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.
|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 –
|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.