It is almost an hour's drive from the last town into this remote monument and the desert that greets you is a forbidding welcome to those accustomed to the grandeur of Yellowstone or Yosemite. The sun has baked the earth dry, and the land bears the scars of a time when livestock were allowed to vanquish the grasses that once grew here.

But this huge national park is remarkable for its surprises, a confluence of three distinctive ecological systems -- the Rio Grande River, the Chisos Mountains and Chihuahuan Desert -- full of unusual plants and wildlife and virtually devoid of the people who have turned other national parks into bumper-to-bumper headaches.

No one stumbles upon Big Bend National Park. It huddles at the bottom of the most barren part of Texas in a country of big skies and arid landscapes. The roughly 800,000 acres that make up the park are a part of west Texas, not so long ago the domain of Judge Roy Bean, who was called the law west of the Pecos River, a title that was earned only after he became known as "the hanging judge."

Big Bend's defining feature is the Rio Grande, which heads southward into Mexico from El Paso, then makes a sharp northeast turn and heads up toward Del Rio, Tex. But its endearing quality is the diversity it offers to visitors. Where else can you walk the river bottom, explore the desert and retreat to the coolness of the mountains in a matter of hours?

Nature rules here at Big Bend, and the flood gauges along the desert highway are a reminder of how quickly the elements can turn the sandy soil into a raging river. Here in the park are mule deer and rattlesnakes, roadrunners and javelina, mountain lions and lizards and soaring turkey vultures -- all within a relatively small area. The desert blooms with wildflowers and flowering cacti in the spring, which is perhaps the best time to visit the park. The winter months are almost as popular because the summer sun makes prowling the desert virtually impossible and the early autumn is the park's rainy season.

We visited here recently and camped in the basin campground in the Chisos Mountains, one of three campgrounds in the park. The basin nestles in the shadow of Casa Grande, a 7,500-foot flat-topped peak that absorbs the evening sun in a spectrum of color, and the camping area is ringed with peaks. When we visited, a full moon popped up over the ridge to illuminate our campsite. The basin is the coolest spot in the park in summer and often the warmest on winter nights, and because of the mountains, it also gets more rainfall than any other area of the park.

The basin is the center of much of the hiking that can be done in the park, including a long trip to the South Rim for one of the loveliest views in the state. A much shorter hike takes visitors from the campground down to the Window, where the sheer rock walls of the mountains narrow to within 20 feet of one another and the water from the basin drains out into Oak Creek Canyon. Every night, the Window frames the setting sun and watching it can be a ritualistic as the sunset parties in Key West, Fla.

Even more spectacular are the rock walls at Boquillas and Santa Elena Canyons at either side of the park. Here, over the centuries, the Rio Grande has cut through the rock to form enormous canyons. At Santa Elena Canyon on the park's west side, the walls rise 1,500 feet straight up -- Mexico to the left and Texas to the right. Most days it is possible to hike to the mouth of the canyon. Almost an hour by car to the east is Boquillas Canyon, also accessible on foot. On the day we visited, recent rainstorms had turned the riverside into a gooey trail of mud, and the shade from the river cane offered us protection from the midday sun. We tromped through water up to our calves before reaching the canyon, and as we left, a young couple had sprawled out along the banks of the Rio Grande to improve their sun tans.

Today much of West Texas seems unchanged from those frontier days of the late 19th century, and there are corners of the park that reflect this untouched quality. One is the little outpost of Castolon, an old cavalry post and later a trading post on the western side of the park that today serves as a ranger station and general store for Mexicans who make the trip across the river from the small town of Santa Helena. The remnants of the cavalry days are evident there on the grounds, as is the rusted hulk of a huge pump used to irrigate the cotton fields that used to bloom near the river.

Another unusual side trip took us across the Rio Grande to the tiny village of Boquillas, just opposite the canyon. We got there the best way possible, in a row boat run by two men from the village who deftly paddled us across the rushing currents of the Rio Grande to the opposite shore. From there, we rented burros for the mile or so trek into the little village, where we refreshed ourselves with beer, Coke and burritos recommended by one of the park rangers.

The day we went there was perhaps the most unusual we spent in the park: driving through desert in the morning, riding burros under the blazing sun into Boquillas by noon, lounging by the river bottom for lunch and in the afternoon, hiking past oak trees and rushing mountain streams to a spot where the mountains reach down and touch the desert floor. It is a place to which we will soon return.