THERE ARE FIVE state parks in Garrett County. New Germany and Deep Creek Lake are the best known. Herrington Manor has a stocked lake. Big Run, in the northeast corner near Savage River Reservoir, has the most beautiful campsites: large, shaded by hemlock and pine and set next to a small rocky stream in the midst of rhododendron thickets.

But Swallow Falls State Park is the gem of them all, with four waterfalls and a 40-acre stand of virgin hemlock and white pine. Nearly a quarter of a million people visit it each year, most on summer weekends.

The park lies along the Youghiogheny River between Oakland and Friendsville. Two of the falls are on the Yough and the others are on tributaries.

A circular trail leads from the parking lots to all four, returning through the virgin forest. The trail starts in a heavy second-growth forest and heads down to Muddy Creek Falls which, with its 64-foot sheer drop, is the highest falls in Maryland.

Wooden steps lead to the canyon floor below, past rhododendron and laurel and wild azalea. The canyon wall is 70 feet of overhanging rock, sometimes chunky, sometimes sheer and sometimes randomly stacked in slabs like books atop a desk. Trees grow thin and tall straight up the wall, roots dug into seams and cracks.

As you descend the steps, the temperature drops and, if the falls is running full, spray wets the steps and the trail; at times water drips down the canyon wall.

The trail follows the creek (its muddy color is caused by the tannic acid in Cranesville Swamp where it rises) and then gradually climbs as it turns upstream along the Youghiogheny. The Yough is still a young river here, feisty and boisterous, running through a heavily forested gorge.

The trail is dark and spongy underfoot. Rhododendron and laurel are everywhere. Mossy rocks, ferns and fallen tree trunks cover the forest floor beneath the hemlock and pine beech. One cantilevered boulder is supported by what looks like stacks of dominoes.

Topping a long climb, you can see the Lower Falls, a single riverwide ledge with a churning, dangerous backwash at the bottom. "We have had six or seven drownings here since 1974," warns Ranger Gerry Sword. "Swimmers slide down it at low water and get trapped in the backwash."

Swallow Falls is just beyond and borders on the anticlimactic with its riverwide, two-step plunge of 25 feet.

The trail turns inland along Tolliver Run to the last of the four falls, Tolliver Falls, a series of small ledges. Such an exquisite miniature should be a movie set.

Just past Tolliver Falls, the trails turn right into the stand of 300- white pine preserved, according to Sword, because a lumber company used the area for company picnics.

This is what much of Garrett County looked like before the lumbermen came: huge trunks with feathery crowns so high and thick they screen the sun, and a soft, springy floor with a sparse covering of fern.

SWALLOW FALLS STATE PARK -- About 10 miles west of Oakland via Herrington Manor Road. It has picnic areas and 125 improved campsites in its 406 acres. It is managed in tandem with Herrington Manor State Park, four miles away. Information on fees and reservations: Park Superintendent, Herrington Manor State Park, Route 5, Box 122, Oakland MD 21550. 301/334- 9180.

HERRINGTON MANOR STATE PARK -- Five miles west of Oakland. Has a 53- acre stocked lake, swimming, boating and picnicking and 20 fully equipped cabins in its 365 acres within Garrett State Forest. Information: same as for Swallow Falls State Park.

NEW GERMANY STATE PARK -- Five miles south of Grantsville on New Germany Road in Savage River State Forest. Its 210 acres include a small lake for swimming and boating, hiking trails, campsites and some cabins. Information on fees and reservations: Park Superintendent, New Germany State Park, Grantsville MD 21536. 301/895-5453.

DEEP CREEK LAKE STATE PARK -- On the north shore of Deep Creek Lake 10 miles northeast of Oakland. Camping, boating, swimming. Information on fees and reservations: Park Superintendent, Deep Creek Lake State Park, Route 2, Box 70, Swanton MD 21561. 301/387-5563.

BIG RUN STATE PARK -- 300 acres in Savage River State Forest, on the northern tip of the Savage River Reservoir. Boating, fishing, 30 primitive campsites. Information: Park Superintendent, Big Run State Park, Route 2, Grantsville MD 21536. 301/895-5453.