Pingo scars
– what on earth is a pingo scar?
On earth is
just what it is, an earthform . Years ago a friend who was a geologist took
several of us to see an array of water-filled pingo scars in the Big Woods and
explained how they were formed:
During the
last 1 ½ million years a succession of glaciers formed in the northern reaches
of Canada and gradually spread southward covering parts of Pennsylvania until
conditions changed and they began melting and retreated northward. The last of
these glaciations is called the Wisconsinan and was present in Pennsylvania
from 22,000 years ago until 17,000 years ago.
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Adapted from Keystone.edu |
South of
the glacier arctic-like conditions prevailed for even longer; the vegetation
resembled that of extreme northern Canada and Alaska and the soil was
perpetually frozen in a condition called permafrost.
When the soil froze, perhaps to a depth of
as much as 25 feet, it increased pressure on the groundwater below. Due to the
increased pressure, warm groundwater was forced upward into the permafrost,
freezing as it encountered colder temperatures, and created scattered mounds of
ice. As these mounds grew, they would push the overlying soil up, some of which
would slide to the sides. When the ice melted, it left a depression, surrounded
by a small mound. The process is shown here –
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By Janet Stone USGS |
Go far
enough north and you could still find active pingos. Here, pingos melted as the
glaciers retreated and now, 10-15,000 years later we have the pingo scars.
They’re shallow saucer-like depressions typically 50-150 feet in diameter and
3-15 feet deep and usually have a low rim.
Many,
probably most, of the vernal pools in the Big Woods are pingo scars – breeding
places for wood frogs and many species of salamanders because they usually dry
up in late summer and theref0re can’t support fish populations.
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Spotted Salamander |
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Wood Frog |