Weekend Escape: Mammoth : Upward Bound : Steep bike trails, trees to dangle from . . . What more can an adventurous family ask for?

Nothing stops a tantrum like turning a bend in the trail and spotting your mom 35 feet off the ground, fingernails sunk deep into lodgepole pine bark.

*

A sappy grin swept our 4-year-old’s tear-stained face. “Be careful, Mom!” Bobby called out, suddenly cheerful again.

My own smile was ambivalent.

In an effort to keep money flowing when the snow pack melts, Mammoth Mountain, the vast Eastern Sierra ski resort, has cooked up an array of new . . . things to do --some of which resemble activities that Marine recruits plead tearfully to avoid.

The moment our oldest daughter, Ashley, saw that the minimum age for the resort’s new “adventure challenge course” was 10, she (10 by two weeks) had to give it a try.

And someone had to do it with her.

And I’d been having so much fun lately . . . .

My wife, Pam, fixed me with a peculiar, not entirely hateful stare, then stepped forward, forked over the money ($40 for her, $30 for Ashley) and began filling out the heap of liability waivers.

As a rule, Pam faces life with equanimity. One fear to which she readily admits, though, is a fear of heights. Her only other phobia I can think of might be called “the fear of icky human potential movement numskullery.” So, as I watched my wife and daughter disappear into the towering forest that morning, eyes closed, hands on strangers’ shoulders in a sort of touchy-feely, trust-inducing conga line, I figured the vacation had taken a bad hop.

Our late August three-night stay had started more serenely. Before even reaching Mammoth, we pulled off California 395 at the Hot Creek Fish Hatchery just outside Mammoth and rumbled along the dirt road to the Mammoth Geothermal Area, where we immersed ourselves in California at its blissful best. Snow-encrusted Sierra peaks jabbed into blue sky to the west; the desolate Glass Mountains floated on the desert-dry eastern horizon.

With a blast-furnace wind huffing up Hot Creek, we slipped from currents cold enough to make trout shiver into pockets where magma-heated water grumbles up through the sand.

Lulled into tranquillity, we were ready for summertime Mammoth--a much mellower place than Mammoth in winter, with its mad scramble of Angeleno skiers intent on maximizing their utilization of the facilities after massive recreational expenditures. The Mammoth Mountain Inn, part of the resort, offers a staggering range of lodging options and activities from approximately late May till mid-October. .

*

Overwhelmed by the choices, we simply rented a suite--a cavernous one-bedroom condo with vaulted ceiling and Murphy bed--for $99 a night. The complete kitchenette allowed us to subsist from dawn till dinner time on Cocoa Puffs and grilled cheese sandwiches, before venturing out to sample the resort’s and the town’s eclectic restaurants.

A new, 36-foot-tall, rock-climbing monolith has become the inn’s centerpiece. All day long, swarms of well-put-together parents barked encouragement as their little Biffs and Brees clambered for foot- and toeholds on the fiberglass wall ($10 an hour, $30 for the day; the wall is open weekends only and closes in early October).

“What does it say about a people,” I wondered as I stood there on the lawn, “when their lives are so free from real threat, so devoid of danger, that they crave ersatz adventure? And then transfer that lust to their kids?”

My reverie was interrupted by my 7-year-old daughter. “Belayer ready?” Emily asked. “Belay on,” I said, hauling in climbing rope as she scrambled up the wall.

Ropes and pseudo-rocks are fine, but the main draw at Mammoth in summer is the one most like skiing. So, on our second day, I rented two mountain bikes for the day ($35 each) and bought two passes to the mountain ($18 for a full-day adult pass, $9 for children).

First, Emily and I took off on a gentle, four-mile ride along a forested trail into town. We had lunch and a milkshake at Berger’s, then loaded our bikes onto the resort’s complimentary shuttle for the return trip.

Ashley and I didn’t do as well. Convinced that she could handle the intermediate trails, we loaded our bikes onto the gondola and got off at the halfway point. But Ashley had never ridden a mountain bike. Squeezing the brakes hurt her hands.

As we inched down a dusty zigzag trail, I had a full two hours to ponder an unpleasant truth: A ski resort without snow looks a lot like an open pit mine.

The mountain does have its allure, though. On my own, I found the wind-rocked gondola ride to the 11,053-foot summit gloriously long and lonely, the peak like a lunar outpost. Exiting the gondola station--unpeopled except for a couple of jumpsuit-attired operators--I straddled my bike and tried to gain equilibrium. Wind gusted to 50 m.p.h., and the distant minarets looked more ominous than peaks within a taxpayer-supported national forest have any right to be.

*

Tires skidding in pumice scree, I edged along the eastern brink of the whole Sierra Nevada, glimpsing a forlorn-looking farewell sign to my left as I rolled:

You are at the start of Kamikaze, the “West’s most outrageous ride.” You are riding at your own risk. . . .

Gripping the brakes almost constantly, I rode with what seemed like extreme caution, jaws jangling, legs absorbing the lurch of rocks that my tires spit out like bullets.

Peeling around a corner, I caught sight of a building. I marveled that I’d hit the halfway point so quickly. Then came a disorienting realization: I was ALL THE WAY DOWN. Even with several brief stops to gasp for air, the 3,100-foot vertical descent had taken only 17 minutes.

My adrenal gland’s most exhilarating workout, however, came in the afternoon when Bobby and I spied Pam, high in a tree on that “challenge course”--a guided labyrinth of ropes, ladders and platforms designed to instill courage and self-confidence (in the off season open only to groups of six with advance reservations). We craned our necks as Pam shinnied from that first tree onto a quarter-inch-thick cable, and wobbled along it 40, then 50 feet off the ground. (Ashley was off trying to leap from a 25-foot pole to catch a swinging trapeze.)

Belayed by a total stranger, and encouraged by the shouts of her newfound comrades, Pam reached another tree and hoisted herself up a skittish rope ladder, then threw herself, upside down, onto a swaying wooden platform approximately six stories off the ground.

What came next, she later said, was the scariest thing she’s ever done. (“And the most fun, right? . . . Right?”)

With Bobby and me watching, Pam clipped a carabiner attached to her climbing harness onto another cable--a 400-foot “zip line” that cuts a harsh diagonal to the ground. For a long, long time, she stood staring into the treetops and shaking.

Then she hurled herself off the edge.

Children tend to take the super-hero status of mothers for granted. For Bobby, the sight of Mom sailing through the pine trees--even with that endless, soul-chilling screech she let loose--erased all doubts.

Budget for Five

Gas, L.A. to Mammoth and back: $76.00

Mammoth Mtn. Inn, 3 nights: $323.00

Adventure course: $70.00

Bike rentals: $70.00

Lift tickets: $27.00

Climbing wall: $25.00

Meals at inn and in town: $174.00

Two T-shirts: $28.00

FINAL TAB: $793.00

Mammoth Mountain Inn, 1 Minaret Road, Mammoth Lakes, Calif. 93546; tel. (619) 934-2581 .