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Margerie Glacier, off Tarr Inlet, was a tributary of Grand Pacific Glacier at the time of the original Harriman Alaska Expedition in 1899. Since then, the Grand Pacific has retreated almost ten miles, and Margerie is now a tidewater glacier independent of the Grand Pacific. (Photo by National Ocean Service, NOAA). |
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Reid Glacier is one of several outlets of the massive Brady Icefield. In 1899, the glacier extended much farther into the fjord, and filled the water with brash ice and icebergs. (Photo by Jonas K. Parker). |
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Johns Hopkins Glacier, is more than ten miles down John Hopkins Inlet. A century ago, the glacier extended all the way to Glacier Bay, but like most glaciers in the park has undergone catastrophic retreat. Notice the local tour boat in the bottom right corner. (Photo by National Ocean Service, NOAA). |
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Grand Pacific Glacier, at the end of Tarr Inlet, has its terminus in Alaska, but most of its bulk extends northwest across the border into British Columbia's Tatshenshini-Alsek Provincial Park. It then turns west and reenters Alaska and Glacier Bay National Park, the only glacier with such international tendencies. (Photo by Jonas K. Parker). |
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As gravity pulls glaciers down out of their mountain birthplaces, the ice grinds away at the mountains, ripping off large chunks of rock and abrading smaller chunks down into rock powder, called "glacial flour." When the glacier meets the sea, the glacial flour colors the seawater an iridescent green (shown on the right), a muddy brown (shown on the left), or sometimes a gray or milky white. Note the ice cave on the extreme left. (Photo by National Ocean Service, NOAA). |
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Tidewater glaciers, such as Johns Hopkins Glacier, often have a terminus that rises a hundred feet or more above the sea. When the glacier calves, it appears that ice is breaking off and falling into the sea, but in reality the ice usually extends far below the sea as well. When the above water ice falls away, the ice below shoots up, the air fills with a sharp boom, and a wall of water spreads out from the calving. (Photo by National Ocean Service, NOAA). |
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On its march to the sea, a glacier often appears to be a solid mass of ice. But viewed from the edge, most glaciers reveal their true nature: a tumbled mass of ice, constantly pressed down to the sea by gravity. Along the way, the ice scrapes away at the mountains, grinding everything down to bedrock, an inhospitable home for plants and other living organisms. (Photo by National Ocean Service, NOAA). |
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As the glacier wears away at the mountains, powdered rock mixes with the ice. Over time, as more ice accumulates on the glacier's surface, the powdered rock gives the glacier a banded look. Like trees, the banding gives glaciologists clues on the age of the ice, and sometimes evidence of prehistoric plant and animal life. (Photo by National Ocean Service, NOAA). |
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Most tidewater glaciers in Alaska are "wet," with meltwater streams underneath the glacier. A glacier cave, such as this, is clear evidence of a meltwater stream. (Photo by National Ocean Service, NOAA). |
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Glacial till is the name given to the undifferentiated silt, sand, rocks and boulders created by glaciers as gravity pulls them down out of the mountains. When a glacier retreats, the till remains, and it often takes quite some time before plants can establish themselves in the crushed bedrock. (Photo by Jonas K. Parker). |
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At the edge of the glacier, where the ice meets the surrounding rock, lateral moraines form. When two glaciers meet, the inward moraines join together and form a medial moraine in the new, combined glacier. Some glaciers with lots of tributaries may have a dozen or more medial moraines, each formed by the merging of two glaciers. (Photo by National Ocean Service, NOAA). |
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For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu |