Beyond the 100th meridian, the dry line that slices southward through the western Dakotas and Nebraska, the abundant rainfalls of the East diminish and the landscape turns arid. The line marks the beginning of the American West, a vast and still-empty land of both awesome grandeur and utter desolation.

Tourists flock to mountain resorts and jam our national parks to view the scenic wonders. But once they have descended from the Colorado Rockies en route to Las Vegas or San Francisco, they roll up the car windows, flick on the air conditioner full blast and bemoan the bleak distances they must cross. And who can blame them?

"Out in the eerie dry lakes, nightmare mountains and charnel rivers of central Utah, the Great Basin begins," writes Rob Schultheis. "Dust devils saraband across the salt flats; beyond lie the hopeless white sepulchers of the Beaver Mountains. It is one of the most dreadful landscapes on earth."

But it is these overlooked wastelands--the hidden West--that hold a fascination for Schultheis, a 38-year-old anthropologist and outdoorsman who lives in Falls Church. "Mystery country," he calls it. "Terra incognita, a land of lost rivers, dead seas, trackless deserts, mountains charged with voodoo, adobe Lhasas at the end of endless roads."

Twenty years ago and just out of an eastern prep school, Schultheis hitchhiked to Denver to get his first view of the West. This short book is an impressionistic account of his adventures and encounters in the American outback in the years since, following, as he says, "a cold trail of rumor and campfire gossip."

It is a haunting, wonderfully evocative appreciation of nature's bleak and inhospitable regions that through Schultheis' eyes become beautiful, if no less dangerous. He is a very fine writer, deftly weaving geology, history and philosophy into his vivid descriptions of this secret land. The landscapes may be desolate, but his imagery is very much alive. His prose is lean, strong, even athletic. Like a hiker setting a fast pace to cover a lot of territory, the book moves swiftly across the West's great distances.

Once a woman friend--a "desert rat"--drew a map for him on a paper napkin in a cafe in El Paso that showed the trail to a particularly lovely side canyon of the San Juan River in southwestern Colorado--"one of the wildest places in the world." He held onto the map for years, and then one recent October hoisted his backpack and with another friend sought out this almost untrodden site.

"We entered a weird zone of fossil mud, petrified dunes, the cliff peeling off in iron-black blocks the size of battleships. There were walls of jackstrawed flood timber twenty feet high blocking the canyon floor; bones and stones and sticks and mud washed down from the world above, piled up in utter maniac confusion. Geology gone berserk."

Day after day, they struggled down canyon sides, backtracking miles from dead-end gullies, drinking from bitter pools of water left from the summer rains. And then, around a corner, they reached the promised white dunes and willows where the San Juan cascades through a twisting gorge. "There is," says Schultheis, "a beauty to such journeys that knocks logic to smithereens."

Another time he lugged his wooden canoe and overnight gear to Mono Lake at the foot of the parched eastern slope of the California Sierras. "If there were a lake on the moon," he says, "it would look like Mono."

But in contrast to the absolute nothingness of the surrounding alkali flats, the lake itself teemed with life, including thousands upon thousands of grebes--"taut, drab little birds with anxious eyes." And beneath the lake's surface, the water boiled with brine shrimp on which the grebes fed.

"Everywhere, almost to the limits of vision, birds swam and dove and fed: the dance of life and death . . . in the sepulchral heart of that stony country, of all places."

On an island of dry brush and alkali dunes, Schultheis stumbled onto an abandoned farm. "It was not a place meant for men to live," he writes, "perhaps that was its special charm." Nonetheless, daft dreams troubled him during his stay on the island, and in the mornings he awoke exhausted and drained, turning gladly to morning chores to clear his thoughts.

Late one summer, he takes in the annual Indian powwow and rodeo in Bishop, Calif., tucked into a Tibetan-like valley not far from the Nevada line. In the surrounding mountains that are "the color of nothing" grow the oldest living things in the world--bristlecone pines more than 4,500 years old.

Schultheis' experiences have given him an unusual insight into the lives of the Indians who once inhabited what he calls America's geographic purgatories. Arrogant historians considered the Great Basin inhabitants "low and beastly" because they dug for roots and grubs, and dubbed them "diggers." But Schultheis sees them as "almost supernatural survivors," able to conjure water "from a sea of dusty stones" and living in harmony with nature in "subtle grace and sophistication."

Schultheis concludes with a warning to the cities of the West, like Los Angeles, that have arisen in the thirsty, fragile land. The best that all great desert cities of history have left, he says, is a lovely set of ruins. A few years of light snowfalls in the mountain ranges that water Los Angeles could make the city vulnerable.

"Hear the dry snicker of dunes drifting over the elaborate canals, in the doors of abandoned pumping stations," he writes. "I wouldn't bet against it: not against the weight of history, the inexorability of entropy and the eternal, inescapable desert."

To the West-bound traveler this summer, Schultheis provides an intriguing introduction to the region not to be found in the standard guides. He may even lure you into an impromptu exploration of your own. After all, as he points out, just a hundred yards off the interstate, nothing has changed in 10,000 years.