of woodland with picnic facilities, fireplaces, hiking, nature trails, shooting range and 132 improved campsites from late April to late November. Open daylight to sunset year round. Fishing is allowed in the 5-acre pond; hunting, forestry demonstration area. P.O. Box 580, Waldorf, MD 20601. 301/645-8000.

CHAPEL POINT STATE PARK -- Scenic access, historical site. Open year round. Chapel Point, MD 20664, turn right from Route 301 to MD 427 at Bel Alton.

FOR MORE INFORMATION -- On individual state parks or forests in Maryland, write directly to the Maryland Forest and Park Services, Department of Natural Resources, Tawes State office Building, Annapolis, MD 21401. For campsite availability, call 301/768-0895, 24 hours a day.

For Virginia parks, drop a line to the Division of Parks and Recreation, 1201 Washington Building, Capitol Square, Richmond, VA 23219. 804/786-2134. For a campground directory, call 804/786-4484 or write to Virginia Division of Tourism, Bell Tower on Capitol Square, 101 N. Ninth Street, Richmond, VA 23219.

PRIVATE CAMPGROUNDS

In addition to camping facilities in state parks, the following private campgrounds -- on both sides of the bridge -- may suit you. Many have scheduled activities and programs. Brochures and information are available from the managers of the sites. Be sure to call ahead for rates and reservations.

AQUA-LAND CAMPGROUND -- Boat rentals, fishing, grocery store, planned activities, laundry, bathhouse. Open year around, located at Route 301 at Potomac River Bridge. P.O. Box 355, Newburg, MD 20664. 301/259-2775.

GOOSE BAY MARINA AND CAMPGROUND -- Boating, fishing, pets on leash, snack bar, grocery store, bathhouse. Open April 1 to November 1. From 301, turn right (west) on Md. Rte. 6 to a left on Blossom Point Road to campground. Mailing address: Route 1, Box 1276, Welcome, MD 20693. 301/934-3812.

PORT TOBACCO CAMPGROUND -- Swimming, boating, fishing, pets on leash, bathhouse, grills available. Open April 1 to October 31. From Route 301 at La Plata, go west on Md. 6 for 3 miles to Shirley Blvd. Port Tobacco, MD 20677. 301/934-9707.

CAMPGROUND 721 -- Near Bowling Green, Va., 4 miles south of the town on Route 301, then 4 miles east on county road 721. Open April 15 through October 25. 804/633-9516.