Go with the floe when exploring glaciers

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The Mendenhall is several miles wide and nearly 13 miles long, but it is only one of some 40 rivers of ice that flow down from the Juneau Icefield. An average annual snowfall of 100 feet has blanketed more than 1,500 square miles of coastal mountains under glacial ice up to 4,900 feet thick. Yet this vast reservoir of accumulated snow ranks only as North America's fifth-largest ice field.

Though shrinking in both size and number in these greenhouse days, glaciers are not rare. They occur throughout the most northern and southern latitudes, and many places in between where cold mountains collect more snow in winter than can melt in the year's warmer months. Over decades and centuries, the snow accumulates and compacts the underlying layers into solid ice so dense that its crystalline structure changes, something like the process that transforms coal into diamond. Eventually, gravity gets that seemingly rigid ice mass to start sliding downhill under its accumulated weight. Once a glacier gets going, nothing can stop it.

At one time, all of the Juneau Icefield glaciers flowed downhill until they met the sea, but, like the Mendenhall, most have now shrunk back so that they now terminate in frigid, opalescent lakes drained by short, rushing rivers of melt water.

As the helicopter moved several miles up the ice sheet, the view was awesome. Clearly visible were the moraines, long, dark ribbons of gray running down the gray white glacial face, the stony rubble gouged from the flanks of mountains through which the glacier grinds. The lateral moraines defined the glacier's outer edges, while the medial moraines designate the lines where two fingers of the glacier flowed together, the inside lateral moraine of each slowly merging into a single band. Woe to the mountain between those two fingers, fated to be filed into a knife-sharp flute by their eternal grinding of its flanks.

After the copter landed on an icy plateau, we stepped gingerly across its snowy surface, tiny beings hopping over rivulets of melt water and peering into deep, blue-walled crevasses.

We heard about weird insects that have adapted to life on this frozen river, and we spotted several mountain goats on a distant ridge.

Other than that, not much seemed to be happening on the glacier. Certainly, there was no sense of movement beneath our feet. Then another copter landed maybe a quarter mile away. A fancy-dressed party hopped out and, while we watched, a couple was married on the ice -- matrimony on the Mendenhall.

Then it started to snow again, so we left.

Glaciers in retreat

Several days later, while cruising up through the open waters of Glacier Bay National Park, we craned our necks and peered through binoculars and cameras at full attention. But for the first few hours of the journey, the only ice to be seen was on distant mountain peaks. There weren't even bergs for passing humpback whales to scratch their backs.

The huge bay is separated from the Gulf of Alaska by the Fairweather Mountain range, which peaks out at just over 15,000 feet. Although global warming is a more recent phenomenon, scientists know that Alaska's glaciers have been retreating for about 250 years. In 1794, when explorer George Vancouver first sailed past the opening of Glacier Bay, its mouth was a barely indented tidewater glacier, which ended at the sea. Estimates figured that this ice floe measured more than 100 miles in length, as much as 20 miles in width, and up to 4,000 feet thick.

Since Vancouver's visit, however, this glacier has retreated at a rapid pace. In 1879, less than a century later, John Muir and his party of Tlingit guides could paddle their dugout canoes 48 miles before encountering the glacier that now bears the naturalist's name.

By 1925, the year Glacier Bay was dedicated as a National Monument, the Grand Pacific Glacier, the most northern of its 12 remaining icy tributaries, had retreated to the head of Tarr Inlet, a full 65 miles from the mouth of the bay and less than a mile from the Canadian border. Since then, the Grand Pacific has actually recovered slightly, advancing several hundred yards down the bay to a point where it almost merges again with the Margerie Glacier.

Though next-door neighbors, the two glaciers presented markedly different faces. Measuring more than two miles from end to end, the leading edge of the Grand Pacific rose about 50 feet above the water, its face blackened with stony debris called glacial flour. Though much narrower in width, the face of the Margerie Glacier towered more than 250 feet into the air, a giant's castle of ice with huge flutes and spires. Equally amazing, 100 feet more lie below the water line.

As the Endeavour idled seven hundred yards from where the two glaciers crumble into the seawater, we listened for the deep-throated groan caused by millions of tons of ice slipping between sheared rock walls. The park naturalist who had boarded when we entered the bay that morning told us that both glaciers were racing downhill at the speed of about seven feet per day, constantly shoving their leading faces past the lip of rock at the ice river's mouth and out over deeper water, where great shards sheer off under their own weight.

Maybe half an hour passed before we heard a sharp crack and rumble, like a lightning bolt, as a spire of ice maybe a hundred feet high fractured off the upper edge face of the Margerie Glacier and tumbled into the sea. Its fall set off a swoop of seabirds and widening waves that upset seals basking on ice floes and rippled out to rock our ship and to elicit cheers from the passengers. We quickly became aficionados of floating ice chunks, and learned to differentiate between the different-sized pieces, the bergs, which range from house-sized hunks to desk-sized chunks. Then there are the mid-sized "growlers" and the smallest "bergy bits."

It was also possible to detect differences in the color of the various floating bergs. White bergs are filled with frozen bubbles. Greenish or blackish bergs may have calved off the glacier's bottom, while dark striped bergs may carry remnants of moraine rubble. Some may even harbor encased boulders.

The blue bergs are the dense ice calved from the glacier's heart. Actually, they are not blue but clear as glass, as decades of intense pressure and weight have squeezed out most of the air.

When sunlight hits this ice, lower-energy-frequency colors -- the reds, yellows and greens -- pass right through, while the higher energy frequencies bounce back, giving the ice its characteristic blue color.

We could even hear the snap, crackle, pop of melting ice shards floating in the slurry beneath our bow and detect the faint fizzy smell as bubbles of ancient atmosphere were released after being locked up for centuries.

Life goes on

Cruising the length of Glacier Bay also provides a visual record of the impact of the advance and retreat of these glaciers on the landscape. We know that their glacial progress gouged gorges, dredged channels and scoured away mountaintops. Their great weight can even depress entire land masses.

When they retreat, however, they leave behind barren rock and mounds of glacial rubble. But eventually, life returns.

This process of revival provides observational evidence that a steady, evolutionary succession of flora takes hundreds of years to run its course. This is neatly laid out as you move down the bay. The upper miles most recently glaciated are sheer and sheared.

A bit farther down, the fractured, scoured rock faces become streaked with a "black crust" of algae, which stabilizes the dust and holds water. Then come the moss and lichen, followed by horsetails and fireweed. As deeper soils accumulate, trees take root, willow, alder, spruce and hemlocks. Animal life also eventually begins to recolonize the landscape, beginning with insects and rodents and moving on up to mountain goats and even bears.

Later that afternoon, as the Endeavour pulled up close to the Lamplugh Glacier, our ranger remembered a kayaking trip the previous summer when he camped on a narrow sill along the beach by the edge of the ice mass. All night long, he could sense it moving and feel the ground tremble, and he awoke several times as chunks of ice peeled off and dropped into the bay. He talked about hiking up along the glacier's flanks toward Mount Cooper.

I marked that down as a possibility for further explorations.

It's only water

My education into glaciers continued three days later in the Tracy Arm, a steep-walled fjord that cuts 25 miles into the Alaskan mainland. The sense of anticipation grew as the Endeavour navigated between cliffs that soared up to 2,500 feet. At one point, the captain nosed the ship in so close to shore that two crew members could stand on its bow and fill pitchers from a waterfall that cascaded off a cliff that plunged into the channel.

Near its end, the channel split in two, each lead terminating in a tidewater glacier: South and North Sawyer. We whiled away an hour at each waiting for a calving, and both times our patience was rewarded with satisfying splashes.

At South Sawyer, two crew members took out one of the Zodiac boats and netted a suitcase-sized bergy bit. Hauled on board, the glacial shard was placed in a galvanized tub in the ship's lounge. Bets were taken on how long it would take to melt completely. Over the next few days, interest in the shard increased and plenty of side bets were made. A vigil was kept to ensure no finagling, and four who remained in the betting pool were present when it finally disappeared at 11:06 on the final night of the cruise.

After their celebrations concluded, the contestants went off to bed. I stood a while longer regarding the tub full of clear water. Any sediment that might have been in the ice now settled invisibly to the bottom. Somehow it seemed to me anticlimactic, considering how long the ice from which it came had been around and traveled.

I dipped in a finger and put it to my lips. There was no particular taste to it. After all, water is only water. It has no memory.

While I was pondering that, two crew members came to empty the tub's contents over the side.