When describing glaciers in Wrangell St. Elias National Park, superlatives are hard to avoid. Within its boundaries are enclosed the nation's largest glacial system. Glaciers cover over 25 percent, or approximately 5,000 square miles, of the park. In summer, these glaciers contribute a significant portion of the rivers' high runoff and heavy sediment load. During the winter, glacial melt ceases and many rivers run with clear water.
Some notable park glaciers include:
Glaciers, perennial accumulations of ice, snow, sediment, rock and water, respond to changes in temperature, snowfall and geologic forces. Several components make up a glacial system: the ice and sediment contained in the glacier; the valleys, fiords and rock features it flows over, on, or around; and the deposits left by its retreat or advance.
Moisture-laden air masses from the ocean are forced to quickly rise when they collide with the high mountains of the Wrangell and St. Elias Ranges. As the air rise, it cools and releases its moisture in the form of deep snowfall. New snow layers create pressure on existing layers of snow and ice. This process, "firnification," slowly squeezes out the air pockets and changes fluffy snow into denser firn, a dense granular snow (like corn snow). After the first season's melt, snow becomes firn. As it is compressed further, firn eventually becomes ice.
As the snow collects over many years, an ice field forms. Icefields feed glaciers. When thick enough, ice will begin to flow downslope, eventually filling valleys, sometimes flowing all the way to the sea. The Bagley Icefield which runs along the crest of the Chugach Range for about 120 miles, is the largest subpolar icefield in North America. It covers most of the core of the Chugach Mountains and nourishes dozens of valley glaciers that drain down both sides of the range.
Glaciers form where more snow falls than melts. A glacier's accumulation area, located at higher elevations, accrues a wealth of snow and ice. The ablation area, located at lower elevations, loses ice through melting (downwasting) or calving. A glacier's terminus or face advances when more snow and ice amass than melt, and it retreats when melt exceeds accumulation. When melt equals accumulation, a glacier achieves equilibrium and its face remains stationary. Whether the glacier's face is advancing or retreating, glacial ice persistently glides down-valley.
Coerced by gravity, ice pursues the path of least resistance. Ice depth and bedrock angle influence the rate of glacial flow. Glaciers contain two zones of ice flow. The zone of plastic flow, ice closest to the bedrock, experiences extreme pressure from the weight of the ice above and conforms to the anomalies in the bedrock. The zone of brittle flow, the upper 150 feet of glacial ice, lacks this pressure and reacts in-elastically to the bedrock features, forming elongated cracks called crevasses which fluctuate with the glacier's flow. Tubular chutes or moulins drain surface meltwater, and formidable spires of ice called seracs reach skyward. Ice plummets over particularly steep terrain creating ice falls. One theory suggests that differences in seasonal flow rates over an icefall create the convex bands called ogives at the base of the falls, which undulate down glacier. The erosive power of glacial flow changes the landscape and scrapes much of the soil and rock from the valley walls that channel its irrepressible flow.
The landscape around a glacier clearly illustrates the effects of Pleistocene and Holocene glaciation. Ice excavates the bedrock, forming bowl-shaped cirques, pyramidal horns, and a series of jagged spires called arête ridges that separate glacial valleys. As glaciers carve U-shaped valleys, rocks plucked from the bedrock and frozen in the ice etch grooves and striations in the bedrock. Rocks scoured from surrounding valley walls create dark debris lines called lateral or medial moraines along the edges and down the center of glaciers. Pulverized rock called rock flour, ground by the glacier to a fine powder, escapes with glacial meltwater producing the murky color of glacially fed rivers and lakes. Glacial recession unmasks trimlines, slightly sloping changes in vegetation or weathered bedrock on the valley walls that indicate a glacier's height at its glacial maximum. Meltwater transports glacially eroded material to the outwash plain, an alluvial plain at the edge of retreating glaciers. Icebergs break away or calve from the faces of glaciers ending in lakes or the ocean.
Cracked pieces of rock, plucked or torn from the bedrock, are carried with other debris in and on the glacier. This debris scrapes the valley walls and floors, leaving grooves and striations. Rock debris is crushed and ground into fine grains, called rock flour.
Hikers should not attempt to cross glaciers without proper equipment including crampons, ropes and iceaxes. Even the gravel covered moraines will turn slick and dangerous during or after a rain.
Please discuss your plans with a ranger before undertaking glacial travel or mountain peak ascents. Guides are available for these activities and can be used to gain experience.
A great way to see the park's glaciers and icefields is from the air. There are a number of flightseeing operators that offer a variety of spectacular tours.
Did You Know?
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