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Cirques, Arêtes, and Horns, Oh My! Glacier National Park

Cirques, Arêtes, and Horns, Oh My! Glacier National Park
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Glacier National Park is filled with cirques, arêtes, and horns. Together these glacial features sculpt the mountaintops to form their famous peaks. It all begins with cirques. Information comes from Mathews (2003 Reference at post base – this is a great field guide if you are in the Rockies a lot, or even a little).

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Glacier National Park Cirque by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This cirque is visible from the top of the Hidden Lake Trail in Glacier National Park as you look east. This one is circular, but many of Glacier’s cirques are square because of the limestone, and the way it breaks and erodes. Note the green at its base. Sediment and water accumulate on the cirque floor, creating a vegetation-rich environment (usually a marshy one).

 

Cirque

  • Cirque means “circle” or “circus” in French
  • It’s a 3-D version of the classic U-shaped glacial valley.
    • There are three rounded sides instead of two, with one open side.
  • In Glacier National Park, the limestone generally forms square cirques.
    • This affects the tarns.
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Tarn Lake by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This tarn lake both forms within its own tarn, and is fed by a stream from another one upslope to the north. A double-whammy. More on the stream in the next photo.

Tarn

  • Some cirques have floor called tarns.
  • Water and sediment collect in these floors, often forming a marsh.
  • Tarn means “small lake” in Scandinavian.
  • In Glacier National Park, the tarn’s water seeps into their limestone base.
  • It forms a spring elsewhere, sometimes feeding a displaced tarn lake.
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Glacial Stream Feeds Pond by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. At the top of Glacier National Park’s Hidden Lake Trail, this spring emerges from nowhere to feed the tarn lake pictured above. The spring probably seeped into the limestone from another tarn to the north (upslope), then flowed through the rocks to form this stream.

Arête

  • Often within Glacier National Park, glaciers within cirques grow so that their tops almost touch.
  • These glaciers carve a knife ridge in the peak top.
  • The sawtooth ridge formed this way is called an arête.
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Arête by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This arête is visible from below the entrance of the Loop Trail in Glacier National Park, to its NW. The razor-top mountain top was carved by two cirque glaciers nearly meeting at the zenith.

Horn

  • Sometimes more than two cirque glaciers grow close.
  • Three to four cirques glaciers can carve their way upward towards a peak, forming a “horn”.
  • Horns are common in Glacier National Park.
  • A famous horn is the Matterhorn (it’s also a great Disney ride).
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Horn in Glacier National Park by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This horn is visible from Glacier National Park’s Hidden Lake Trail, just as Hidden Lake becomes visible, looking to the E-SE. The horn formed when three or more cirques grew together near the top, carving its pointy peak.

I love Glacier National Park.

Reference

Mathews, Daniel

2003   Rocky Mountain Natural History: Grand Teton to Jasper. Altitude Publications Canada Ltd., Raven Editions, Portland, Oregon.

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Cirques, Arêtes, and Horns, Oh My! Glacier National Park by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.


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