"Dak!"
"Yo, dawg."
"You bring anything to eat?"
"Nah."

Dakota Blackhorse-von Jess, 25, finished off the quart of oatmeal he'd brought along to the base of the South Sister, Oregon's third-highest mountain. Then he chucked the dish into my car, strapped some skis onto his backpack, and began swaggering up the trail, a stocky national-class cross-country ski racer at ease in the great outdoors. I followed in his dust, along with the lanky and bearded Reitler Hodgert, 18, his teammate on the elite Bend Endurance Academy Nordic Team. It was early September and hot. Forest fires smoldered below us, and the acrid smoke nagged at our lungs as we wended our way up through ponderosa pines.

Left: Access to late-season backcountry snow is complicated by, among other things, crevasses. Right: Heading toward the bowl of Broken Top Mountain in central Oregon.

Not even Blackhorse-von Jess (whose name reflects his Nez Perce and German ancestry) had skiied in September before, and every other skiier I'd asked to join me on this late-summer tour had demurred. Conventional wisdom holds that September snow, when it exists, is dirty and wrecked by weather. But I wanted to ski it. I'd recently grown semi-serious about c ross-country ski racing—a sport that depends on getting a feel for snow in its myriad, morphing varieties. A good skier knows, for example, that cold, dry, sharp-crystalled snow—the sort that squeaks underfoot—skis far more slowly than hard-packed old snow basking in the relative warmth of 25 degrees Fahrenheit, and that a cold, clear day poised to warm up requires a complex layering of three different waxes.

I wanted to learn snow. And in isolated pockets of the western United States, spring and summer offer ski enthusiasts something special. The wide-open slopes of the high mountains, buried deep in loose powder all winter, thaw and freeze, and thaw and freeze a bit more. A firm but not icy crust forms on the surface that you can glide atop, sometimes "crust cruising" for miles. It's as if the streets of a city were suddenly coated with ice, delivering residents the magical possibility of skating anywhere they pleased.

The dry, sunny days and crisp, cold nights of central Oregon's Cascade Mountains provide perfect conditions for good crust. Even there, however, crust cruising—like surfing—involves waiting for conditions to cooperate. Last May, when I'd hoped to make my first-ever foray out onto the crust, a series of blizzards blew through the Cascades. It was like January up there, even as kids played Little League baseball in the valleys below. Finally, in mid-June, I got an e-mail from my skiing coach, J.D. Downing, director of XC Oregon. Subject line: "Crust."

We met early the next morning in Bend, drove into the mountains, and herringboned up through sparse woods toward Broken Top, a 9,175-foot peak five miles east of South Sister. The snow was rock hard, and when we crested a ridge and could finally go fast—on an immense, gently sloping plateau that climbed to the Tam McArthur Rim—Downing quipped that we were "waterskiing in a hurricane." Indeed, the frozen moonscape was pocked with grapefruit-size divots, hellaciously uneven and bumpy, making for a knee-jarring ride.

THE DIVOTS WERE "SUN CUPS," tiny craters carved out by the sun. Later I'd learn that—among snow hydrologists at least—they are the subject of spirited debate. "We're not really sure how these inhomogeneities in the surface begin," said Timothy Link, a hydrologist at the University of Idaho. "Some people say they're caused by ripples from the wind. Others think that it's a matter of the sun penetrating into the snow and refracting off crystals and melting them." What often happens, in any case, is that a piece of debris—a scrap of bark, say, or a fleck of dirt—blows into the nascent divot and then acts as a solar absorber. Sun cups grow on clear, sunny days. And, mercifully, they also become softer. As Downing and I skated along toward the rim, the snow grew slower but more pliable.

Water running beneath summer snow—a potential death trap.

It was 9 a.m., the golden hour, roughly speaking, for crust cruising—it marks the transitional period between ice and slush—and the snowfields were swarmed by high school kids, young hotshots in Bend for a weeklong cross-country ski camp. Many were skiing without shirts and treating the parabola-shaped slopes of Broken Top as a sort of launch ramp. I stood and watched at least 10 adolescent males careen down from the highlands, knees wobbling as they worked their way over the cups. They held their poles wide and clenched their fists as they assumed the archetypal about-to-eat-it stance. Miraculously, they all reached the flats unscathed, summarizing the experience with remarks meant to convey a cool savoir faire.

"Yeah, bro!"
"Awesome!"

As the summer progressed, the sun bore down and the cups grew bigger, the crust bumpier and less navigable. Gradually it became the province almost exclusively of gonzo youth. It was week one of the NFL season when I found myself packing my boards up South Sister, listening in for three hours as Blackhorse-von Jess schooled Hodgert on the relative merits of various Norwegian ski racers. I scrambled behind them to the top of a ridge, where, in a ripping warm wind, we looked down at the steep slopes of the glacier. I figured that the snow would be radically different from what I'd skied a few miles away in June. Link, the hydrologist, had told me of a certain red algae, Chlamydomonas nivalis, that lives in high mountain snowfields during warm months. It can be so profuse, he said, as to make garishly pink "watermelon snow." The algae also serves as food to snow fleas—minuscule blue-black bugs that leap about on their hindquarters, looking like millions of hopping grains of pepper.

This September day, however, there were neither visible algae nor hopping fleas, and even the sun cups weren't that bad. The glacier had the pitch of a black-diamond alpine slope, although its heavy, mushy snow was ungroomed, and we were on free-heel skinny skis without metal edges—a circumstance that Blackhorse-von Jess found sublime. "Eh, it's not that steep," he said, and then began schussing down, carving giant turns in the dirt-flecked slush. "This is where Dakota always hurts himself," Hodgert snickered, "trying to be a hero."

Skiing on summer crust is like 'waterskiing in a hurricane.'

Then Hodgert and I began floating downhill. Remembering alpine skiing skills I'd learned long ago, I leaped my way through some wide step turns. My descent was still a vaguely controlled free fall. In time, we reached a tiny, emerald green pond—pure snowmelt—backed by a heap of volcanic rock.

"Pond skimming, Dak?" Hodgert said.

Blackhorse-von Jess maintained that gliding across at high speed wouldn't be hard. "It's waterskiing, basically, but here"—he gestured toward the rocks—"the landing would be a little harsh."

We climbed 100 yards to a rocky outcropping, took off our skis, and picked our way over some boulders. Reaching snow once again, we sailed along a steep side slope. The crust was rougher here, and a little bit cupped. Blackhorse-von Jess remarked that crust skiing was excellent training. "It's good for agility and balance," he said. "If you can work your way through variable terrain like this, you can work your way through anything." I saw that his hands weren't in the handle grips of his poles, but lower down, choking the high neck of the shaft. "It's so if I fall in the soft snow, I won't break a pole, or a wrist," he said. "But it's no big deal, really."

There are no rooster tails in crust skiing.

We descended—10 or 15 more turns apiece. When we got down to the edge of the snow, we stopped and, leaning over our poles, looked up to behold the swooping S shapes of our tracks. "Good stuff," Hodgert said. "Good stuff." And it was a satisfying sight. We had skied for only 25 minutes or so, but we had stolen something sweet: We had found decent snow in September.

We hiked down and got some tacos in Bend. On the drive home to Portland, I got out of the car on the pass going over Mt. Hood. It was night now, and cold out—there was a definite bite in the air. In only six or eight weeks, it would start snowing again.