Tongass National Forest, National Geographic Magazine: Ice Tunnels In Glacier

Equiped with crampons and emergency equipment, a hiker crawls through an ice tunnel formed in the Mendenhall Glacier. As the glaciers in southeast Alaska melt, ice is exposed thousands of years after being buried. Some tunnels in the 1,500-square-mile Juneau Icefield are connected to ice caves, which formed as the glacier moved across uneven surfaces.

During the Pleistoncene Great Ice Age several climate fluctuations created glacial advance and retreat, and vast sheets of ice covered nearly a third of the Earth’s land mass and one half of Alaska.

As the climate warmed during the Holocene, ice retreated remaining in Alaskan at high elevations. The most recent variation in advance and retreat created the Juneau Icefield formed 3,000 years ago and ending in the 1700’s. Mendenhall Glacier has flowed for 250 years for 13 miles ending in a lake at its’ base.

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