page_title: Denali National Park and Preserve
section_title: Glaciers
hierarchical_section_title: Denali National Park and Preserve / Geology / Glaciers
caption_attribution_description: English: Denali National Park and Preserve, Alaska, USA, Ruth Glacier and the Great Gorge
context_page_description: Denali National Park and Preserve is an American national park and preserve located in Interior Alaska, centered on Denali, the highest mountain in North America. The park and contiguous preserve encompass 6,045,153 acres which is larger than the state of New Hampshire. On December 2, 1980, 2,146,580-acre Denali Wilderness was established within the park. Denali's landscape is a mix of forest at the lowest elevations, including deciduous taiga, with tundra at middle elevations, and glaciers, snow, and bare rock at the highest elevations. The longest glacier is the Kahiltna Glacier. Wintertime activities include dog sledding, cross-country skiing, and snowmobiling. The park received 594,660 recreational visitors in 2018.
context_section_description: Glaciers cover about 16% of the 6 million acres of Denali National Park and Preserve. There are more extensive glaciers on the southeastern side of the range because more snow is dropped on this side from the moisture-bearing winds from the Gulf of Alaska. The 5 largest south-facing glaciers are Yentna (20 miles (32 km) long), Kahiltna (30 miles (48 km)), Tokositna (23 miles (37 km)), Ruth (31 miles (50 km)), and Eldrige (30 miles (48 km)). The Ruth glacier is 3,800 feet (1,200 m) thick. However, the largest glacier, Muldrow Glacier (32 miles (51 km) long), is located on the north side. Nonetheless, the northern side has smaller and shorter glaciers overall. Muldrow glacier has "surged" twice in the last hundred years. Surging means that it has moved forward for a short time at greatly increased speed, due to a build-up of water between the bottom of the glacier and the bedrock channel floating on the ice (due to hydrostatic pressure).
  At the upper ends of Denali's glaciers are steep-walled semicircular basins called cirques. Cirques form from freeze-thaw cycles of meltwater in the rocks above the glacier, and by glacial erosion and mass wasting occurring under the glacier. As cirques on the opposite sides of a ridge are cut deeper into the divide, they form a narrow, sharp, serrated ridge called an arête. As the arête wears away from glacial ice erosion, the low point between cirques is called a col (or if it is large, a pass). Cols are saddle-shaped depressions in the ridge between cirques. A spire-like sharp peak, the horn, forms when cirques cut back into a mountaintop from three or four sides.
  Glaciers deposit rock fragments, but the most notable of the depositions are the erratics, which are large rock fragments carried some distance from the source, found on glacial terraces and ridge tops in many places throughout Denali. Headquarter erratics are made of granite and can be the size of a house. Some erratics (like those from the Yanert Valley) are located 30 miles (48 km) away from their original location.
  Large amounts of rock debris are carried on, in, and beneath the ice as the glaciers move downslope. Lateral moraines are created as debris accumulates as low ridges of till that ride along the edge of the moving glaciers. When lateral moraines adjacent to each other join, they create medial moraines, which are also carried down on the surface of the moving ice.
  Braided meltwater streams heavily loaded with rock debris continually shift and intertwine their channels over valley floors. Valley trains are built up as streams drop quantities of poorly sorted sediment. Valley trains are long, narrow accumulations of glacial outwash, confined by valley walls.
  Kettles are formed when glacial retreat and melting is rapid, and blocks of ice are still buried under till. When the ice under the till melts, the till slumps in and forms depressions called kettles. When kettles fill with water, they are known as kettle lakes. Kettle lakes are visible near the Polychrome Overlook, the Teklanika rest stop and near Wonder Lake.
caption: Ruth Glacier and medial moraine - the dark stripe of debris down the middle
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