At the tail end of the last Ice Age 14,000 years ago--a heartbeat in geological time--the southernmost tendrils of the Cordilleran ice sheet marched into Idaho and sealed the Clark Fork Valley with a plug of ice several miles wide and nearly half a mile high. A glacial lake backed up into Montana. As the waters rose over decades, pressure built behind the ice dam until, inevitably, spectacularly, it failed. Some 500 cubic miles of pent-up water--equivalent to roughly half of Lake Michigan--screamed across what is now the U.S. Northwest, carrying away anything smaller than a mountain, as it sought equilibrium with the Pacific Ocean.

When the cataclysm reached The Dalles in the Columbia River Gorge six hours later, people wandering around what is now Portland's Marine Drive probably heard a distant murmur. Maybe they looked up, wondering what was up 60 miles down stream. Within minutes, the murmur became a roar, which became a howl.

Then the shockwave, a shaft of compressed air driven by a piston of water perhaps 500 feet high, exploded into Portland with the force of 22,000 Hiroshima bombs. Constricted at Kalama, the deluge retreated up the Willamette River past Oregon City, where it spilled west into the Tualatin Basin, filling the Willamette Valley like a 3,000-square-mile bathtub. And all was quiet.

Until it happened again. And again. Every half-century for 2,000 years. During one of these floods, an iceberg as tall as the Multnomah County Central Library floated into town, carrying 160 tons of metamorphic argillite on its back. Before it reached present-day Sheridan, the giant ice cube melted and tipped, and the boulder sank like, well, 160 tons of metamorphic argillite.

Today, the Belleview erratic (the biggest of more than 300 glacial "erratics" that rafted from Montana during the Bretz Floods), sits at the crest of a lonely ridge overlooking Highway 18, six miles east of Sheridan. If you're not actually looking for the signpost, you'll probably zip right past it.

A pleasant break from the monotony of 18, the half-mile paved trail offers a short but relatively steep trek through a fallow field alive with starlings hunting seeds and bees laden with pollen. Once you crest the hill, the 5-foot-tall, anvil-shaped boulder will probably seem smaller than you might imagine from the above description.

That's because it is. The Belleview erratic weighed 160 tons when geologists first measured it in 1950, and just 90 tons when they came back in 1980. Today, "tourist attrition" has whittled it down to a mere 40 tons.

So the next time you're headed to the coast on Highway 18, pay your respects to the Belleview erratic. Before long (at the present rate, by 2019), the magnificent boulder that outlasted nature's most cataclysmic flood will disappear forever, carried away piece by piece, in pocket after pocket.

Erratic: a geological term used to describe a rock found a considerable distance from its place of origin.

Tourist attrition: a geological term used to describe the everlasting damage wrought by hammer-wielding souvenir hunters.

If you'd like to read more about the Bretz Floods, pick up a copy of

Cataclysms on the Columbia

, by John Eliot Allen and Marjorie Burns (Timber Press, $14.95).

For directions to the Erratic Rock State Natural Site, call 1-800-551-6949 or visit .