A serac (originally from Swiss French sérac, a type of crumbly white cheese) is a block or column of ice formed by intersecting crevasses on a glacier. Often house-sized or larger, they are dangerous to mountaineers since they may topple with little warning. Even when stabilized by persistent cold weather, they can be an impediment to glacier travel.

Seracs are found within an icefall, often in large numbers, or on ice faces on the lower edge of a hanging glacier. Examples of the latter in the Alps are found on the north-east face of Piz Roseg, the north face of the Dent d'Hérens and the north face of Lyskamm. They are a well known obstacle on the Kanchenjunga.

firn at 10,000 feet (3,050 meter) on the Winthrop Glacier of Mount Rainier in Washington, USA

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