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Glossary of Glacier Terminology

Firn

An intermediate stage in the transformation of snow to glacier ice. Snow becomes firn when it has been compressed so that no pore space remains between flakes or crystals, a process that takes less than a year.


Photograph showing a scientist collecting snow and ice samples from the wall of a snow pit. Fresh snow can be seen at the surface and glacier ice at the bottom of the pit wall. The snow layers are composed of progressively denser firn. Taku Glacier, Juneau Icefield, Tongass National Forest, Alaska.

Oblique aerial photograph of the lower reaches of an unnamed Alaskan valley glacier, located east of Yakutat Bay, St Elias Mountains, Tongass National Forest, Alaska. The transition from bare glacier ice to snow-covered glacier is subtle. It can be found by noting the appearance of medial moraines.

Firn line

A line across the glacier, from edge to edge, that marks the transition between exposed glacier ice (below) and the snow-covered surface of a glacier (right). During the summer melt season, this line migrates up-glacier. At the end of the melt season the firn line separates the accumulation zone from the ablation zone.


North-looking oblique aerial photograph of the lower reaches of an unnamed Alaskan valley glacier, informally known as Five Stripe Glacier, showing the abrupt transition from bare, blue glacier ice to snow-covered glacier. This is the firn line. Also show are lateral and medial moraines, trimlines, outwash sediment and many other related glacial features, Chugach Mountains, Alaska.

Fjord

A glacially eroded or modified U-shaped valley that extends below sea level and connects to the ocean. Filled with seawater, depths may reach more than 1,000 feet below sea level. The largest Alaskan fiords are more than 100 miles long and more than 5 miles wide. Also spelled Fiord.


Oblique aerial photograph of College Fiord (the right arm) and Barry Arm, two fiords located in western Prince William Sound, Chugach National forest, Chugach mountains, Alaska.

Photograph of the terminus of Lamplugh Glacier, Glacier Bay National Park, Alaska, showing numerous folia exposed in cross-section. Note the complex character of the folia, especially their intersecting and truncated character. The field of view from top to bottom is ~ 200 ft.

Foliation

The layering or banding that develops in a glacier during the process of transformation of snow to glacier ice. Individual layers, called folia, are visible because of differences in crystal or grain size, alternation of clear ice and bubbly ice, or because of entrained sediment.


East-looking oblique aerial photograph of the middle reaches of Sherman Glacier, an Alaskan valley glacier, located east of Cordova. Folding, and subsequent melting of the glacier has revealed a nested stratigaphy of concentric folia, Chugach National Forest, Chugach Mountains, Alaska.

Fountain

A glacial spring, generally discharging supercooled water with a significant hydrostatic head.


North-looking photograph of a large fountain composed of rock-flour-rich, cold glacial meltwater. The fountain is bubbling to the surface of an unnamed ice-marginal stream adjacent to the debris-covered western terminus of Malaspina Glacier, St Elias Mountains, Wrangell-St. Elias National Park, Alaska.

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