NOTHING YOU HAVE read can quite prepare you for the impact of Fallingwater, that remarkable house that Frank Lloyd Wright built at Mill Run, Pa., for the Edgar Kaufmanns of Pittsburgh. This is Wright's masterpiece, given to the Western Pennsylvania Consevancy by the Kaufman's son upon their death. For this act alone, he should have a crown in heaven since part of the agreement was that it be thrown open to the public.

Fallingwater sits astride the falls of Bear Run in Fayette County, in the midst of a mature forest studded with rhododendron and massive sandstone boulders. The maps call this country the Laurel Highlands, and the roadside fruit stands on Rte. 40, the National Pike running through it, call it the Pennsylvania Alps. Either way, views suitable for framing are offered at every turn -- and the road has plenty of turns. Making its leisurely way from Cumberland, Md., to the Ohio border, Rte. 40 twists and humps its back like a catepillar in no hurry to get home.

The Kaufmanns told Wright they wanted to live with nature rather than dominate it high on a hill, and the house he built for them seems to spring directly from a rocky ledge where the water rushes. House and site blend as if they had grown there together. The rock, the trees and the water are all part of the house. The floors are waxed stone to give the feeling of the wet rocks outside, and from the living room a stairway descends straight into the water below.

Workmen casting the concrete trays dramatically cantilevered over the water whispered that the design was overbalanced and would never work. When it was time to remove the wood frames that supported the concrete, Wright stood beneath the watch just in case any doubters should remain. It must have been quite a moment.

Fallingwater was built in 1936 for $155,000, including Wright's fee, and almost everything about it was innovative. The fluorescent strips installed along the windows to smudge the distinction between in and out was the first residential use of this lighting. The foam rubber in the furniture was another first, and surely no other house has ever used for a hearth a boulder emerging from the water below, left by request of Mrs. Kaufmann as jagged as nature made it.

There is an occasional rug in this house, but no curtains or even blinds on any of the windows, except in the guest room where the Kaufmanns felt guests might want more privacy. The furniture was all Wright's selection, with the exception of the three-legged antique dining room chairs, which Mrs. Kaufmann bought in Florence, and a waxed stump table in the guest house of which Wright never approved. Tiffany lamps and glass appear often and so does Mexican and Peruvian pottery. Even some of the reading lights were designed by Wright, and the Japanese art was his personal gift to the Kaufmanns. Only the kitchen failed to interest him. He roughed it in simply as "work-space."

Edgar Kaufmann Jr. once remarked that, after years of living in fallingwater, it still held surprises for him. This is understandable since, though the main theme is horizontal, there are circles, half-circles and quarter-circles tucked away everywhere in unexpected places, nowhere more dramatically than in the mammoth Cherokee, red cast-iron kettle hung besides the fireplace. The Kaufmanns used this kettle only once, finding it a bit of a chore to clean since it requires two just to lift the lid.

Everywhere you are aware of the water. You hear it roaring by in the stream below in every room in the house, and it trickles from hidden springs into unexpected places like the tiny pool presided over by an anicent Buddha in a passageway. Each room has its own terrace for a closer view of the water and the house above the house has its own spring-fed pool.

The Western Pennsylvania Conservancy does a first-class job of handling the showing of Fallingwater, from the small size of the tour groups to the really fine gift shop. Most impressive of all are the compost public toilets, which make the restrooms spotless and odor free through the use of aerobics and vacuums, returning waste to the earth in proper ecological fashion.

Reserve a place on the guided tour ahead of time. Everyone knows horror stories of visitors who crossed the seas to see Fallingwater and were turned away because of inadequate notice. Twenty-four hours is minimal and more is safer. Call 312-329-8501. The fee is $3, and if you bring children under 10, you must leave them at the Child Care Center where the fee is 75 cents an hour per child. The tour is designed for adults.

It's about 4 1/2 hours to Fallingwater from Washington and, after the mountain roads, a nice inn for the night is essential. The Century Inn in Scenery Hill, farter along rte. 40, fills the bill spendidly.

The Century, one of the oldest continuously operating inns on the National Pike, has played host to a plethora of travelers. One wonders how they ever got over the mountains, which strain even today's six-cylinder automobiles, but they did. General LaFayette, on his tour of America, stopped here with his retinue for breakfast in 1825, and Andrew Jackson spent the night on the way to his inauguration.

The inn today is a national landmark and is a delight inside. The cherry highboy in the room to the left of the front hall was made in 1750 and the cozy bar, once the innkeeper's bedroom, has its original floor and woodwork. Hand-plastered walls have been painted from stencils cut from early 1800s tracings.

You cannot get into this delightful inn without thinking well ahead. By mid-June it was booked for weekends through mid-July, but it is worthwhile waiting for one of the six antique furnished bedrooms. But even if you can't stay here, chances are you can get a reservation for lunch or dinner in one of the five attractive dining rooms. Don't rely on your credit cards, though the inn might take your personal check. The food is traditional American.

At any rate, go upstairs to see the doll room where owners Megan and Gordon Harrington have installed what-surely must be one of the definitive collections of antique dolls. Dolls sit around the bed on the old patchwork quilt, lounge on the veranda of the Victorian dollhouse, sprawl on the floor or in carriages. Don't trip over Brandy, the inn cat, who sleeps on the landing.

For after dinner, there's a nice little tree-shaded patio and even a library with nary a Reader's Digest anthology. Rooms vary between $28 and $35, plus tax. Call 412-945-6600.

If there's no room at the Century, motels abound in nearby Uniontown, or you might want to try Mount Summit Inn atop Mount Summit, where the fog is habitually so dense that permanent signs warn of danger to motorists. Mount Summit Inn boasts that on a clear day you can see 30 miles, but you couldn't see 200 yards when I was there. Nevertheless, it is a nice place to sit around on wonderful wrap-around porches, and it serves meals to other than over night guests. A double here, modified American plan, is $34.50 per person and you can bring your small dog (his bill will be $3 per day, European plan).

If you have any time left before going back over the mountains, take a look at Nemacolin Castle in Brownsville. This is not really a castle but an old home built in 1789 on the remnants of Old Fort Burd, a French and Indian War fort. The Bowman family lived here for more than 200 years, adding on as each generation replaced the last.

The house is rather fun since many of the bedrooms are occupied by department store dummies attired in costumes of the period, and the Bishop's bedroom, though empty of dummies, has a mammoth tester bed that could surely hold its own for size in the Guinness Book of World Records. On the ground floor a 70-foot well hand-dug by soldiers in 1759 has survived, and everywhere are relics of later periods. I loved the bearskin muff flung over a chair in the master bedroom and the wonderful black Floradora hat with white ostrich plumes swamping the chair by the dressing table. Nemacolin is free, with a plea for badly needed donations.

We saw Fallingwater on Sunday and, finishing up about 11:30 a.m., inquired in the gift shop where we could eat lunch enroute home. We were steered to Glisan's, four miles east after you turn from the Mil Run Road onto Rte. 40. We hit Glisan's just before noon, in time to share tales with the post-church local society, all dressed up and accompanied by scrubbed children. Glisan's serves family style, with plastic tablecloths and overworked waitresses, but you can eat yourself sick for $4. The special the day we were there was roast pork, mashed potatoes with gravy trapped in the center, corn and homemade bread. Don't go if you're counting calories, but Norman Rockwell would have loved to get some of the clientele onto his easel.